- w _

— . . —

HANDBOOK~TO BRITISH

STANDARDB58110:1985STRUCTURALOF CONCRETE

USE

I--

PREFACEI It has been a tradition since the first DSIR Codefor reinforcedconcrete,publishedin

1934, for an explanatoryhandbookto be prepared.This work was undertakenby theteamof Scott, Glanville andThomas.andthe versionof theHandbookto CP114:1965,published in 1965. is still relevant. Similarly, for prestressedconcrete, a guide toCP115:1959was preparedby XValley andBate andpublishedin 1961.

With the combinationof the various codesinto the Unified Codeof practicefor thestructural use of concreteand the incorporation of limit state design procedures.theCodedraftingcommitteeexpressedthe desireandneedfor thetradition to becontinued.However,the scopeandcontentof CPI 10 necessitateda somewhatdifferentapproachfrom that in the pastin that. firstl . therewas a needto involve more authorswho hadbeen intimatelyconcernedin preparingthe draft clausesfor the Codecommitteeand.secondly,the sheervolumeof materialprecludedthe inclusionof the actualcodeclauses.The Cement and ConcreteAssociation.having alreadytaken over responsibilityforpublishingthe existingHandbookandGuide,agreedto publish the Handbookto CP11O.and an appropriateteam of authorsagreedto undertakethe task of producing thematerial. An editorial group.consistingof Drs Bate.Cranston.Roweand Somerville.integratedandcorrelatedthe material.

Now that the revisedversionof CPIIO hasbeenpublishedas BS 8110.a neweditionof the Handbook was required and Palladian Publications Ltd has assumedtheresponsibility for publishing it. As before.a group of authorswas assembledand aneditorial teamappointed— this consistedof Dr Rowe. Dr Somervilleand Dr Beebyofthe CementandConcreteAssociationtogetherwith Dr Menziesof theBuildingResearchEstablishment.

Noteon numberingat’ TablesandFigures

TablesandFiguresin thisHandbookareprefacedby ‘H~ (e.g.FigureH3.19) todistinguishthem from Tablesand Figuresin the Code itself, which are referredto by the numberalone(e.g.Table 3.1). TablesandFiguresin Part2 of the Handbookarealsoprefacedby (2) (e.g. Figure H(2)3.1).

[IFOREWORD

D. D. Matthews,MA. DEng. FEng. FICE. FiStructE.FAmSocCE

Chairman of the Code Committee

It may be recalledthat for over halfa centurytherehasbeena Handbookto the currentBritish ConcreteCode. First therewas Scott and Glanville on the DSIR Code, later rScott. Glanville and Thomason CPI14 and \Vallev and Bate on CPuS. This practice Lwascontinuedfor CP110:1972by the Handbookproducedby the CementandConcreteAssociationunderthe generalauthorshipof DrsBateandRowe.TheDraftingCommitteeCSB/39 in its preparationof BS 8110:1985welcomed the proposalof the currentHandbookunderthe generaleditorship of Dr Rowe.Director-Generalof the Cementand ConcreteAssociation and currently Chairmanof the Structural CodesAdvisoryCommittee of the Institution of Structural Engineers. Dr Menzies of the Building rResearchEstablishment,and Drs Somervilleand Beebyof the CementandConcreteAssociation.The draftingof a British Codeof Practicefor the StructuralUseof Concreteis necessarilydependenton the contributionsprovided by the serving panelsof the rStructuralCodesAdvisorv Committeeof the Institution of StructuralEngineers,by the LBuilding ResearchEstablishmentandthe CementandConcreteAssociation.

The explanationsof the changesbetweenCPI10:1972and BS 8110:1985should beinvaluableto readersinterestedin the up-to-dateart andscienceof practicalstructural

concrete.It is a pleasureto recommendthe Handbookto the readerbecauseit supplements

the Code with the highestpossibleauthority and is written in a mannerwhich reflects Fthe successfulinteractionbetweenthe authorsand the othermembersof the Drafting 1:Committee.

CCCI;

LIULLU

CONTENTSPART 1 — CODE OF PRACTICE FOR

DESIGN AND CONSTRUCTION

Section one. General

5.3 Structuralconnectionsbetweenprecastunits

5.4 Compositeconcreteconstruction

11 Section six. Concrete: materials,ii specification and constructionii

Section two. Design objectives andgeneral recommendations2.1‘ 2.22.32.4

&ction three. Design and detailing:reinforced concrete3.1 Design basisandstrengthof materials3:2 Structuresandstructuralframes] Concretecover to reinforcement3.4 Beams3:S Solid slabs supportedby beamsor walls

Ribbedslabs (with solid or hollow blocks11 or voids)3.7 Flat slabs‘ 3.8 Columns3.9 Walls3.10 Staircases3.11 Bases3.12 Considerationsaffecting designdetails

151617172020

222530344749

515765696971

6.1 Constituentmaterialsof concrete6.2 Durability of structuralconcrete6.3 Concretemix specification6.4 Methodsof specification,production.

control andtests6.5 Transporting,placingandcompacting

concrete6.6 Curing6.7 Concretingin cold weather6.8 Concretingin hot weather6.9 Formwork6.10 Surfacefinish of concrete6.11 Dimensionaldeviations6.12 Constructionjoints6.13 Movementjoints6.14 Handlinganderectionof precastconcrete

units

Section seven. Specification andworkmanship: reinforcement7.1 General7.2 Cutting andbending7.3 Fixing7.4 Surfacecondition7.5 Lapsand joints7.6 Welding

Section four. Design and detailing:prestressed concrete4.1

J 4.24.34.44.5

4.6

4.9-j 4.10

Design basisStructuresandstructuralframesBeamsSlabsColumnsTensionmembersPrestressineLoss of prestress.other than friction lossesLossof prestressdue to frictionTransmissionlengthsin pre-tensionedmembers

4.11 Endblocks in post-tensionedmembers4.12 Considerationsaffecting designdetails

Section five. Design and detailing:Precast and composite construction5.1 Design basisandstability provisions5.2 Precastconcreteconstruction

8384859898989899

102103

104104

Section eight. Specification andworkmanship: prestressing tendons8.1 General8.2 Handlingandstorage8.3 Surfacecondition8.4 Straightness8.5 Cutting8.6 Positioninaof tendonsandsheaths8.7 Tensioningthe tendons8.8 Protectionandbond of prestressing

tendons8.9 Groutingof prestressingtendons

PART 2— CODE OF PRACTICE FOR

SPECIAL CIRCUMSTANCES

Section one. General1.1 Scope

110 1.2 Definitions111 1.3 Symbols

1.1 Scope1.2 Definitions1.3 Symbols

115

121

Basisof designStructuraldesignInspectionof constructionLoadsand materialpropertiesAnalysisDesignsbasedon tests

127131135141

143

144145145145146146147148148

154156156156156157

158160160160161161161162

163

167167167

7

Section two. Non-linear methods ofanalysis for the ultimate limit state2.1 General2.2 Design loadsandstrengths2.3 Restrictionson use2.4 Torsional resistanceof beams2.5 Effective column height2.6 Robustness

168168169169170171

Section three. Serviceability calculations3.1 General3.2 Serviceability limit states3.3 Loads3.4 Analysisofstructurefor sex-viceabilitvlimit

states3.5 Material propertiesfor the calculationof

curvatureandstresses3.6 Calculationof curvatures3.7 Calculationof deflectionLI Calculationof crack width

i~ ~stlon four. Fire resistance4.14.2

GeneralFactorsto beconsideredin determiningfireresistance

4.3 Tabulateddata(method 1)4.4 Fire test (method2)45 Fire engineeringcalculations(method3)

173174178178

178

178179179

183186

188189189

Section five. Additional considerations inthe use of lightweight aggregate concrete5.1 1915.2 1925.3 1925.4 1925.5 1935.6 1935.7 1935.8 1935.9 1935.10 193

GeneralCoverfor durability andfire resistanceCharacteristicstrengthof concreteShearresistanceTorsionalresistanceof beamsDeflectionsColumnsWallsAnchoragebond andlapsBearingstressinside bends

Section six. Autoclaved aerated concrete6.1 General6.2 Materials6.3 Reinforcement6.4 Productionof units6.5 Methodsof assessingcompliancewith limit

state requirements6.6 Erection of units6.7 Inspectionandtesting

Section seven. Elastic deformation, creep,drying shrinkage and thermal strains ofconcrete7.17.27.37.47.5

GeneralElasticdeformationCreepDrying shrinkageThermalstrains

Section eight. Movement joints8.1 GeneralL2 Needfor movementjoints83 Typesof movementjoint8.4 Provision of joints8.5 Designof joints

Section nine. Appraisal and testing ofstructures and components duringconstruction9.1 General9.2 Purposeof testing9.3 Basisof approach9.4 Checktestson structuralconcrete9.5 Loadtestsonstructuresorpartsofstructures9.6 Load testson individual precastunits

Tables

Figures

[I195195195195196

196196

197198198198198

199199199200200

201201201201201202

203

204 [

LL[LU

PART 1. CODE OF PRACTICE FOR~ DESIGN AND CONSTRUCTION

~-. - ;::-..‘. - -.fl

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SECTION ONE. GENERAL

1.1 Scope

1.2 Definitions

Seerelevantsections.

1.3 Symbols

The hugenumberof variableswith slightly different definitionswhich haveto be usedin Codesof Practice.make notationa difficult problem. To list a different symbol forevery possiblemarginally different parameterwould result in a totally unwieldy system.The BS 8110 and CP1IO Committeestook an alternativeapproach.using a conceptemployedin computerprogrammingof local’ and~lobal’ variables.On this basisit wasdecidedthat wherea symbol was usedonly in a particularclauseor equation.it couldbe definedwithin that clausewithout it implying any meaningto the symbol in a moregeneralsense.Thiswasdevelopedfurtherby adoptingthe Americansystemof providinga list of symbolsat the beginningof eachsectiondefiningthesymbolsusedin thatsectionratherthan a generallist at the startof the Code. An attempthasbeenmadeheretogiveagenerallist of symbols. In a numberof casesthe list appearsto containambiguities.However, as the Handbookis designedto be used in conjunctionwith the Code, thereaderwill find that no ambiguitiesactuallyoccur in use.

A areaof tensile reinforcementor prestressingtendonsareaof concreteareaof concretein compressionareaof steelrequiredto resist horizontalshearareaof prestressingtendonsin the tensionzone

A, areaof tensionreinforcementA

6b areaof bent-upbarsareaof compressionreinforcement,or in columns.the areaof reinforcementareaof compressionreinforcementareaof tensionreinforcementprovidedatmid-span(atsupportfor acantilever)areaof compressionreinforcementprovided

As.req areaof tensionreinforcementrequiredatmid-spantoresistthe momentduetodesignultimate loads(at support for a cantilever)

Asr areaof transversesteel in a flangeareaof shearreinforcement,or areaof t~vo legsof a link

a deflectiona distancefrom the compressionface to the point at which the crack width is

being calculated

centre-to-centredistancebetweenbars (or groupsof bars)perpendiculartothe planeof benddistancefrom thecrackconsideredto thesurfaceof thenearestlongitudinalbarangleof internal friction betweenthe facesof the jointdeflectionof column at ultimatelimit stateaveragedeflectionof all columnsat a given level at ultimatelimit statelength of that partof a membertraversedby shearfailure plane

b width (breadth)or effectivewidth of sectioneffectivesectiondimensionof a column perpendicularto the v axisbreadth of the compressionface of a beam measuredmid-way betweenrestraints(or the breadthof the compressionface of a cantilever)

be breadthof effectivemomenttransferstrip (of flat slab)width of sectionat the centroidof tensionsteel

Ii

Handbook to BS8J]O.-1985 [Iwidth (breadth)of sectionusedto calculatethe shearstressbreadthor effective breadthof the rib of a beam C

C torsionalconstant,or cover to main reinforcementCave effectivecoverC~, C~ plan dimen~ionsof column 1!c width of columncmin minimum coverto the tensionsteelc~, c~ plan dimensionsof column, parallel to longer and shorter side of base

[respectivelyd effective depth of sectionor, for sectionsentirely in compression,distancefrom most highly stressedface of section to the centroidof the layer ofreinforcementfurthestfrom that face [depthto the compressionreinforcement

dh depthof the head(of a column)depth to the centroid of the compressionzone Cdepthfrom theextremecompressionfibre eitherto the longitudinal barsor tothe centroidof the tendons,whicheveris the greaterstaticmodulusof elasticityof concrete

Ecq dynamicmodulusof elasticityof concrete Cstaticmodulusof elasticityof concreteat age

E,ff effective(static)modulusof elasticityof concretenominal earthload [modulusof elasticityof reinforcementmodulusof elasticityof concreteat the ageof loading

modulusof elasticityof concreteat ageof unloading Cinitial modulusof elasticityat zerostresse eccentricity,or the baseof Napierian logarithmsea additionaleccentricitydue to deflections

resultanteccentricityof load atright anglesto the planeof the wall Cresultanteccentricitycalculatedat the top of a wallresultanteccentricitycalculatedat the bottomof a wall

F total designultimate loadon a beamor strip of slabFb designforce in a bar used in the calculationof anchoragebondstresses

designburstingtensileforce in an anchoragezoneFbf tensile force dueto ultimate loadsin a baror groupof barsin contactat the

startof a bendF, force in a bar or groupof bars

basicforce usedin definingtie forces/ stress Cbond stressfbu designultimate anchoragebond stress

maximumcompressivestressin the concreteunderserviceloadsconcretestrengthat transferdesigncompressivestressdue to prestressdesignstressat distancex from the endof membercharacteristicstrengthof concrete

fpb designtensilestressin the tendonsdesigneffectiveprestressin the tendonsafter all lossescharacteristicstrengthof a prestressingtendon I/5 estimateddesignservicestressin the tensionreinforcementf maximumdesignprincipal tensilestress

f~. characteristicstrengthof reinforcementcharacteristicstrengthof shearor link reinforcement [G shearmodulus

Gk characteristicdeadloadH storeyheighth overalldepthof thecross-sectionmeasuredin theplaneunderconsideration

effectivesectiondimensionin a direction perpendicularto the x axismaximumsize of the coarseaggregate CIz~ effectivediameterof a column or column headdepth (thickness)of flange

~. ..

~ I . -.-‘~ --I .... . I---. -. -.. . - . -~ -. - -a Handbook to BSSIIO:1985

hmax larger dimension of a rectangular sectionhmin smaller dimension of a rectangular sectionI second moment of area of the sectionK coefficient. as appropriateL span of member or, in the case of a cantilever, lengthspan or effective span of member, or anchorage length‘a clear horizontal distance between supporting membersbreadth of supporting member at one end or 1 .8m, whichever is the smaller‘b.2 breadth of supporting member at the other end or 1 .8m. whichever is the smallerdimension related to columns (variously defined)4 effective height of a column or wall‘ex’ 4. effective height in respect of the major or minor axis respectivelyeffective dimension of a head (of column)4, clear height of column or wall between end restraintslength of prestress developmentdistance between centres of columns, frames or walls supporting any twoadjacent floor spans1, floor to ceiling height4 transmission length4,4, length of sides of a slab panel or base4 distance between point of zero momentpanel length parallel to span. measured from centres of columnsI: panel width. measured from centres of columnsM design ultimate resistance momentM~dd additional design ultimate moment induced by deflection of beaminitial design ultimate moment in a column before allowance for additionaldesign momentsmoment necessary to produce zero stress in the concrete at the extreme tensionfibredesign moment transferred between slab and column~ maximum design moment transferred between slab and columndesign moment of resistance of the section~ design ultimate moments about the x and y axis respectivelyMs-, M~- effective uniaxial design ultimate moments about the x andy axis respectivelyM1 smaller initial end moment due to design ultimate loadsM2 larger initial end moment due to design ultimate loadsm~, m1~ maximum design ultimate moments either over supports or at mid-span onstrips of unit width and span 4 or 4 respectivelyN design axial forceNbaI design axial load capacity of a balanced sectionNd number of discontinuous edges (0~ N ~4)design ultimate capacity of a section when subjected to axial load onlyn design ultimate load per unit area, or number of columns resisting sidesway ata given level or storey (in 3.8.1.1)number of storeys in a structuredesign ultimate axial loadP0 prestressing force in tendon at the jacking end (or the tangent point near thejacking end)prestressing force in tendon at distance x along the curve from the tangent pointcharacteristic imposed loadR restraint factor (against early thermal contraction cracking)r internal radius of bendradius of curvature1 curvature at mid-span or. for cantilevers, at the support sectionrb1 shrinkage curvaturecurvature at x

13

[IHandbook to BSSIIO:198S

first moment of area of reinforcement about the centroid of the cracked or Cgross section

spacing of bent-up barsspacing of links along member

T torsional moment due to ultimate loads4, effective thickness of a slab for fire resistance assessment

thickness of non-combustible finish (for fire resistance)u length (or effective length) of the outer perimeter of the zone considered

effective length of the perimeterwhich touchesa loadedarea CV shear force due to design ultimate loads, or design ultimate value of a

concentrated loadVb desi2n shearresistanceof bent-upbars

design ultimate shear resistance of the concretedesign ultimate shear resistance of a section uncracked in tiexure CVcr design ultimate shear resistance of a section cracked in flexure

V~ff design effectiveshearforce in a flat slabV1 design shearforce transferredto column rv design shearstressvc design shear stress in the concrete

designconcreteshearstresscorrectedto allow for axial forces~max maximum designshearstress Cv~, v5,, designend shearon strips of unit width and span 4 or 4, respectively and

consideredto act over the middle three-quartersof the edgetorsional shear stress Uminimum torsionalshearstress.above which reinforcementis requiredmaximum combinedshearstress(shearplus torsion)

P4 characteristic wind loadx neutral axis depth. or dimension of ashearperimeterparallel to the axis of C

bendingx1 smaller centre-to-centre dimension of a rectangular link

Yo half the side of the endblock rhalf the side of the loaded area

Yi larger centre-to-centredimensionof a rectangularlinkz lever arm Ca coefficient of expansion,oranglebetweenshearreinforcementandthe planeof beamor slab

a~.l,aC.2 ratio of the sum of the column stiffnessto the sum of the beamstiffnessatthe lower or upperendof a column respectively r

acm,n lesser of ~ and ~ae modular ratio (E1IE~~~)~ ~ bendingmomentcoefficientsforslabsspanningin two directionsat right angles. r

simply supportedon four sides L/3 coefficient,variouslydefined,as appropriate

partial safetyfactor for loadYm partial safetyfactor for strengthof materials Lz~t differencein temperaturee strain

final (30 year) creepstrain in concrete Lfree shrinkagestrainstrain in concreteat maximum stress

Cm averagestrain at the level where the cracking is being considered

LCr thermal strain assumedto be accommodatedby cracksshrinkageof plain concretestrain at the level considered,calculatedignoring the stiffening effect of theconcretein the tensionzone [coefficientof frictionproportionof solid materialper unit width of slab

p areaof steel relativeto areaof concrete [Icreepcoefficient. or diameterd~e effective bar size

14 [

- - --..j-..-—~$’~- U

SECTION TWO. DESIGN OBJECTIVES AND GENERALRECOMMENDATIONS

2.1 Basis of design

2.1.1 Aim of’ designThe aim or purposeof designis explicitly statedandhenceshouldensurethat all thecriteria relevantto safety, serviceabilityand durability are consideredin the desicnprocess.Thesecriteria are relatedto the performanceof the structureor, equally, itsunfitnessfor use andeach is associatedwith a limit state.Thus the aim of designis toprovide an acceptableprobability that the structure,or part of it, will not attain am’specific limit stateduring its expectedlife.

The intendedlife of the structuremust,obviously, be consideredatthe outsettogetherwith the defined,or likely, maintenance.Further, changesin use, in environmentandin ownershiparealmostinevitableduringthe normal life of buildingsandstructuresandthus imply that the designertreatseachaspectof theperformancewith bothjudgementand an awarenessof the imponderableaspects.

As with otber’structuralmaterial~kn6~’)ledgcis not yetadequateto allow concretestructures~tobe~designedfora specificdurabilit~handlife. Structuresdesignedandbuiltaccordingto therecommendationsin theCodemaynormallybeexpectedtobesufficientlyresistantto the aggressiveeffectsof the environmentthat maintenanceandrepairof the~oncretewill not berequiredforseveraldecades.i.e.~;iife beforesignificantmaintenance~ ~

It is for the client, designer.specifier,manufactureror contractor,as appropriate,tomake the choicesnecessaryfor the constructionof a specific structure.Thesechoicesshouldbe madefollowing considerationof theuncertaintieswhicharelikely tobe presentin particularaspectsof the designandconstructionphasesand also of the subsequentuseandenvironmentof the structurein service.Wherea greateruncertaintythanusualis judged to be presentin a particular aspectit should be offset by adoptinga morecautious,or stringent,approachor by introducingalternativesafeguards~

Wherea higherthanusualdegreeof assuranceof durabilityis required,choicesshouldbemadewhich ensurethatthe structureandits maintenancewill be of higherthanusualquality.

2.1.2 DesignmethodThe limit stateconcept has ~i*iedinternatioi l’hcceptaiice(2’1~22’-3~ but. in particular,hasbeenadoptedwithin the EuropeanEconomicCommunityas the basisfor the draftEurocodes.Theacceptableprobabilitiesfor thevariouslimit stateshavenot beendefinedor quantifiedby the CodeCommitteebut carehasbeentakenthroughoutthe Codetoensurethatstructuresdesignedin accordancewith the Codehavesensiblythesamelevelof safetyas thosedesignedin accordancewith the previousCodes.Furthermore,muchmoreattentionhasbeendevotedto the serviceabilityrequirementsof structures,whichform an integralpart of the limit statedesignprocess.

The durability of structureshas cometo the fore in recentyearsand it should berecognisedthatdurability hasto be designedinto astructureattheconceptanddetailin2stages,thedesigner’sintentsmustbe clearly expressedandthen implementedeffectivelyin practice.

2.1.3 Durability, workmanshipandmaterials

2.1.4 Designprocess

15

Handbook to BS8IlO:198S

2.2 Structural design

2.2.1 GeneralThe limit states to be considered fall into two categories,namely ultimate andserviceability limit states.The criteria given in Part2 for the serviceabilitylimit statesare thosewhich aregenerallyapplicablebut obviously, in certaincircumstances,moreor less stringent criteria may be specified by a controlling authority or client, or be Ideemednecessaryby the engineer.In Part1, all the criteria aredealtwith by deemed-to-satisfyclauses.

2.2.2 Ultimate limit state(ULS)This limit state is concernedwith the strengthof the structurebeing adequatein thesenseof giving anacceptableprobabilityof its not collapsingunderthe actionof defined rdesignloads:as suchit is treatedby appropriateformal calculationswhich takeaccountof bothprimary andsecondaryeffectsin the membersand the structureas a whole.

The possibility of collapse being initiated by foreseeable.though indefinable and [perhapsexceedinglyremote,effectswhich are nottreatedformally in designe.g.explosivepressure.vehicleimpact. shouldbe consideredin design either

(a) by adopting a structuralconcept(including layout) or form of constructionwhichcan acceptadecreasein. or completelossof. the structuraleffectivenessof certainmembersalbeit with a reducedlevel of safety for the structureas a whole; or

(b) by the provision of appropriatedevicesto limit the effects of these accidentaloccurrencesto acceptablelevels. e.g. the useof controlled venting,crash barriers.

For special-purposestructures,theremay well be particularhazardswhich, in effect,require a speciallimit state to be considered.In thesecases.unlessthe hazardcanberspecifiedin sensibleandeffectiveloadingterms,theassessmentof whatwill be acceptablet.is left to the engineer.

2.2.3 Serviceability limit states(SLS)

2.2.3.1 General [2.2.3.2 Deflection due to vertical loading F:2.2.3.3 Responseto wind loads

2.2.3.4 Cracking L2.2.3.4.1 Reinforced concrete.The evidenceon the significanceof crack width on the~corrosion of reinforcing steel is conflicting but it is generallyacceptedthat, for theLenvironmentalconditionsobtainingfor moststructuresin the United Kingdom, asurface

crack up to 0.3mm wide may exist from both aestheticand performanceviewpoints Lprovidedthat thequalityof theconcreteandthecoverto thereinforcementarecontrolled(3.3). Moreinformationon acceptablecrackwidthscanbefound in CEB Bulletin 166’~~-

It must be emphasizedthat cracking is influenced by many factors and is a variablephenomenon;hence,absolutelimits to the widthsof crackscannotbegiven or compliedfwith andthe requirementsgiven in the Codemerely providean acceptableprobabilityLof the limiting widths not beingexceeded.

2.2.3.4.2Prestressedconcrete.Thecriteriagivenfor Class1 and2structuresareessentially Lthe sameas those in the previous codes.For Class3 structures,which correspondtowhat havebeen termedpartially prestressedstructures,the limiting width of crack i5~j0.1mm fo: “very severe”and “extreme” categoryenvironments,and for all other~conditionsis 0.2mm. Thus, thereis a progressionfrom Class 1. 2 and 3 to reinforcedconcretestructures.

I

Part I: Section 2

2.2.3.5 VibrationIn the majority of structures,the stiffnessprovidedto comply with the requirementsofthe deflection limit state will be such thatAno further considerationof vibration isnecessary.Where specific considerationof vibration is required by virtue of knownrepeatedloading, the following shouldbe included:

(a) the dampingcharacteristicsof the material(b) the dynamicmagnification.effectson the structuralmembers(c) the sensitivity of humanbeingsto vibration.

Steffenst5~reviews the problem andgives a detailedbibliography. BRE Digest No278. 1983.Vibrations: building andhuman response,is alsorelevant.BS 6472:1984Guideto evaluation of human exposure to vibration and shock in buildings (1 Hz to 80 Hz)gives further guidance.

2.2.4 DurabilityThis is a function of the conditionsof exposure,the quality of the concreteas placed.the cover to the steelandthe crackwidth if significantly greaterthan0.3mm. The firstthreeof thesearecontrolledby the requirementsof 3.3 andTable 3.4. The quality oftheconcretein turn is controlledby therequirementsof 6.2 to ensureadequatedurabilityin the variousexposureconditions.

In design.both strengthanddurability requirementshave to be satisfiedandso thequality of concretechosenwill dependon which of thesetwo criteria governs. Inconditionsof severeexposure,a high minimumcementcontentmaybe specifiedanditmat’ well thereforebeappropriateto utilize thestrengthassociatedwith this in design.

Where exceptionally severeenvironmentsare encounteredwhich are outside thecategoriesindicatedin Table 3.2, referenceshouldbe madeto Leat?6).

2.2.5 FatIgueftFatigue~ loadingis extremely unlikely 9nmlost.’Mructures,particularly fatigue loadingwhich is appreciablein relation to the characteristicimposedload. Even in veryspecialcaseswherethe primary loading is of a fatiguetype, the behaviourof both reinforcedandprestressedconcrete(Class1. 2 and3). designedin accordancewith 3. 4 and 5, issuch that the endurancelimit is of the orderof millions of cycles. The only significanteffectsare on the widths of cracksandthe deflections,d~eseincreasingby between20

~and”25%-comparedwith equivalentstatic loading. More detailedinformation may befound in~Yeferences2.7,2.8 and2.9.

2.2.6 Fire resistanceSee3.3.6.

2.2.7 Lightning

2.3 Inspection of construction

Seereference2.10.

2.4 Loads and material properties

2.4.1 Loads

2.4.1.1 Characteristicvaluesof loads

2.4.1.2 Nominal earth loads, E~

2.4.1.3Partial safetyfactorsfor load, VtStrictly speaking.‘y~ is thepartial safetyfactor for loadsandloadeffectsas indicatedby

17

Handbook to 8S8110:1985

the effects it embraces.The design load for each limit state is the productof thecharacteristicload andthe relevantpartial safetyfactor y~: hence.y~ may be consideredas coveringthe following:

(a) (i) Possible unusual increasesin the actual load not covered in deriving thecharacteristicload.(ii) reduced probability thatexistforcombinationsof loadsall at characteristicvalue

(b) Assumptions made in design which affect the distribution of stresses. or load effects.in the structure. It is implied that the assumptions normally madeand given in 3. 4and 5 give an acceptable accuracy in the assessmentof the effectsof loading.

(c) The dimensional accuracy achieved in construction. It is implied that the tolerancesdefined in the relevant clauses of 3. 4 and 5 are complied with.

(d) The nature of the limit state and its significance as assessed from the economicconsequences of attaining it and the safety aspect with regard to human life associatedwith it. In strict limit stateterminology(.I ~ this particularaspectis coveredby aspecialpartial safety factor ~ For simplicity, and becausethe economicand socialconsequences cannot as yet readily be quantified. the Code implicitly assumes y~ isunity.

2.4.1.4 Loads during construction

2.4.2 Material properties

2.4.2.1 Characteristicstrengthsof materialsThe characteristicstrength of materials is defined on the basis of test results.fromappropriatestandardtestspecimens.as that valuebelowwhich not morethan5% of allpossibleresultsfall. i.e. the 5% fractile. For.anormal,or Gaussian.distributionof testresultsin which themeanvalueiS/rn andthestandarddeviationiss. thenthecharacteristicvalue fk is given by

Ac = frnl.64S

2.4.2.2Partial safetyfactorsfor strengthsof materials, VmThe partial safety factor for materials. Yin, is necessaryto relatethe strength of thematerial in the actualstructureandits members.which is a function of the constructionor production process.to the characteristicstrengthderived as above. Vrn also takesaccountof modeluncertaintiesi.e. in the calculationmodelsfor the strengthof sections.Its definition implies a certainstandardof constructioncoveredin the caseof concreteby 6 and for steel by 7 and 8. Thus, the designstrength is obtainedby dividing, thecharacteristicstrengthby the relevantvalueof Yni’

2.4.2.3 Stress—strainrelationshipsIn analysis,the responseof the structureis governedby the averagepropertiesof thematerialsthroughout the structure:for convenience,however,it is assumedthat thecharacteristicstrength,andthe propertiesassociatedwith it. will obtainsince thesehaveto be specified by the designer.This assumptionwill be conservativebut it does impiythat a single analysiswill suffice for all limit statesthussimplifying the design process.The designstrengthsof the materialsare relevantonly when considerin2the behaviourof cross-sectionswithin thestructureandit is thenthat therelevantvaluesof y~ obtain.

The stress-straincurvesgiven in Figures2. 1—2.3havebeenderivedfrom the availabledatato be representativefor designpurposes.For concrete(Figure2.1), thecurvediffersfrom that given in reference2. 1 by having a variable strain at the intersectionof theparabolaand straight line, which is a function of the strengthof the concrete,and adefinedtangentat the origin. This is moreconsistentwith the availabledata,particularlYfor the higher concretestrengths.althoughslightly more complicated:it is also moreusefulin the non-linearanalysiswhich may becomemore importantin the future.

The elasticmodulusfor concret~isa function of the significant parametersaffectingit is discussedin Section7 of Part2 (particularlyTable 7.2). The moduli given for the

varioustypesof steelare typical and are accurateenoughfor all design calculations.For—‘---II ~..-.

[IFr

CCF[LI.’L[U

18

.— — — . — — . — . —. —. -- -.. - r<-- - vr’ - ---7,. -. - . . - --

. . - .

IPart 1: Section 2

reinforcement,the modulusof~clasticfty rangu~betwecn.2OOand 2O5kN/mm2.For prestressing tendons, it is advisable to use the actual secant modulus of elasticity

in determining the expectedextensionduring stressing.

2.4.2.4Poisson’sratio for concrete

2.4.3 Values of loads for ultimate limit state (ULS)

2.4.3.1 Design loads

2.4.3.1.1 General.The selectionof the yr factors for the various combinationsof loadhas beengovernedlargely by the considerationthat structuresdesignedin accordancewith the Code should have sensibly the samedegreeof safetyand serviceability asstructuresdesignedin accordancewith previouscodesandwill havethe samegeneralstandardsof workmanshipand quality control. Thereforethe global factorsof safety.the product y~X’yi’, rangesfrom 1.15 x 1.4 = 1.61 to 1.15 x 1.6 = 1.84 for structuressustainingwholly deadloadto wholly live load.

Although the three load combinations1, 2 and 3 should be consideredfor all thestructuralmembers.of the three,loadcombination1 will governdesignin mostbuildingstructures,particularly sincefor this load combinationthe minimum design deadload1.0 Gk hasto be consideredt2[U; combination2 will govern in thosestructureswherethe wind loading is the primary imposedload.e.g. chimneys,cooling towers. etc.. andfor this casethe minimum and maximumvalues of v~ are 1.0 and 1.4 respectively.

Theultimatestrengthof sectionsandthe ultimateloadof structuresarenotsignificantlyaffected by the effects of temperature.creepor shrinkagebecausethe deformationsproducedby thesecausesaremuchlessthanthoseassociatedwith thecollapseconditions.Theseeffectsare thereforeconsideredonly for the serviceabilitylimit states.

2.4.3.1.2 Partial factorsfor earthpressure.SeeSection2 of Part 2.

2.4.3.2Effectsof exceptionalloadsor localizeddamage

2.4.3.3 Creep,shrinkage and temperatureeffects

2.4.4 Strengths of materials for the ultimate limit state

2.4.4.1 DesignstrengthsThe values of y~ are relevant to the control and workmanshiprequirementsgivenelsewherein the Code. Obviously,wheredataexist. e.g. in themanufactureof precastunits, thesemaybe usedto justify lowervaluesfor Ym forconcrete~21~.Whenconsideringmisuseor accident,the reducedprobability of occurrenceis reflectedin the lower valuesof 1.3 and 1.0 for y~.

2.4.4.2 Effects of exceptionalloadsor localizeddamage

2.4.5 Design loads for serviceability limit statesSee3.3 of Part2.

2.4.6 Material properties for serviceability limit states

2.4.6.1 GeneralSince for these limit statesit is deformation in general which is the criterion, and thisdependson the behaviourof the structureas a whole. the propertiesof the materialsare takenas thoserelatedto the characteristicstrengthalthough.strictly speaking,themeanstrengthshould be used,but it is the former that is known and specified;henceYin 15 1.0 in general. For the treatmentof individual sectionssuch as in cracking, thelocal propertiesof the concreteassumea greaterimportanceandso Ym is takenas 1.3to ensurean acceptableprobability. See3.2 of Part2.

1’-)

Handbook to BS,3110:1985

2.4.6.2 Tensile stresscriteria for prestressedconcrete F2.4.7 Material properties for durabilityTo achieve appropriate durability for the expectedlife of the structure requires anintegrated approach to d~i~fl7~$ecifications and construction as mentioned earlier(2.1.2S.The specific material relatedaspectsare itemizedherewith forward referencesto sectionsand clausescoveringtheir treatment;then the clauseprovidesa check list. [Summarizingthe subjectis possiblesuccinctlyby the 4 C’s. namely:

constituentscompactioncuringcover

to which could be addedcontrol. checkingandcare. C2.5 Analysis

2.5.1 GeneralThe morefundamentalapproachesare thosewhich arebasedupon the moment-curvatureand moment-rotation relations for reinforced and prestressedconcrete sections; [descriptionsof theseare given in references2.12and2.13.

2.5.2 Analysis of structureOf the assumptionspermitted for the stiffness of membersin elastic analysis, thatassociatedwith the grossconcretesection(a) will for obviousreasons,generallybe used:(b) and(c) may berelevantwhencheckingexistingstructuresfor newloadingsassociatedwith changeof use.

Someusefulsourceson yield-line theoryandthe strip methodfor slabsare references2.14—17.

2.5.3 A.nalvsis of sectionsfor the ultimate limit state

2.5.4 Analysis of sectionsfor serviceability limit states C2.6 Designs based on tests [

2.6.1 Model testsSeereference2.18. [2.6.2 Prototypetests ISeereference2.19. L

It shouldbe emphasizedthat,appropriatetestinghaving beencarriedout to establisha designprocedurefor a structureor structuralmembers.this is equivalentto design bycalculationin accordancewith 3. 4 andS.Thereis. therefore.no further needfor testingthe productionunits other than for quality control. or assurance.purposes.

REFERENCES L2.1 co%In’E ELRO.E’.1~ERNATIONAL DL BETON - FEDERATION INTERNATiONALE DE LA PRECONTRAINTE. MOdCI

Codefor concretestructures.1978.2.2 I~TER.NATIONAL STANDARDS ORGANIZATION Generalprinciplesfor the verificationof the safetyof

structures.Geneva.12 pp. InternationalStandard2394:1973. L2.3 RowE. RE. c~NS~rON w a. and aes-r.ac.New conceptsin the designof structuralconcrete.The

Structural Engineer.Vol.43. No.12. December1965. pp.399-4)3.Discussion.Vol.44. No.4.April 1966. ppl27-133.FurtherDiscussion. Vol.4-I. No 11. November1966. pp.411-2l

2.4 ~OMI-~EEURO-INTERNATIONAL DL’ aETON Draft CEB guide to durableconcretestructures.MaY L1985. 2~ pp. CEB Bulletin 166.

2.5 sm~v~s.R ~. Structural vibration anddamage.London.HMSO. 1974. ‘6 pp. [

Parr 1: Section2

2.6 i..e~. p.,.i. The chemistryof cementandconcrete.Third edition. London. Edward Arnold.1970. pp.659-676.

2.7 SNOWDON. L.C. The static and fatigue performance.of concretebeamswith high-strengthdeformed bars. London. ConstructionIndustry Researchand Information Association,August1970. 31 pp. CIRIA Report24.

2.8 BATE. s.c.c.A comparisonbetweenprestressed-concreteand reinforced-concretebeamsunderrepeatedloading. Proceedingsof the Institution of Civil Engineers.Vol.24. March 1963.pp.331-358.

2.9 s~ra\ms.R.F. Testson prestressedconcretebeams.Concrete.Vol.3. No.11. November1969.pp.452-457.

2.10 INST~Tt-~ON OF sRucrUkAL ENGINEERS. Inspectionof building structuresduring construction.London. 1983. 22 pp.

2.11 DEEB~. AW. andCItANsTON. we. Influenceof loadsystemsonsafety.Civil EngineeringandPublicWorks Review. Vol.67. No.797. December1972. pp.1251-1253.1255. 1257-1258.

2.12 BAKER. ALL. Limit-state design of reinforced concrete. London. Cement and ConcreteAssociation.1970. 360 pp. Publication12.037. (Now out of print.)

2.13 coMITE ELTROPEEN DLJ BE-roN. Appendixto InternationalRecommendations(1970)— HyperstaticStructures.Paris. 1972.

2.14 bo~s.L.L andwooo,RH. Yield-line analysisof slabs.London.Thames& Hudson.Chatto&Windus. 1967. 405pp.

2.15 JOHANSEN. K.W. Yield-line formulae for slabs.Translatedfrom Danish. London. ViewpointPublications.1972. 106 pp. Publication12.044.

2.16 wooo. RH. and ARMER. OsT. The theoryof the stripmethodfor designof slabs.Proceedingsof the Institution of Civil Engineers.Vol.41. October 1968. pp.285-311.

2.17 ARNIER. OST. The stripmethod:a newapproachto the designof slabs.Concrete.Vol.2. No.9.September1968. pp.358-363.

2.18 ROWE. RE. and BASE. GD. Model analysisand testing as a design tool. Proceedingsof theInstitutionof Civil Engineers.Vol.33. February1966. pp 183-199.

2.19 5oN4ERvILLE. G. Developmenttestingfor structuralconcrete.Engineering.Vol.205. No.5321.12 April 1968. pp.558-559.

21

SECTION THREE. DESIGN AND DETAILING: F’REINFORCED CONCRETE [3.1 Design basis and strength of materials 17

3.1.1 GeneralThe methodsof analysisanddesignhavebeenchosenprimarily for theirsimplicity in use Fand the wide rangeof conditionsover which they produceacceptableresults. Other 1.methodsarepermittedbut it is suggestedthat they shouldnot be morecomplicatedinusethan thoserecommended. F

The limitationsof the methodsshouldbe fully recognized:in many cases.limitationsare stated.but often it will be necessaryto rely on the experienceandjudgementof theengineercarrvin~outthe design.It is for this reasonthatall designshouldbe supervisedby suitablyquaiAedandexperiencedengineers. I3.1.2 Basisof designfor reinforced concrete rIn general.the initial analysisanddesign of a reinforcedconcretestructurewill be tor [the ultimate limit stateand the final stageswill involve checking for the serviceabilitylimit states.It will be prudent.however,whenassessingpreliminaryproportionsfor thestructure to perform a rough check on the ratios of the span to effective depthin Laccordancewith 3.4.6.3.

3.1.3 Alternative methods (serviceability limit state)Theseare discussedunderPart 2. Section5

3.1.4 Robustness— general requirements [Sincethe partial collapseof the RonanPointblock of flats andof a precaststructureatAldershot, it hasbecomeacceptedthatdesigninga structureto withstandth~p~cifjed.designloadsis nor sufficient in itself to guaranteea structurewith the requiredlevel ofsafer~. An additional property is requiredof the structurewhich can be describedasrobustness.This can possiblybest be definedas an ability to withstandall the minorunforeseenoccurrencesand accidentsto which the structu.remay be subjectedduring I’its life without major consequences.Itt particular~..itis essentiaLto avoi~tniduraL [solutionswheredamaget~ onemembercanleadto amajorcollapse(progressivecollapseor a ‘house of cards’ type of failure). The general requirement for robustnesswasexpressedin CP11Oas follows: I

“No structurecanbe’ expectedto be resistantto theexcessiveloadsandforcesthatcould arisedue to an extremecause,but it shouldnot b~ damagedto an extentdisproportionateto theoriginal cause L

Clause2.2.2.2in BS 8110attemptstoexpressthesameideain aslightly differentform:“Structuresshouldbe . not unreasonablysusceptibleto the effectsof accidents.In particular.situationsshouldbe avoidedwheredamageto smallareasof astructureor failure of single elementsmay leadto collapseof major parts of the structure.” L

While it is generallyunderstoodwhatis meantby a robust building. the developmentof designruleswhich will ensurethis hasbeenless easysincerobustness,unlike strength.is not a conceptthat canbe expressedmathematically. I

Attempts havebeen made to define what constitutes~majorparts of the structurethoughnot in BS 8110. Building RegulationA3 statesthat, if one memberis consideredremovedby an accidentthen: L(a) structural failure consequenton that removal would not occur within any storey

other than the storey of which the memberforms part, the storey next above(ifany)and the storey nextbelow (if any); and L

(b) any structuralfailure would be locatedwithin eachsuchstorey.It is further statedthat (b) may be “deemedto be satisfied if the areawithin which

structural failure might occur would not exceed70m or 15% of the areaof the storey L

L.—.—.

‘I

Part 1: Sectwn3

(measuredin the horizontalplane).whicheveris the less”. It is understoodthat, undertherevisedBuilding Regulations,similarlimits will begiveninan‘Approveddocument’.

The~Codespecifiesthreebasicmeasuresto-’~nsizre1hata designis robust:

1. ensurethat thereis noinherentweaknessin thestructurallayout(see3.1.4.1below);2. fot~buj14~gs,~yherethe.windIoads~areJ~~ensurethat the structurecan withstand

~at1east~nominajponzonraij9adequ tpI.5%~of’the deadweight(3A.4.2); and3. e ensure that the lossoConemember will not result in the collapseof a major part of

the structure.

This last provision may be met in one of two ways: either each membermay beconsideredto be removedin turn and the remainingstructurecheckedto ensurethat itwill remainstandingwhensubjectedto the accidentalcombinationof loadsor a ‘deemedto satisfy’ solutionmay be adoptedusingties.

Methods‘for designingfor element removal are setout in Part 2. Section 2.6 and willb~ discussedmore fi4ly later2’

It is recognizedthat it may occasionallybe impossibleto avoid havingsingle memberswhich supportwhatmight be consideredto be a ‘major part of the structure’andwhosefailure would thus automaticallycausethe collapseof a ‘major part of the structure’.Suchelementsareclassedas‘key elements’..I.f possible,theyshouldbeavo~ded.However,wherethey are.unavoidable.special rules for their design are given in Part2, Section2.6. The Codedoesnot explicitly defineakey elementbut it would seemreasonabletoassumethat suchan elementwas onewhich supportedmorethan 70m or 15% of theareaof a storey.

The ‘deemedto satisfy’ provisionsfor ties areset out in 3.12.3but, for completenessof this generaldiscussion,the principlesof the methodwill be outlined here~

The basicelementof the tie systemis aperipheraltie or continuouslyreinforcedstriparoundtheentireperimeterof eachfloor. Internalties, continuousoverthe wholelengthor breadthof thebuilding andanchoredto the peripheraltie at both ends,areprovidedto maintainthe integrity of the wholefloor shouldsomepart of it sufferseveredamage.Similarly, vertical ties are provided over the whole height of vertical load bearingmembers.Wheresuchmembersarelocatedoutsidethe peripheraltie, theyaretiedbackinto it. Figure H3.1 illustratestheseprinciples.

-1

vertical tie fixed backto peripheral tie

3.1.4.1 Generalcheckof structural integrityThis and other codes ask for a robust and stable planform without specifyine whatconstitutessucha structurallayout. It is notpossibleto give an exactdefinition of whatthe Coderequiresbut£gme.general-guidelinescart i~eEiven. Thereseemto be two basicprincipleswhich can be statedhere:

1. ~theoverallform q~b.~4xa wreshould be chosensothat it is notexcessivelyflexibleto any modeof.deformation:

Figure H3.I. Schematicillustration of tying s~’ste,n.

Handbookto BSSIIO:1985 [I2. the form of thestructureshouldbesuchthat thecentreof resistance6f thestructure F

to a particularloading shouldbe closeto the line of action of the loading.Theseconceptscan beexplainedmorefully by examples.Considerthe building layout

sketchedin Figure H3.2(a).This layout is reasonablystiff relative to uniform lateralloading but would havea very low torsionalstiffness.Wewouldnot. of course,designthis structurefor torsion but torsions will inevitably occur (possibly due to gusting,eddyingof wind arounda structureetc) and this planformcould hardly be considered

____________ [to be robust.

(a) lack of torsional stiffnm ~ [17

WiNO 17~ ~LI

line of action of load

0 0 0 0centroid of

resistanceIl~I 0 0 0I structure• resisting

•wind

1W llne~of lanai load andiesistanee not coincident

Figure H3.2: Poor structural layouts 1:(a) lack of torsionalstiffness(b) lines of action of load and resistance not coincident L

Figure H3.2(b)showsasituation-.*herethe~ceticroidof the structure resisting lateral,load is well away frQm theliz~c of actiQaof the load. It will beseenthat the lateral loadcan only be transferred rojthe structure-designed--to carry it by inducing substantial-torsioQaldistortion in thestructure.4gain. this could not be consideredto be a robust Lstructurallayout.

3.1.4.2 Notional horizontal load L3.1.4.3 Provisionof ties I-.See3.12.3.

3.1.4.4Key elementsandbridging structures LSee2.6 in Part2.

3.1.4.5 Safeguardingagainst vehicular impact [3.1.4.6Flow chart of designprocedure

3.1.5 DurabilIty and fire resistanceof reinforced concrete [1As far as designis concerned.durability is dealt with largely in termsof the choiceof

[. —.—

. .-<. ... ‘- .- - - S - ~ —

I —

— I

-. ~ .... ..,;...: - . ~ - - - - - - - . - N

] Parr 1: Section 3

a suitableconcretequality andnominal cover.This is coveredin Section3.3 with more

I generalinformationin6.2. The Handbookwill dealwith the backgroundto theseclausesunder6.2. Fire resistanceis coveredat two levels. Section 3.3 gives simple, safe rulesforcoverandmembersizewhile Part2, Section4givesamuchmorethoroughtreatment.

The fire resistanceof a reinforcedconcretestructureis treatedon an elementalbasisIi.e. column,beam,slab.wall etc.The tablesin this Coderefer to widths or thicknessesof sectionsandthe amountof covernecessaryto main andsecondaryreinforcement.

Part1 of theCodecontainssimplified tabulardatafor generalusein ordinaryreinforcedI concreteconstruction.Wherethe requirementof the designis not encompassedby therangeof values given in the tablesthen the designershould refer to Part2. Section 4

for a moredetailedtreatment.

3.1.6 Loads

3.1.7 Strengthof materials

3.1.7.1 General

3.1.7.2 Selectionof compressivestrengthgradeof concreteTheBS 5328Table of preferredgradesis reproducedasTable H6.5. Note that C25 canonly be usedunderratherspecialcircumstancesandthatprobablyC35 is alikely commonminimum.

3.1.7.3Age allowancefor concreteDatafrom watercuredcubesshouldnotbetakenasevidenceof thepotentialdevelopmentof strength.Concretewhich is allowed to dry below a relative humidity of 85% will0ceaseto gain in strength.Concreteswhich are exposedto the UK weather,exposedbutprotectedfrom rain or effectively sealedfrom the time of castinge.g. the centreof alargepour,will all continueto gain in strength.The rateof gain in strengthwill dependElon the exposure,the quality of concreteandthe typeof cement.

3.1.7.4 Characteristicstrengthsof reinforcement

3.2 Structures and structural frames

)3.2.1 Analysis of structures

3.2.1.1 CompletestructuresandcompletestructuralframesIt will rarely be necessary,or advantageous,to attemptan analysisof the completestructurewhere vertical load only is considered.It maymore frequentlybe necessaryto considerthe completestructureof an unbracedframe,except in thecaseof basicallysimple structures(e.g. portal frames),or certain specialstructures(an example mightbe oneof the morecomplexforms of grandstand).

3.2.1.2 Monolithic frames not providing lateral stability

3.2.1.2.1 Simplification into sub-frames.The frame can be consideredby breaking itdowninto sub-frames;the sub-framespermittedare illustrated in FigureH33. Analysesof thesesub-framesunder the prescribedloadingsgive results which do not differsignificantly from those obtainedfrom analysesof the complete frame. The simplersub-frames(FigureH3.3(c)) have the advantagethat an explicit solutioncan be writtendown in terms of the rotationsof the joints at the ends of the beamconsidered.Theequationsfor thesetwo rotationsare:

6’=k3o’. MFI +k4u.M~

— —— — — ~ —

— —

i-I-

U LW 4J.U U.d .~A LW &~

Id) beam only

Handbook to B5811LJ:1985

o’.MFI and~ are respectivelythealgebraicsumsof thefixed-endbeammomentsOneither side of the joints at ends 1 and2 of the beamunderthe loading considered.~and62 are the rotationsof joints 1 and2 andk1. k.. k3 andk~areconstantswhich dependon the relativestiffnessesof the membersconnectedto eachjoint. They aregiven by:

k1=K2/B

k4= K1IB

k2=k3~—KbI2B

where BKIKa—Kb2/4

K1= the sumof the stiffnessesof all membersconnectingto joint I

K-= the sum of the stiffnessof all the membersconnectingto joint 2

Kb= the stiffnessof the beam(4E111)

The momentsat ends I and2 of the beamare given by:

MIMFI—Kh(6! +0.56:)

1W~=M~—Kh(6:±O.56l)

MFI andM~2 are respectivelythe fixed-end momentsat ends I and2 of the beam.In the interests.ofsimplicity. the concretesectionshouldbe usedfor assessingrelative

stiffnesses.Of course,the gross or transformedsecii6nscan be usedonly if a priorestimateof reinforcementareaisavailableor whereareassessmentof anexistingstructureis being carriedout.

In moststructuralframes,the beamsare integralwith the floor slabsanddesignedasT beams.In suchcases,the beamstiffnesscanbe assessedby takingthe effectivewidth.as describedin 3.4.1.3to apply overthe entirespan.

(al full frame

r

K

Ib) I h

stiffnesa halved

II

I

L‘— atiffneaa halved

Figure H3.3: Permissible simplification of a frwne for analysis.

-. ~; -:.~.K. ::<:. - - ‘ - - :-.

[IFVr[r

[FCI:C

t

CCI’.L

le) column only

—-IPart 1: Section 3

3.2.1.2.2 Choiceof critical loading arrangemea.s.The load patternsarea simplificationof previouspractice.The old ‘altcrnatc,spansloaded’ and ‘adjacent spansloaded’ loadpatternscould~requirea-+~4,ca~esto be considered,wheren is the numberof spansofa continuousbeam.The provisionsof this clauserequire a maximum of 3. The actualdifferencemadeto the design momentsis fairly small: maximum saggingmomentsareunchangedbut maximum supportmomentsare reducedby a few percent. Assuming

somedegreeof ductility, it canbeshown’3’1 thatoverallsafetyis not significantlyaffectedby this changesince the ‘alternate spansloaded’ pattern definesthe critical failuremechanisms.The only realeffect is a ~ight ipcreasein stressnearthe supportsunderserviceconditions. This is not of practicalsignificance.

3.2.1.2.3Alternative simplificationfor individual beams.This hasbeendealtwith under3.2.1.2.1.

3.2.1.2.4 ‘Continuous beam’ simplification. While the simplestanalysisto cam’ out. itshould be notedthat it can be very conservative,especiallywhen thereare fairly stiffcolumnsin theframe.The possibledegreeof conservatismmaybeseenfrom FigureH3.4.

3.2.1.2.5Asymmetrically-loadedcolumns where a beam has been analysedin accordancewith 3.2.1.2.4. The momentsin columnsmay be assessedusing the formulae given inTable H3.1.

Table H3.1 Moments in columns

Momentsfor framesofonebay

Momentsfor framesoftwoor morebays

F=temal(andsimilarlx- loaded) colwnns

Moment at foot of upper column

Moment at head of lower column

K,K

1 +K,+0.5K1,

K1K1+K, +05K1,

Internal columns

Momentatfootof uppercolumn

Momentatheadof lowercolumn

K,,

K + K,—~-- K~

K

K, +K1,

K,Me,,

K1 +K,—l--K1,1--s--K1,~

A-I

Figure H3.4: Comparison of analyses including and ignoring thecolumns.

—V.—I

Handbook to BSSIIQ:1985 Fwhere A’f~, is the bendingmomentat the endof the beamframing into the column.

assumingfixity at both endsof the beam:~ is the maximum difference betweenthe momentsat the ends of the two

beamsframing into opposite sides of the column. each calculatedon theassumptionthat the endsof the beamsare fixed andassumingone of thebeamsunloaded:

Kb is the stiffnessof the beam:Kb, is the stiffnessof the beamon oneside of the column;Kb-, is the stiffnessof the beamon the otherside of the column;K, is the stiffnessof the lower column:K~ is the stiffnessof the uppercolumn.

For the purposesof this Table.the stiffnessof a membermay be obtainedby dividinthemomentof inertiaof a cross-sectionbx’ the length of the member.providedthat thememberis of constantcross-sectionthroughoutits length.

The equationsfor the moment at the headof the lower column may be use.dforcolumnsin a topmoststoreyby taking K~ as zero.

Where the bendingmoment is calculatedin the internal columnsit is permissibletotake into accountthe reduction in load resulting from the beamon one side of (hecolumn being fully loadedandthe beamon the otherside being loadedwith deadloadonly.

3.2.1.3 Framesproviding lateral stability [3.2.1.3.1 General. The division betweenframesproviding lateral stability (sometimescalled unbracedframes)and framesnot camin~horizontal loads(bracedframes)is. at I..course,somewhatartificial in manycases.Any monolithic frame will redistributethehorizontalforcesbetweentheverticalmembersas a function of their relativestiffnesses. [It so happensthat elementsof structuresuch as lift shafts.stair wells and walls arecommonlyso stiff relative to columnsthat it is a legitimatesimplification to assumethaiall the horizontal loads are carried by the stiff vertical elements and ignore anycontributionfrom thecolumns.ThisSection is concernedwith framesin structureswhichdo not containsuch very stiff verticalelementsof structureso that the influenceof thehorizontal forces on the frame cannot be ignored. Occasionally,the designermayneverthelessarbitrarily decidethatcertainmembersin such structureswill be designedto carry thelateral loadsandthe remainderbe consideredas elementsof bracedframes--Thisapproachshouldbeviewedwithconsiderablecautionif thestiffnessesof theelementschosenas bracingelementsdo not greatly exceedthe total stiffnessof the remainderThe lateral loadswill be sharedin proportionto the stiffnessesandwill only be shedto [

LL11LiiU[

Figure H3.S: Alternative treatmentof lareral1v-loa~ledunbracedframe.

- a-

Part I: Section 3the membersdesignedto carry them after excessivecrackingor failure of the ‘braced’elementshas greatlyreducedtheir stiffness.

3.2.1.3.2 Sway-frameof three or more approximatelyequalbays.Studieshaveshownthatthe methodssuggestedin this clauseare often inadequatewhen applied to one- ortwo-bay frames.For these,or for frameswith very unequalbay sizes,a morecompleteanalysisshouldbe employed.An approximatemethodwhich seemsto give reasonableresults is to adopta modified version of the sub-framedescribedin 3.2.1.2.1.This issketchedin FigureH3.5.

3.2.2 Redistributionof moments

2E

— ultimate support moment

Figure 113.6: Developmentof bending momentsin an encasrr~beam.

3.2.2.1 GeneralReinforcedconcrete behavesin a mannerintermediatebetween the elastic-plasticbehaviourof the reinforcementand the behaviourof the concretewhich, in normalcircumstances,is capableof very little plasticdeformation.The exactbehaviourdependson the relativequantitiesof the two materialsandtheir properties;however,it may beconsideredto be roughly elastic until the steel yields andthen roughlyplasticuntil theconcrete fails in compression.The concretefailure limits the amount of this plasticbehaviouror, morespecifically, it limits the amountof rotation which a plastichingecan undergo.Thus a systemof analysisis requiredfor indeterminatestructureswhichwill allow for plastichingesforming as the collapseload is approachedwhile controllingtheir rotation.

Figure H3.6 showshow the bendingmomentsdevelopin an elastic-plasticmember.As the load is increased,the beambehaveselasticallyuntil the plasticmoment of oneor more critical sectionsis reached(in Figure H3.6(a)the supportmoments).Furtherloading causesthesehingesto rotate while the momentsdo not change.The extramoment required to balancethe load is carried by other parts of the member.Thiscontinuesuntil the mid-spansection reachesits plastic moment,when the structure

— ultimate apan moment

(a)

(bi

(bI

theoretical cut-off point.ultimate condItions only

elastic

service load b.m.d.

redistributedmoment diagram

29

Handbook to BSSIlOx’985

becomesa mechanismandcollapses.Figure H3.6(b) shows the final bendingmoment rdiagramwhenall the critical sectionsarecarrving theirplasticmoments.Fromthedesignpoint of view, this samebendingmomentdiagramcan be obtainedby calculatingtheelastic bendingmoment diagram under the collapseloading, and then reducingthe Psupportmomentswhile increasingthe mid-spanmomentby a correspondingamountto I -

maintain equilibrium (Figure H3.6(c)). This operationis known as redistribution otmoments.The percentageby which a moment is reducedfrom the elastic value is a rmeasureof the rotation of the hinge. Design can thereforeconvenientlybe done b~ jcarrying out an elastic analysisand then applying a limited amount of redistribution.The design of the critical sections must then be such that they can carry the rotations -

implied by the redistribution. rThe effect of condition 2 is to limit the neutral axis depthat ultimate to 0.3 of the

effective depthif the full 30% reductionin momenthas beenmade:as the neutralaxisdepthis increased,the amountof redistributionis reduced.Wherethe neutralaxisdepth rexceeds0.6 of the effective depth,no reductionsin momentare permitted.

Condition3 is requiredto dealwith serviceabilityconditions,whereanelasticresponsewill be appropriate.From Figure H3.6(c). it will be seenthat serviceloading in this casewill produce hogging moment in region ~x- Ultimate load conditions require noreinforcementhereandplainlyverywide crackingwoulddevelopin thisregion.Supplyingreinforcementto carrvat least70% of the maximummomentmeansthat the structuralresponsewill remain roughly elasticat loadsequalto or less than70% of total ultimate 17load. The loading correspondingto the serviceabilitylimit state is al~vavsless than this.and thus the possibility of wide crackingis ruled out.

The limitation of 30% in reducingmomentsis to restrictthe rotationwhichwill developat critical sections.It should be noted that no limit is placedon the amount by whichmomentscan be increased.

Redistributionwill alsoaffect theshears.Dependingon howtheredistributionis done.thesemay be eitherdecreasedor increased.It is consideredthat it is prudent to take Lthe worst of the redistributedandunredistributedshearsasthe designvalues.A reasonfor this is that, if the redistributionreducestheshearsand.possiblybecausethestrengthof the reinforcementwasaboveits design value, the redistributidndid not takeplace rthendesignfor the reducedshearscould lead to the structurehavinga reducedsafetyfactor (note that an increasein steel strengthwould increasethe flexural strengthof asectionbut not its shearstrength).

The questionof appropriatestrategiesfor redistribution from the point of vie~v of Feconomyof designandeaseof constructionis not simple. It is consideredin somedetail I’in reference3.2.

3.2.2.2 Restriction to redistribution of moments in structures over four storeys wherestructuralframeprovideslateral stability

The limitation of 10% for tall unbraced frames is because the formation of plastic ~-

hinges at the onset of redistribution may induce a prematurefailure due to frameinstability1331.

3.3 Concrete cover to reinforcement

I.Concretecover has to perform the following functions:

1. provide adequate bond between steel and concrete for load transference L2. protectthe steelfrom corrosionandthe effect of aggressiveagencies3. protectthe steelfrom fire.

It mustalso meetotherpracticalrequirementse.g. in relationto placingconcrete.All tsuchrequirementsare givenin this clause(togetherwith 4.12.3 for prestressedconcrete~ Lbecausecover influencesthe position of reinforcementwithin the overall cross~5ectionand hencethe design resistanceof the section. L3.3.1 Nominal cover

30 L

. . . . . . .

. - .- -. -... . ..

Parr I: Secnon 3

3.3.1.1 General’H BS 8110 usesthe term ‘nominal cover’ in an attempt to clarify what, in the past,hascausedsomeconfusion.The vital factoris that ‘nominal cover’ is the specifiedcover toall reinforcement including links. It is envisagedthat. in line with the recommendationsI of the joint Institution of StructuralEngineers/ConcreteSociety reporton detailingt341,spacersshould be fixed on the links andnot on the main barsin a beamor column. Inthe past.where it was the normalprocedureto fix the spacersto the main bars, thecover to the links could be very much lessthanintended.This hasresultedin numerous] caseswherelinks havecorrodedandhavecausedlocal spallingof theconcrete.All limitsto covergiven in Part 1 of BS 8110 arequotedas nominal covers. It should be noted,

Ihowever,thatthis is not thecasein Section4of Part2 (Fire resistance)whereadefinitionmore appropriateto fire resistanceconsiderationshasbeenadopted.

The referenceto surfacetreatmentswhich reducethe effective depth of protectiveconcreteis a furtherexampleof the wide rangeof considerationsin a properlyintegrated

I design. In 7.1 attention is drawn to the possibility of using coatedor stainlesssteelreinforcement in special circumstancessuch as may occur when protection ofreinforcementagainstcorrosioncannotbe achievedin the usual way, i.e. by provisionof an appropriatedepthandqualityof concretecover,or whenanexceptionalassuranceU.againstcrackingof the concretearisingfrom reinforcementcorrosionis required.

3.3.1.2 BarsizeThereare two reasonsfor ensuringthat the cover to the main bars is not less thanthebardiameter.Thefirst is thatsmallercoversmaymaketheproperplacingandcompactionof theconcretearoundthe barsdifficult. The secondisrelatedto structuralperformance.Bondstrengthis verydependenton the ratio of cover to barsize, though this variabledoesnot appearin thebond provisionsin this Code.andthe safetyfactor againstbondfailure could ~vellbe reducedto an unacceptablylow valueif the coverwas significantlyless than the bar size. Furthermore,local bond stressescould lead to cracks formingalongthe line of the bars which might posea corrosionrisk.

3.3.1.3Nominal maximumsize of aggregateThe requirementthat the covershouldnot be lessthanthe nominal maximumaggregatesize hasbeenintroducedto helpensurethe properflow aroundthe barsof the concreteandhencepropercompaction.

3.3.1.4ConcretecastagainstunevensurfacesClearly a much larger uncertaintyexists aboutobtainingthe necessarycover in thecircumstancesand so a substantiallygreatertoleranceneedsto be provided for. Theprovisionsof this clauserepeatthosegiven in CP2004.

3.3.2 Endsof straightbarsThis clausecoversa distinction not madein CP11O but suggestedin Section 3.11.2ofthe Handbookto CP11O. It is in effect a relaxationfor a specific limited applicationwhich shouldnot be extended.The possibility of rust stainingduring constructionwill,of course,haveto be considered.Experiencesuggeststhat somecorrosionof the endsof bars will occurbut that this is not progressiveor harmful.

3.3.3 CoveragainstcorrosionThe essenceof this clauseis containedin Table 3.4. The advancefrom CP11O which itrepresentsis the direct linking of mix design parametersto cover and conditionsofexposure.in effect a mergerof the previousTables19 and48. togetherwith a generalstrengtheningof the recommendations.

The stimulusto this integrationwas increasingevidencethat concretehasnot alwaysbeenspecifiedand placedto the necessarythicknessandquality to provide reasonableassuranceof adequatelong-termprotectionto reinforcement,in the absenceof otherdeleteriousor particularlyextremeeffects.

In particular.sufficientcementcontentandlimited watercontentare essential,alongwith propercompactionand curing, to ensurea concretesufficiently impermeabletothe ingress of carbon dioxide which leads to carbonationand loss of th~e alkaline

31

Handbookto BS8IIO:1985

characteristicswhich inhibit corrosion(seealso6.2).3.3.4and3.3.5deal withvariousaspectsof Table 3.4(which is basedon 20mm nominal

maximumsizeof aggregate)andthe technicalbasisfor theTableis discussedunder6.2.

3.3.4 Exposureconditions

3.3.4.1 GeneralThe first point of entryto Table3.4 is assessmentof the exposureconditions.Thesearedescribedin Table3.2 andshouldbe consideredas designparameters.It hasnot beenpossibleyet toquantifythe conditionsassignedto the five environments—mild, moderate.severe.vervsevereandextreme— so thedefinitionscomprisedescriptionsof the exposureconditions.

This Table is reproducedbelow, modified to include examplesof most of theseconditions.Theseexampleswere not includedin the Code becauseit is the designefsresponsibility to assessthe environmentin the light of all relevant factors. which mayindicatesometimesa moresevereenvironmentthan theexamplesindicate.Nevertheless.theyarelikely to covermostcommonsituations.

The four moresevereenvironmentsarenow aligned~viththosein BS 5-UJO for bridgesandincorporatethat involving chloridesfrom externalsources.

Table 3.2 Exposure conditions

Environment ExposurecondItions Example

MODERATE

MILD Concretesurfacesprotectedagainstweatheroraggressiveconditionsbutconditionsmaybeaggressivefora briefperiodduringconstructionConcretesurfacesshelteredfromsevererain orfreezingwhilstwetConcretesubjecttocondensationConcretesurfacescontinuouslyunderwaterConcretein contactwith non-aggressivesoil(seeclassI ofTable6.1)

SEVERE Concretesurfacesexposedto:severerain,alternatewettinganddryingoroccasionalfreezingorseverecondensation

VERY Concretesurfacesexposedto: SeawaterSEVERE spray,dc-icingsalts(directlyor

indirectly)Corrosivefumesorseverefreezingconditionswhilstwet

EXTREME Concretesurfacesexposedtoabrasiveactione.g.seawatercarryingsolidsorflowing waterwith pH ~4.5ormachinervor vehicles

Internalconcretee.g.shops.offices

Undersideof suspendedgroundfloors

Reservoirs

All foundationsexceptthosedescribedin 6.2.4.1Mostexternalconcreteabovezround.washingandcookingareas(pipesandducts:seeBS5911)

Concreteadjacentto thesea.carparks.

structuresadjacenttocarriageways

Swimmingpoolsandhalls

All partsofmarinestructuresincludingpiersor harbourworks.moorlandwater,with abrasion

3.3.4.2Freezingand thawing and de-icing saltsSeeSection6 (6.1.5 and6.2.3.2).

3.3.5 Concrete materials and mixesTheseclauses(3.3.5.1-6)providequalificationsto the useof Table 34 to take accountof practicaldifficulties in demonstratingcompliancewith requirementsforcementcontentand free water/cementratio, of benefits of specially well controlled conditions ofmanufacture,of different sized aggregates,of the use of pulverized-fuelashor groundgranulatedblastfurnaceslag and of sulphateresistingcement.The rationale for theadjustmentsor restrictionsis given in Section 6.2 of the handbook.

[IrF[

I!

[U[CI;[

L

32

L[L -,

I[

—~ ~ .

- ~ - —. .~ 2.~-~~ —-.-- - ~- - --.- ~- - - ...; ---......--——. - --. - -. - ... - .

- —.. -- --- - - . . - ..--.. . .• --;->-•-.--. - -— . .

Parr I: Section 3

3.3.6 Cover as fire protectionSection 10 of CP110: 1972 gavevaluesof cover to main reinforcementagainstminimumwidths or thicknessesof sectionswithout any opportunityto varv coverwith increaseordecreasein sectiondimensions.

Since 1972 international fire testing of concreteelementshas demonstratedthatmoderateincreasesin beamandrib widths can lead to small but significant reductionsin concretecover to maintain the requiredfire resistance.Also the Building ResearchEstablishmenthas relaxed its view on the depth of coverat which it is necessarytoincludemeasuresto control spalling.CP11O:1972 requiredsupplementaryreinforcementto be inserted in beams and ribs whenever the cover to the main reinforcement.irrespectiveof the aggregateused,exceeded40mm.This Code now requiresadditionalmeasuresto reducethe risk of spallingwhen the nominal coverexceeds40mm in denseconcreteelementsand50mm in lightweight concreteelements.

Thesetwo developments(i.e. variation of coverwith sectionsizeandthe coverdepthrequiringanti-spallingmeasures)arethe principal changesin this new Code.

Variation of cover with section sizeThesketchesand dimensionsin FigureH32 indicatethe differing approachesto beamcoverrequirementsin the 1972 and 1985 codes,for two hourfire resistance.

cPiio esaiio

_ - [J~45mm dO—10—3Omm

iT180 mm zoo I

Minimum beam width—180 mm Minimum beam width—200 mmCover to main steel—AS mm Nominal cover—30 mm

(representing 40 r’m cover to main steel heldby 10mm stirrup)

Figure H3. 7: Comparison of CP 110 andBS 8110for twohoursfire resistance.

A comparisonbetweenthe section sizes/coversgiven in Figure 3.2 and the tabulardata for elementsin Part 2. Section 4 revealsdifferencesin the minimumwidths forbeamsandribs.

Figure3.2 bringsthe minimum width of a beamor rib up to a dimensionwhich suitspracticalconsiderationson sitesuchasblockworkinfilling betweenfloor andbeamsoffitsandthe settingout dimensionsfor troughandwaffle floors. By increasingthewidth ofabeamor rib a correspondingsmall reductionin the coverto the main reinforcementcan be madein accordancewith Table 4A in Clause4.3.5 in Part2 of the Code.

Referring to the example in Figure H3.7, for two hour fire resistance,continuousconstructionanddenseconcrete,Tables4.1 and4.2 in Part2 can be usedas follows:

Minimum increasein width 50mm (from 150 to 200)Decreasein cover10mm (from 50 to 40)After allowancefor 10mm stirrupnominal cover is 30mm

The nominal coverof 30mm is the figure to be found in Table3.5 for a two hourfireresistantcontinuousreinforcedconcretebeam.

Additional measuresnecessaryto reduce the risk of’ spailingTable 3.5 setsout the nominal coversto all reinforcement(including links) to meetspecifiedperiodsof fire resistancefor denseconcrete.Thesketchin FigureH3.8 illustratesthe differing approachesto cover in CP11O: 1972 andnominal cover in BS 8110: 1985.

The useof fine meshassupplementaryreinforcementto control spallingindicatedinCP110 hasmet with almostuniversalcondemnationasdetrimentalto the placingof good

33

LIHandbookto 858110:1985

BS8IIO F- I

I I 10mm 17H—nre mesh

[20 mm

1~ ~L. — --~ ~~~T>40mm1~~,

20mm >40mm [Figure H3.8: Comparisonof CP 110 and BS 8110fire provisions.

homogeneousconcretearoundthe reinforcement.Consequentlythe new CodedoesnotCadvocatethis methodas a meansto control spalling.

To maintainthe conceptof nominal coverin the new Codewhilst still relatingbasiccover to main tensilesteel,it has beennecessaryto make allowancefor the thickness Fof a stirrupin beamandcolumnconstructions.Consequentlyastirrup thicknessof 10mm Lhas beenusedasan averageof the range8 to 12mm usedin practicein the majoritvofconstructions. [

The valuesof nominal cover in Table 3.5 thereforereflect the stirrup allo’vanceof10mm in comparisonwith the tabulardatain Section4 of Part 2. Other constructions.suchas ribs, floorsand walls which do not incorporatestirrups.generallyhavenominalcoversequalto the coversto be found in Section4 of Part2. LIThe horizontallinesdrawnthroughTable3.5separateconstructionsthatdo not requireanti-spallingmeasureswhereabovethe line anddo requireadditionalmeasurestoreducetherisk ofspallingwherebelowtheline. Suchmeasuresareoutlinedin Section4of Part2.

3.3.7 Control of cover

The importanceof workmanshipto realisethe designintentionsis re-emphasised.

3.4 Beams

3.4.1 General CFor consideringthe designof elements,the Code considersdifferent classesof elementin turn (i.e. beams,slabs,columns. etc). It is generallyclearhow a particularelement rshouldbe classifiedbut not alwaysso andit shouldbe understoodthat. in the limit, the Lboundariesbetweenthe different typesof elementare entirely arbitrary. For example.the elementsketchedin Figure H3.9 could possiblybe classifiedas eithera beamor a

LiLi

[UL

~ - -

j7z40 mm

40 mm

V

Figure H3. 9: Is it a ivall, beam,colwnnor slab?

I— --. .

Parr 1: Secuon3

column or a slab or a wall. Frequently,the Code doesnot provide specific definitionswhichwill allow a directanswerto be obtainedin particularcircumstances.The designerin thesecircumstancesshould use his commonsensein judging the most appropriatedesignrules to useratherthan looking for quasi-legalinterpretationsof the wording.

3.4.1.1DesignlimitationsThe basicassumptionsaboutbeam behaviouronly hold wherethe span is reasonablylargecomparedwith the depth. Theirvalidity certainlydoesnot hold wherethe clearspan is lessthantwice the depth.For the designof suchmembers,referencecould bemadeto CIRIA Guide2: The designofdeepbeamsin reinforcedconcretet35).

3.4.12 Effectivespan ofsimply-supportedbeamsTheobjectiveof thisprovisionis tomakeallowancefor theinfluenceofwidesupports.

3.4.1.3 Effective spanof a continuous memberWide supportswill also influence the behaviourof continuousbeamsand the sameprovisioncouldbe appliedasfor simply-supportedmembers.Thiswould,however,makepracticaldifficulties in the analysisof continuousbeamsas the spansusedwould differfrom thoseshownon drawings.It wasfelt thattheresultingconfusionwasnotjustified.

3.4.1.4 Effectivelength of a cantileverThe clauseis draftedto giveconsistencywith theprovisionsof 3.4.1.2and3.4.1.3.

3.4.1.5Effective width offlangedbeamThe conceptof an effectivewidth to a flangedbeamis a device which will permit an

LZ

(a)

(ci

equivalentuniform stressover effectiveflange width

(e,a

Figure H3.10: Effectiveflange width concepts.35

[IHandbook to BS8IJO:1985

essentiallythree-dimensionalproblemto be consideredas a two-dimensionalone.Thebehaviourof a beam with a wide flange is illustrated in Figure H3110. At a point ofcontraflexure,the compressivestressis clearlyzero. With increasingdistanceinto thesaggingregion,the compressiveforceincreasesasthe momentincreases.However,stresscan only get into the flangesby the action of shear.This increasesthe breadthof slabsubjectto significant compressionwith increasingdistancefrom a point of zeromoment,The effective flange width is the width of flange which, if assumedto have the samestresscondition at all points acrossits width, will be equi~--alentto the actualbehaviour(seeFigure). It will be seenthat the effective flange width is not constantandwill be 1.~atits maximumatthepointof maximummoment.TheCodeapproachgivesaconservativeestimateof this maximumwidth. r3.4.1.6Slendernesslimits for beams,for lateral stabiir-.~Theserulesprecludefailure by sidewaysbendingandbuckling. Lateral restraintshould

Cnormally be providedby constructionattachedto the compressionzoneof the beam.Inthe caseof parapetbeams,lateral restraint may be assumedto be provided by slabsattachedto thetensionzone.providedthat the slabthicknessis at least one-tenthof theeffective depthof the parapetbeamand the parapetbeamsthemselvesdo not project rabovethe slab more than ten times their width. The limit to the value which needbe Ltakenwas introducedso that lightly stressedmembersshouldnot be penalised.

The limits arederivedfrom work by Marshall’~’. F3.4.2 Continuousbeams

3.4.3 Uniformly-loaded continuous beamswith approximately equal spans:momentsand FshearsThe factorsgiven in Table 3.6 areclose to thosewhich would be obtainedfrom accurate [analysisof a numberof equalspanson point suPports.In a practical case.with thepermittedvariation in span.calculatedmomentsin excessof thosegiven can arise. Inaddition.where load is transferredto the beamfrom a slab. no allowanceis madeforthe typeof distributionof loadingspecifiedin 3.5.3.7:someallowancefor redistributionhasthereforealreadybeenincludedin Table 3.6 andthis is why no further redistributionis permitted.Hence it will be prudentto limit the neutralaxis depthat critical sectionsto 0.5d(3.4.4.4). r3.4.4 Designresistancemoment of beams

3.4.4.1Analysis of sections LiThese assumptionsare now well establishedand needlittle comment.The simplifiedstressblock in Figure 3.3 gives answerswhich aregenerallyvery close to thosewhich rwouldbeobtainedusingtheparabolic-rectangulardiagram.It will. therefore.beadequate Lfor all practicalpurposes.The limit to the lever arm may be consideredto serve twopossiblepurposes:it providesa limit to the maximum tensilestrain in reinforcementof0.028andit avoidsrelianceon what mightbe poor quality concreteat thetop of abeamor slabsection. L

The presenceof a small axial thrust of up to 0At~ times the cross-sectionalareaactually increasesthe calculatedultimate moment. if taken into account.but at theexpenseof considerableaddedcomplexity in calculation. 1.-

3.4.4.2 Designcharts IFull detailson the designchartsare givenin Part3 of the Code.Theyhavebeenprepared Lfor a rangeof concretegradesandare basedon the stress-straincurve for concretegivenin Figure 2.1.

Where redistributionhasbeendonein the analysisfor beamsor slabs.the chartsmay ~be usedto designsectionswhich complywith the neutralaxis limitations givenin 3.2.2.1- -LLines are markedon the chartsfor neutralaxis depthsat ultimate load of 0.3. 0,4 and05 of the effective depth. Any point on the chart to the left of one of theselines f ‘

correspondsto a section which will havea neutral axis depth at failure less than the L36 value appropriateto the line.

Li

Part I: Section 3

3.4.4.3Symbols

3.4.4.4Designformulaefor rectangularbeamsThe derivationof theseformulae assumethat the tensionsteel will be yielding at theultimate limit state.The limit to the valueof K’ ensuresthat the neutralaxisdepthatfailure doesnot exceedhalf the effective depth. This will ensurethat the tensionsteelwill haveyieldedfor all currently availablegradesof reinforcing steel.The derivationof the formulaeis given below. FigureH3.11 illustratesthe assumptionsused.

// - - . / /

/ / /- // /

/

b,.

• 0

Figure H3.11: Flangedbeam.

Considerfirst a singly reinforcedsection:

by equilibrium forces:

0.87A.A~=0.45x0.9bxf~

by equilibrium moments:

M=0.S7f~A.:

where:=(d—0.45x)

Fromthesetwo equations,

x=(0.87f~A1)/(0.45x0.9bf~)

A~=M/0.87f~~

Hence

x=Ml(045x0.9XbXzXf~)

andhence.

z=d—(MIO.9 bzf~~)

SubstitutingK=M/bd2 f~ gives

z=d (1—Kd/09z)

The solutionto this quadraticin is:

z=d(0.5-t-V(025—Kd/0.9)]

Turningnowto the limitations appliedby the redistribution.Clause3.2.2.1condition2

statesthat:x<(13b—OA)d

Writing the momentequilibrium equationas:

M=(d—0.45x) 0A02bxf~~

andsubstitutingfor x’ gives:

M~0A02bf~(l3b—0.4)d[d—0A5 (/3b—0.4)d]

writing K’=M/bd2f~ correspondingto the limiting value of z~ gives:

K’—0402 (Pb—O--4)—O.18(~h—04)

(Note: 067fJL5=0.4467J~but the figures in the above derivation haveonly been37

F’Handbook to BSSIIO:1985

written to two decimalplacesas 0.45f~. The moreaccuratefigure has, however,been rusedin the arithmetic.)

Substitutinginto this equationgives the valuesin Table H3.3 for K’ for variousamounts Pof redistribution.

Table H3.3 Values of K’ corresponding to various amounts ofredistribution [

%redistrjbu~ioo I3~, K’

< 10 0.9 0>15615 0.85 0>14520 0.8 0.13225 0.75 0t1930 0.7 0>104

When K exceedsK’. the simplified equationsprovide for the addition of sufficientcompressivesteelto ensurethat the neutralaxis remainsat the level correspondingtoK’. The compressionsteel is assumedto be at yield andthis will be so provided:

0.0035(x—d’)/x>0.87f~/200000

or

d’/x<1 f.s’805

assumingf~=400. this zivesd’/x < 0.43The extra momentabovethat which would correspondto a singly reinforcedsection

with the requiredneutralaxisdepthis (K— K’)f~bd2 . By momentsaboutthetensionsteel,

(.K—K’)f~bd=0.87d—d’)AVJ

hence,

A‘1=(K—K)fcubdi0.87(ddV)fv I

Theareaof tensionsteelis that requiredfor the momentK’f~bd2 plusA’

1. This can be K

seento be given by:A1(K’f~bd/0.87f~z)--A’, C

The equationsin 3.4.4.4thus follow directly from the basicassumptionsandthe neutralaxis limitations of 3.2.2.1. r

Table H3.4 maybe usedfor the calculationof steelareasfor reinforcingrectangular ~sectionsas T-sectionswhere the neutralaxis lies within the flange.

Table H3.4 Design parameters for rectangular sections [K <0047 0.05 0.075 0.10 0.125 0.15 0.156

x/d 011 0.13 0.20 0.28 0.37 0.47 0.50rid 0.95 0.94 0.91 0.87 0.83 0.79 0.78

0.87fz/d f~=250f~.=460 206.6380.2 204.7376.6 197.5363.5 189.8349.3 181.2333.5 171.5315.6 169.0311.0

A~M/0.87f~z K=.~I/bd:f~

3.4.4.5 Design ultimate momentsof resistance(offlangedbeamswhereneutral axisfallsbelow theflange)The equations 2iven in this clause are not exact but are slightly conservativeapproximations.Equation 1 is derivedas follows:

Momentsaboutthe centreof the flangegives:

M=087f~(d—h~/2)A1—(0.45 —h~/2) 0.45 x 0.9xb~f~

Rearranginggives:

38 A,=(M-s-(0.45x—hf/?) 0.402b~xf~,j/(0.87A,f~(d—h~/2))

4 - . ..-

LLiLLC:,

Parr 1: Section3

x will notexceed0.Sdsosubstitutingthisvaluewill giveagenerallyconservativeestimateof A1. This eives:

A1= (M+0.1f~b4 (0--45d—h~))/(0.87AJ’~(d—h~/2))

Equation2 canbe deriveddirectly by taking momentsaboutthe centroidof the tensionsteel.

3.4.5 Designshear resistanceof beamsCalculationsfor the strengthof reinforcedconcretebeamsarebasedon comparingtheaverageshearstresson a sectioncalculatedfrom equation3 with a nominal value ofultimate shearstressv~ given in Table 3.9. The shearstressesgiven in Table 3.9 arebasedon an extensivestudyof test data(FigureH3.12). Whenthe averageshearstressis treaterthan the nominal stressfrom the Table, shearreinforcementis providedinproportionscalculatedon the assumptionthat the reinforcementforms the tensionmembersof one or moreseriesof pin-jointedtrusses.This approach,commonlycalledthe trussanalogy, hasbeenshown in teststo be conservativeand the contribution tothe shearstrengthfrom the concrete.v~bd, is thereforeassumednot to be lost if v>v~.If v is only just greaterthan v~, it will be necessaryto increasethe amountof shearreinforcementrequiredfrom the trussanalogyto theCodeminimum permittedamount.When v is lessthanv~, minimum reinforcementis still requiredin manycases:seeTable3.8.

3—

2

oe~10

Figure H3.12. Shearstrengthof beamswithout shearreinforcement.

Where large shearsarecarried,it is possiblefor the diagonalcompressivestressestocausecrushingof the web concrete.For this reason.the maximumshearcarriedby thebeammust be limited: in the Code this limitation is given as 0.8Vf~~ or 5 N/mm

2whicheveris lesser.

3.4.5.1 Symbols

3.4.5.2Shearstressin beamsThe actual way in which shearis carriedby a section is highly complexandthe valuegiven by equation3 shouldnot be viewedasastresswhichactuallyexistswithin asection:it is merely a mathematicaldevice used for the interpretationof test results in thederivationof an empirical designmethod.

3.4.5.3Shearreinforcement:form, areaand stressThe intentionsof Table 3.8may besummariseddiagramaticallyin Figure H3.13.

2 3 4 5 6 7 8

39

Handbook to BS8IO:198S

Miniumum shear reinforcementmay be omitted in somemembers of minor importance

//

/ Minimum shear

/

P P

reinforcement //

//

/

v, v,---OA

A----

rThKK’ LU

K/

/ /

08vf04 5 Nimm

2

Figure H3.13: Provisionofshearreinforcementin beams.

3.4.5,4 ConcreteshearstressSincethe shearstressis a function of the amountof tensionreinforcementat the sectionconsidered,it is clearly importantto establishjust what reinforcementcan be included.This causedsomeconfusion in CP110 and an attempthas beenmadeto give clearerrules. In general.anybar which extendsmorethan an effectivedepthon eithersideofthe sectionbeingconsideredcan be included.This definition will not be satisfactoryata single supportbut thereis ampleevidence,to showthat the full areaof bottomsteelat the supportmaybeusedin this caseprovidedit is anchoredaccordingto the normalrules. The final paragraphof 3.4.5.4aimsto clarify whichreinforcementshouldbeusedwhennominal top steel is providedat a notionalsimplesupport.It is believedthat thereis a misprint in the final sentencewhichshouldread“This steelshouldextendinto thespan for a distanceof at leastthreetimes the effective depth.” Clearly, it shouldalsobe fully anchoredinto the support.

3.4.5.5Spacing of linksThe trussanalogywould suggesta maximumspacingequalto the leverarm: 0.75diS alower boundvalue for this. The logic behindthe limit on lateral spacingof the legsofstirrups is less clear but experimentalevidencesuggestsa reasonwhy such a limit isvaluable.Oneof the majorfunctionsof stirrupsis to inhibit ~dowel’failureof thetensionsteel.This is a tearingout of the bars in the way sketchedin FigureH3.14.

Figttre H3. 14: Normal modeofshearfailure.

a40 (1

. . . ..

. .,.. . —.

A~(0B7f,,J

0.4

Fr

[CI

CII

stirrup controlsthis

failure crack

[LL

bottom reinforcement pushed down.tearing out tension bars

_ . _ .71-- - - - i.—.. -NT--’...- --~-:--:.---- ---- -- - ---- -- ,-. - -- -.-

Part I: Sectwti S

Clearly the effectivenessof the stirrup in achievingthis will reduce‘~‘ith increasingdistanceof the vertical leg from the bar considered.Clearly, if a bar is placedfurtherthan 150mmfrom astirrup leg, it canstill be usedto provideflexural strengthbut shouldbe ignoredin assessingVc. The situationin slabscanbe usedto extendthe interpretationof this clausefurther. In slabs,stirrups are not required until v exceedsv~. It seemslogical to arguefrom this that the requirementrelating to the spacingof stirrup legsisto ensurethat ~c can be maintainedin circumstanceswhere v exceedsv~. It thereforeseemsreasonableto concludethat the limitations on lateral spacingmay be ignoredwherev is less thanv~ both in beamsandslabs.

3.4.5.6 Shearresistanceof bent-upbarsThe trussanalogyassumesthat the tensionsin the trussarecarriedby longitudinal andstirrup reinforcementand the concretecarriesthe thrust in the compressionzoneandthe diagonalthrustacrossthe web.

Equation 4 is derived directly from considerationof the equilibrium of this system(FigureH3A5).

/Ii .

I, ,

A A

2/I-Ia A

A. ~ /~~- ~1

Figure H3.iS: Trusssystemsfor shear.

3.4.5.7 Anchorageandbearing of bent-upbars

3.4.5.8 Enhancedshearstrengthof sectionscloseto supportsWhenthe ratio of shearspanto effectivedepth of a beamis reducedbelow2, theshearcapacity is considerablyincreased.

Figure H3>16 shows a plot of test results reportedin references3,7. 3.8 and 3,9illustrating the relationshipbetweena/d and v. for beamswithout stirrups. The lineshown on the graphis straight for all valuesof a~Id greaterthan 2 when v/vt is 1. Thisclausedefinesthe parabolicline shownin the Figure.

The strengthof short beams dependsto a large extent upon the detailing of thereinforcement.Adequateanchoragemustbeprovidedto the main tensilereinforcement,Vertical stirrupsarenot very effective in beamsin which a.,./d is lessthan 0,6. in whichcasehorizontalstirrupsparallel to the main tensionreinforcementare recommended,

The resultssho~vnin FigureH3.16 derive from testson short-span.point-loadedbeamsbut the resultswill hold for any failure wherethefailure planeis constrainedto form atan anglegreaterthan tan’ (1/2) to the horizontal,The enhancementin strengthcanthereforebe appliedfor anysectioncloserto a support than2d.

3.4.5.9Shearreinforcementfor sectionscloseto supportsEquation5 derivesfrom the assumptionthat the effect of the enhancementis only onv,, anddoesnot affect the efficiencyof shearreinforcement.Applicationof trussanalogymight be seento suggestthat the increasedtrussangle implied by a failure close to asupportwould increasethe efficiency of shearreinforcement.This maybe so, in whichcase, equation 5 is conservative, but, while the experimental evidence for theenhancementof v~ is clear(FigureH3.16), it is doubtful if sufficient exists to show theeffect of a~.Id on shearreinforcementunequivocally. 41

Handbook to BSSIIO:]98S

10

9

8

7

6

v

vc 5

FigureH3.16: Ultimateshearstressesfor beamsloadedclosetosupports:Ur takenfrom Code

3.4.5.10 Enhancedshearstrengthnearsupports(simplifiedapproach)At a distanced from thesupport.FigureH3. 16 will showthat the capacityof the sectionis increasingvery rapidly. So muchso that it is mostunlikely that the shearforce willbe increasingmorerapidly. The rule given in this clausewill thus normally give a safeway of gaining the advantageof the strengthenhancementfor minimal effort.

3.4.5.11 Bottom loadedbeamsA loadapplied nearthe bottom of a beamcould breakthe bottom out as sketchedinFigure H3.17. The load.effectively, hasto be transferredto thetop by links beforethedesignmethodis valid.

4, possible mode offailure

Figure H3.!7: Loadson thebottotn of beams,

3.4,5.12Shearand axial compressionIn dealingwith the commentson the draft of BS 8110 circulated for public commentitbecameapparentthat therewas a considerabledemandfor guidanceon the treatmentofshearandcompression.particularlyin columns.Equation6 in this clauseis anentirelyempirical attempt to make allowance for the increasedshear capacity given bycompressiveaxial load. Truly applicabletest dataare not easyto find but Figure1-13.18comparestest results from several seriesof tests‘vith the proposedformula- -Inthecomparison.Vm has beentakenas 1.0 whencalculatingv~.

[IFU[I!I

C

El

4

3

2

0

a’,d

UIICCCI[

U42

- - - . -- - -

. . ... . .

-- ..-- - -- - -. - . -

-J.1

I-aLU~

4 LU

LLU~

LU CO CO —

0

Figure H3. 18: Shearand axial compression.

3.4.5.13 TorsionWhen thesystemisstaticallydeterminate,ultimate torsionalmomentsmustbe calculatedand providedfor. In indeterminatestructures.it will in mostcasesnot be necessarytoconsidertorsionat the ultimate limit state.but it may be necessaryto considerit at thelimit stateof cracking.FigureH3.19showsanexamplewith edgebeamsin whichtorsionis statically indeterminate,i.e. it arisesbecauseof an imposeddeformation and itsmagnitudewill dependon the relativetorsionalandflexural stiffnessesin thestructure.Lack of torsional strength in such caseswill not causecollapsesince the memberswillcrackanddeformwithout developingthe ultimate torsional condition.

However, in somecasesthetwist imposedon amembermax’ causeexcessivecracking.This can happen,for example,in anedgebeamwherethespanof theslabatright-anglesto the beamis large,or as in Figure H3.19 in the short lengthof edgebeambetweenthe trimming beamandcolumn wherethe imposedrotation from the secondarybeam

IParr1: Section3

2

cALcULATED SHEAR STRESS (N/mm2)

not usually seriousSerious torsional cracking likely

beam

Figure H3J9: Examplesoftorsion dueto imposedrotation.43

[IHwtdbookro BS8IIO:1985

hasto be accommodatedin quite a short length. For the purposeof designingthereinforcementanddeterminingthe forcesexertedon adjoiningmembersthe method ft)r

calculatingthe imposed torque is basedon testson crackedreinforcedbeams’3”” andis generallyconservative,

Torsional stiffnessesare generallysmall with respectto flexural stiffnessesandcantherefore.be ignored in assessingthe imposedtwist. This will give an upperlimit andonewhich will not be too conservative.

Explicit design for torsionis dealtwith in Part2 of the Codeandwill be discussedinthe relatedsectionof the Handbook.

3.4.6 Deflection of beams

3.4.6.1 GeneralWhere the conditionsof serviceof a beamare knownwith precision. its detlectioncanbe calculatedreasonablyaccuratelyby usinganyof anumberof semi-empiricalequations.However,the calculationsare tediousandtime-consuming.It is not practicableto checkthe sufficiency of all normal memberswith respectto deflectionby direct calculation,Furthermore,as is discussedin the commentaryto Part 2 Section3, the calculationofa deflectionrequiresconsiderationof manyfactors,a numberof which may well not hedefinableat the designstage.

For this reason,the Tablesof ratiosof span to effectivedepthhavebeendevisedandit will besatisfactorytousethemfor all normalmembers.Considerationshould.however.be given to the possibility of calculationwherethe conditionsare in any way unusual.

3.4.6.2 Symbols

3.4.6.3 Span/effectivedepthratio for a rectangular or flangedbeamUse of the Tablesshouldbe largely self-explanatory.The category‘continuous~in Table3.10 may be taken to apply to any situationwhere at least one end of the beamiscontinuous,Thus, the endspanof a seriesof continuousbeamsmay be considercdas [continuous.

The derivationof the clauseis discussedfully in references3.11—3,13;however,abriefdiscussionof the principlesinvolved will be given here. CTo see how the setting of limits on the ratio of span to depthcan be expectedtocontrol deflections,considerthe caseof a fully elastic rectangularbeam supportingauniformly distributedload (g~+q~).

FIf the maximumpermisiblestressin the material is f, the momentwhich the sectioncan withstandis given by:M = fbh/6 = (gk+qk)12/8 3.1

The deflectionof the beamis given by: La = 5(gk+q~)l4/384E1

Forminganexpressionfor (g~+qk) from equation3.1 andsubstitutingthis into3.2gives:(5fl24E) (1/h) = (all) L

Thus for a given elasticmaterial, if the ratio 1/h is keptconstant.the ratio of deflectionto spanwill remainconstant.By settingalimit to theratio of spanto depth.thedeflection 1~.~will be limited to agiven fraction of the span.This is what is requiredby the Codesince.in general.the deflection limits are given as fractionsof the span. It should be Lk~--Ir

that. if an absolutelimit is set on deflection, the ratio of span to depth must decreasewith increasingspan.This is the case where a limit of 20mm is specified. 3.4,6.4 is Lnecessaryto copewith this condition,

If the engineerconsidersthat limits otherthanthosefor which the Tableshavebeenproducedare more appropriate,the Tablesof basic ratioscan be modified to suit the Lchosenlimits. This maybe doneby multiplyingthe basicratio by theratio of the requireddeflectionto the deflectionfor which the Tablewas derived.Thus.if the total deflectionis to be limited to span/B instead of span/250,the figures in Table 3.10 shoUld bemultiplied by 250/B. Similarly, if the deflectionoccurringafter the constructionof thepartitionsis to be limited to some absolutevaluea’, ratherthanto 20mm. the factor in3.4.6.4can be adjustedby multiplying it by a’/20.

‘S.-

p

Span/depthratios providea rigorousmethodfor controlling dflh Parr]:Sectionas long as thematerial from which the beamis madeis elastic.Unfortunately,reinforcedconcreteisnot: the stiffness of a beam dependsto a large extentupon the steel percentageandupon the stateof cracking. Thus, if span/depthratios are to be used for reinforcedconcrete,someway of correctingfor its actual behaviourhasto befound. The first step

-in this processis to use ratios of span to effective depth rather than span to overalldepth.andthe secondis to introducemodifying factors.

The basicratiosgiven in Table 3.10 derive from experience.Theycan be seento besimilar to thosein previousCodeswhich experiencehasshown to be of the right order.Theymay be consideredto apply to averag&beams.The factorsin 3,4.6.5and3.4.6.6modify this averagefigure up or down as a function of the level of steelstressandthestateof loading of the section.

The logic behindthe factorsin Table 3.11 canbe seenfrom consideringthe four casessketchedin FigureH3.20.This figure showsthe straindistributions in four sections.tworeinforcedwith 250gradeandt~vo with 460 gradesteel:of eachpair. one hasahigh andthe othera low steelpercentage.All sectionsare loadedto their full designserviceload.For this example.the effectsof the concretein tensionon the stiffnessareignoredandtheanalysisis basedon acrackedsection.Thedeflectionof abeamisdirectly proportionalto the curvatureat the critical sectionand this is given as:

curvature= 1/r =e~/(d—x)

4=400 4=250

lowpercentage

highpercentage

values of curvature x effective depth(—din

400

4

250

Steel lowPercentage

high

~

j~,~0jf,j56X10,,,,j

Figure H3.20: Logicbehindtensionsteelmultipliers.

The figure illustratesclearlyhow steelpercentage.which definesthe neutralaxis depth.andsteelstrength,which definesthe strain,influencethe deflection. From the point ofview of span/depthratios. the higher the curvature. the lower must the permissiblespan/depthratio be in order to limit deflectionsto a constantproportionof tj~e span.InCPI 10,atableof factorswas includedwhich was afunction of steelpercentageandsteelstress.The effect of steelpercentageoverthe full practical rangeis illustrated in FigureH3.21(a). In practicethis was found inconvenientto use since deflectionscould not becheckeduntil afterthereinforcementhadbeendetailed.Sincethereis adirectrelationshipbetweensteelpercentage,steelstrengthandK(M/bdi. thetablein CPl lOis reformulated

45

0.00078

Handbook to B58110:198S

--‘n

1.8 ~~‘1 [~1

1.6 \\\~.\ I ii F~j

1.4

1.2 — ~,c.;~---—--—4 —----—-----~.~—

1.0

0.6__,III~

0.4 I’ll

0.2

0

f,—15O

~II_ -inn — ~

4--.20a

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 10 22 2.4 2.6 2.8 10PERCENTAGE OF TENSION STEEL

Figure H3, 21 (a): Modificationfactorsas a function ofsteelpercentage.

z0F4

1,00’~

ILL.

Figure H3.21 (b): Modificationfactorsfrom Table3.11.

to give factors as a function of K and steel stress,Thus it is now possible to checkdeflectioris as soonas an estimateof the design ultimate momentscan be made. Forconvenience,Table 3.11 is presentedgraphicallyin Figure H3.21(b).

The analysisillustrated in Figure H3,20 ignoresany influenceof the concretein thi.tensionzone.In fact. this concreteaddsconsiderablyto the stiffness(seePart2. Section3) andthis is allowedfor in the factorsgiven in the Code.Clearly, ho~vever,in the caseof flangedbeams,this stiffening will be reduced.The basic ratiosgiven in Table 3.l()for flangedbeamsreflect this fact.

3.4.6.4Long spansSeediscussionof 3.4.6.3above,

3.4.6.5 Med~flcation of span/depthratiosfor tension reinforcementSeediscussionof 3.4.6.3above.

3.4.6.6 Modification of span/depthratiosfor compressionreinforcementThe effectsof compressionreinforcementon deflectionsare twofold:

. .

[I

z0F4

0~~

~u.

2.0

rCr

CCCI

[I-;L[I:

M

46U

., .. - -- .-. - -. -~ . -

‘.—..—.—.. .

IParr]:

(a) compression steel reduces the neutral axis depth and hence the curvatures (Figure

Fl

1-13.20)

(b) compression steel signifi canrlv reduces the effects of creep and shrinkage and thus

has a substantial effect on the long-term deflections.

1 These two factors are taken into account in the values given in Table 3A2

3.4.6.7 Deflection due to creep and shrinkage

TI This clause is self-explanatory, More information on the effects of creep and shrinkageis given in Part 2 Section 7.

13.4.7 Crack control in beams

See Section 3.12.11.2 for discussion.

] $5 Solid slabs supported by beams or walls

3.5.1 Design

3.5.2 Moments and forces

3.5.2,1 General

Single-way slabs may be analysed as beams taking account of the simplified load

arrangement. Most two-way solid slabs in practice are rectangular. are cast monolithically

with supports and are covered by 3.5.3. As the provisions here are based on Johansen’s

yield line method i~ is suggested that 3.5.3 be applied in all such cases. It follows that

determination of moments and forces will be necessary only for non-rectangular slabs.

Elastic theory for such cases is rather complex and so the Johansen or Hillerborg methods

are recommended’3’~15’.

3.5.2.2 Distribution of concentrated loads on slabs

The empirical rules given for the effecti~--e widths of slab that resist bending moments

due to such loads are based on loading tests to failure.

3.5.2.3 Simpl~ficatwn of load arrangements

The justification for a single-load case of maximum design load on all spans or panels

is given by ~~~~y(3.lbl, The major part of the report is taken up with justifying this

simplification in terms of current knowledge of slab behaviour and imposed loadings on

floors. It is reasonable for normal occupancy but cannot be rigorously proved to be so.

It is not considered valid for structures designed for storage or ~vhere the live to dead

load ratio is large. In such circumstances pattern loading should be considered.

The effect of redistributing the moments by 20% is shown in Figure H3.22.

Redistribution of moments affects the shear forces and these should be calculated to

satisfy static equilibrium.

-— Elastic moment diagram

Final moment diagram

20% reduction

column

Figure H3.22: Developmentof bendingmomentenvelopefor slab.47

Handbook to BS8IIO:198S F!It shouldbe notedthat the effectof redistributingmomentsby 20% affectsthe neutral Faxis depth limit and hencethe value of K’. The value of K’ in 3.4.4.4 15 reduced(CI

0,132. This is not generallya limitation for solid slabs.The momentreductiondoesnot. of course,apply to cantilevers, FWhere a cantileverextendsfrom the last support of a continuousslab it is essential

to considerthe loadcaseof cantileverloadedandadjacentslabpanel-orspanunloadedto ensuresufficient anchorageof the cantilevertop steel into the adjacentslab,

3.5.2.4 One-wayspanning slabsof approximatelyequal span

3.5.3 Solid slabs spanning in two directions at right angles: uniformly distributed loads

3.5.3.1 General

3.5.3.2 Symbols

3.5.3.3 Simply-supportedslabsThe coefficients in Table 3,14 arederivedfrom the Grashof—Rankineformulae,

3.5.3.4Restrainedslabs

3.5.3.5 Restrainedslabswherethecornersarepreventedfrom lifting andadequateprovisionis madefor torsion: conditionsandrulesfor the useof equations14.and 15 - LIThe coefficients in Table 3.15 have been derived from yield-line analvsis’3’7~. Theparticularvaluesof ~ andj3~ dependon thechoiceof yield lines, the relationshipforwhich is given in equations16 to 18. The valuesof /3~ were chosento an accuracyoftwo decimalplaces.This has led to small differencesoccurringbetweenthe values forf3~ and~ for squarepanels.

3.5.3.6 Restrainedslab with unequalconditionsatadjacentpanels I3.5.3.7Loads on supportingbeams

ITheestimationof loadson supportingbeamshasbeenchangedsignificantly frompreviouscodes.The new Table of coefficients(Table 3.16) providesan estimateof the load Onthe supportingbeamswhich makesallowancefor the supportconditions.If a panelhasoneedgecontinuOusandtheoppositeedgediscontinuous,thenmoreloadwill beattractedto thecontinuoussupport(in the sameway as the shearcoefficient in Table 3.13for theLfirst interior support is greaterthan that for an end support). The ne~v Table (Table3.16) takesaccountof this effect in developingcoefficients for the shearforcesin the

Fslab along the line of the support.Those shearforcesconstitutethe loading on thesupportingbeams.When analysing the supporting beams,the loading sho~vn in Figure 3.10 gives ~

maximumfree bendingmomentof 0.117v51

2anda fixed end momentof 0.076v51. [

3.5.4 Resistancemoment of solid slabsWhere the single loadcasehasbeenused with 20% redistributionof supportmoments [(3.5.2.3) the valueof K’ in 3.4.4,4is reducedto 0.132.

3.5.5 Shear resistanceof solid slabs IThe problem of defining what is a slab for the purposeof sheardesign is not easytoresolve. The simple definition that any memberwith a ratio of width to depth greaterthan4 is aslab andany memberwith a smallerratio is a beam— is a first rough way ofseparatingthe types of member, In some circumstances,the way in which a slab.50 Ldefined,is designedor loadedmay require it to be treatedas a beam.Oneexample i~

that of treatingareasof slabs,which mayor may not be locally thickened.as beamstflp5betweencolumns;suchstripsshouldbetreatedasbeams.This discussiondoesnot applyflto the ribs of waffle typefloors: thesedo not have to be consideredgenerallyas beattISLJ

(3.6). -j

. ...-, — — — .

Part 1: Section3

5 3.5.5.1 Symbols

3.5.5.2Shearstresses

13.5.5.3Shearreinforcement

TI 3.5.6 Shear in solid slabsunder concentratedloads (3.7.7)

3.5.7 Deflection

1 Note that, with theexceptionof cantileverslabs,it is alwaysthemid-spanmomentwhichis usedin assessingthefactor from Table 3.11. It is worth bearing in mind that the ratiosof span to effectivedepthhave beenarrangedto ensurethat the deflectionsof flexuralmembersdo not exceedthe specifiedlimits relative to their supports.If the support for

1a slab consistsof beamswhich will themselvesdeflectunder load, the total deflectionof the whole systemmight not be satisfactoryeven though each memberconsideredindividually wassatisfactoryrelative to its own supports.It might be unwise, therefore.to design both the beamsand slabs in such a systemto be on the limit of the ratios ofspan to effectivedepth.

Wherecompressionreinforcement is usedin slabsTable 3.12 may be usedto modifythe span/effectivedepthratio.

3.5.8 Crack controlSee3.12.11.Specificcrack control measuresare unlikely to be required providedthespacing of barsconformswith the rules given in 3.12.11.2.7.

When reinforcementis neededpurely to distribute cracking arising from shrinkageand temperature effects the recommendationgiven in 3.9.4.19and 3.9.4.20for plain

0walls shouldbe followed. Further information is given in Part 2 Section3.8.4.

B3.6 Ribbed slabs (with solid or hollow blocks or voids)

3.6.1 General

3.6.1.1 Introduction

3.6.1.2Hollow or solid blocks andformers

3.6.1.3Spacingandsize of ribsWhen calculating the sectionresistancethe maximum flange width assumedshouldnotexceedthat specifiedfor T-beams(3.4.15).

Where the slab is arrangedto span in one direction, it is suggestedthat, in additionto the condition specifiedin the clause,a minimum of five ribs may be provided.

The last paragraphappliesto trough slabswhere there maybe practical difficulties inensuring the positioning of main reinforcement or where the rib spacing is such thateachwill behaveas a separatebeam.This is likely if the spacing exceeds1.5m.

It is not considerednecessaryto providelinks in waffle slabsunless theyare requiredfor shearor fire resistancepurposes.Fixing of the bars in one direction can be achievedby tying them to the bars in the other direction.

3.6.1.4Non-structural sidesupport.This clause neglects to point out that cracking in the adjacent flange is likely andconsiderationshouldbe given to introducing ribs at right anglesto the support.

3.6.1.5 Thicknessof tapping usedto contribute to structural strengthAlthough a nominal reinforcementof 0.12% is suggestedin the topping (3.6.6.2),it isnot insistedupon, and the topping is thereforeexpectedto transfer load to the adjacentribs without theassistanceof reinforcement.The modeof transferinvolvesarching actionand this is the reasonfor the insistencethat the depthbe at leastone-tenthof the cleardistancebetweenribs. .49

Handbook to BS8IJO.-198S [I3.6,1.6Hollow block slabswhere topping not usedto contribute to structural strength r3.6.2 Analysis of structureWhenconsideringthe effectsof concentrate.dloadsin one-wayslabs,the width assumed flto contributeto the supportof the load shouldnotexceedthe width of the loadedareaplus four times the rib spacing;in addition, it shouldnot be takengreaterthan 0,251on eitherside of the loadedarea. r

For waffle flat slabs the sectionpropertiesshould be basedon the stiffnessof the ~waffle section. If the breadthof the solid areaat the columnis at leastonethird of thesmaller dimensionof the surroundingpanels. it should be taken into accountin thesectionproperties. r

Similarly for a flat slabwith drops,the dropshouldbetakeninto accountin the section ~propertiesif it is atleastone-thirdof thesmallerdimensionof thesurroundingpanels.

Hencewhenanalysingby equivalentframethesectionpropertiesshould be basedonthe hatchedareasas shownin Figure H3.23. h

T~T

at mid-span

WAFFLE SLAB

3.6.3 Design resistancemoments

3.6.4.2 One or two-wayspanning slabs

5(1

at mid-span

[

CI£CLiLIC[U

/,/,/ ,/7/ -, /

at support /

FLAT SLAB WITH DROPS

FigureH.3.23: Areasto be consideredfor sectionpropertiesin equivalentframeanal.vsis

The lateral distribution of momentsin waffle flat slabs is similar to solid flat slabsexceptthat the solid areaarounda column may be consideredas a drop (3.7.1.5. 3.7.3and 3.7.2.10).The presenceof the solid areacausesstressconcentrationat the outercomers.Forthis reasonthe concentrationof reinforcementrequiredgenerallyby 3.7.3.1doesnot apply. The reinforcementshouldbe placedevenly acrossthe column strip.

The torsionalstiffnessof waffle slabsis small andwhenanalysingby grillage or finiteelementmethodsit is reasonableto neglectits effectsexceptin solid areas.

A more detailedmethod of grillage analysis coveringwaffle flat slabs is given byWhittle13 18)

The alternativesuggestionsgiven in 3.6.2 for situationswhere it is impracticabletoprovide sufficient reinforcementto develop the full designsupport moment apply tosingle andtwo-way ribbed slabs.For flat slabs3.7.2.3.7.3 and3.7.4 apply.

3.6.4 ShearIt is suggestedthat wherethe rib spacingexceedsIm. nominalstirrupsshouldbeprovidedin the ribs in accordancewith 3.4.5.3.

3.6.4.1 Flat slab constructionWhereshearlinks are requiredin the ribs theyshouldbe extendedinto the solid sectionto allow the concentratedstressesto disperseinto the solid section(an effective depthsay). The tensionreinforcementshouldextenda further tensionanchoragelength.

3.6.4.3 Shearcontribution by hollow blocks

L

at support

.~ -—:.. .-. .

Par:): Section.3

3.6.4.4 Shearconributionfrom solid blocks

3.6.4.5 Shearcontributionbyjointsbetweennarrow precastunits

13.6.4.6Maximum designshearstress

3.6.4.7 Area of shearreinforcementin ribbed, hollow block or voidedslabs

] Advantagemay be taken of enhanced shear strength of sectionsclose to supports asgiven in 3.4.5.8—3.4.5.10.

13.6.5 Deflection in ribbed, hollow block or voidedconstructiongenerally

Note that if a slab is designedas simply supported it must be consideredas simply

-supportedwhen checkingdeflections.

3.6.5.1 GeneralIf a slab is designedas simply supportedaccording to the rules given in 3.6.2 then itshouldbe treatedas simply supportedfor the purposesof checking the ratio of span toeffective depth.

3.6.5.2 Rib width of voidedslabsor slabsof box or I-section units

3.6.6 Arrangementof reinforcement3.6.2 allows that a continuous ribbed slab may be designedas simply supportedwith

a so-calledanti-crack steelprovidedover-the supports.The systemof treating continuousslabs as simply supported has arisen in practice becauseof the difficulty, or evensometimesimpossibility, of fixing enough top steelin the ribs oversupportsto resist the

El momentswhich would arise from treating the slabs as continuous. From the point ofview of safety, this is likely to be satisfactory. However, from the point of view ofserviceability,its sufficiencyis more doubtful. Effectively, designingin this way is askingfor a very large redistribution in the support section.This meansthat, evenunder dead

ciload, the support steel will yield if the concretecracks, and it cannot therefore acteffectivelyas anti-crack reinforcement.It maywell be that cracks in thetop surfaceofslabsover thesupportsare often not serious.thecracks beingcoveredby floor finishes

N or partitions. The engineershould neverthelessbe awarethat this.methodof designdoeshave risks of seriouscracking associatedwith it.

The necessityof providing internal ties in accordancewith 3.12.3 may on occasions

LI influence the application of thesedetailing rules.Wherewaffleslabshavebeendesignedasflat slabs.situationswill arise whereparticularspacingsof ribs will lead to thoseof the middlestrip requiring more reinforcementthan-those in the column strip for positive moments in the span (Table 3.21). In such

ii circumstancesthe reinforcementrequired for the total momentshould be spreadevenlyacrossthe middle and column strips.

0 3.6.6.1 Curtailment of bars

13 3.6.6.2 Reinforcementin toppingfor ribbedor hollow block slabs

3.6.6.3 Links in ribsSeecommenton 3.6.1.3.

U The ribs along the external edgesof waffle slabs should be provided with nominallinks to help control torsional cracking.

U3.~ Flat slabs

3.7.1 GeneralThe designof flat slabs by the empirical method given in CP11O was generally lessconservativethan that of more rigorous elastic methods.Researchin recent yearshas

51

Handbookto BS8IJO:J985

shownthat patternloadingfor the rigorousmethodsis overconservative.For this reason rthe simplified single load case(3.5.2.3) has beenintroduced.In general.designsnowcarriedout in accordancewith the simplified ruleswill be moreconservativethanthose

madeusingequivalentframe or grillage methods.New shearclauseshavebeenintroduced as a result of researchby Regan~3•’9 and

Long et alt30~. These clausesare consideredinternationally to provide the closestrelationshipwith the test resultsof all the existingconcretecodes.

3.7.1.1 Symbols

3.7.1.2 DesignIn order to satisfy the serviceabilitycriteria, elastic methodsof analysis are likely tocontrol the design of flat slabs. The use of computersenablesequivalentframe andgrillagemethodsto becomeincreasinglypopularfor modellingflat slabs.Grillage methodscanprovidereasonableestimatesof deflectionprovidedcareis takenin selectingsectionpropertiesanddueaccountis takenof the effectsof crackingandcreep.Detailedmethodsare given by Whittle~3181.

The distributionof momentsacrossthe width of slabfor negativemomentalterswith Frespectto the aspectratio of the panel.Figure H3.24.takenfrom Regan43’91showstherelationship.

F -

80

80 — [PERcENTAGE OF 50LONG-SPANNEGATIvEMOMENT — [

20 —

10 r..0. L

1.0 15 20ASPECT RATIO

Figure H3.24.- Distribution of long-spannegativemomentin internal panelofflat slab. L3.7.1.3Column head

(3.7.1.4Effectivediameterofa columnor columnhead I.

3.7.1.5 DropsWhen checkingthe shearresistance.two critical perimetersshouldbe considered.One LLSdd from the face of the column andthe other I.Sd. from the outeredgeof the drop(dd=effectivedepthof drop andd

5=effectivedepthof slab). L3.7.1.6 Thicknessofpanels

3.7.2 Analysis of flat slab structures LAnalysisof flat slabsis normally carriedout by usingone of threemethods:yield line.

E

1

Parr 1: SectionS

equivalentframe, or grillage analogy. Yield line methodswhilst providingthe mosteconomicsolutiondo not provideinformationconcerningthe mostsuitablearrangementof reinforcementfor working loadconditionswith consequentimplicationsfor crackinganddeflection.

Elasticmethodsare morelikely to predictthebehaviourunderworkingloadconditionsandtheycan be extendedto ananalysisattheultimate limit state.The equivalentframeapproachprovidesa reasonablerepresentationof the behaviourof the floor by a systemof columnsand beamsanalysedseparatelyin eachspandirection.

One misconceptionheldbysomeengineersis to considera reducedloadwhenanalysingthe slabin onedirection.A flat slabsupportedon columns,otherthanperimeterbeams.can fail as a one-waymechanismjust as a single-wayslab, andit should be reinforcedto resistthe momentfrom the full load in eachorthogonaldirection.

The use of computersfor the analysisof flat slabs is becoming more common andgrillage programsare now often used to solve routine designproblems.Thesecan givereasonablepredictionsof deflectionprovidedcareis takenin selectingsectionpropertiesand due accountis takenof the effects of crackingand creep.Detailed methodsaregiven by Whittle~~181.

3.7.2.1 GeneralThe justification for a single loadcaseof maximum designloadon all spansor panelsis given by Beebv~310~.Although it is a reasonableassumptionfor normaloccupancyitcannotbe rigorouslyproved.It is not consideredvalid for structuresdesignedfor storagewherepatternloading maybe a real possibility.

3.7.2.2 AnalysisIt should be realisedthat although the equivalentframemethod of analysisprovidesareasonableset of momentsandshearforcesit will normally over-estimatethemomentsat the edgecolumns. The lateral distribution of momentsat edges is normally veryrestrictedas describedin 3.7.4.2.

3.7.2.3Division offlat slab structuresinto framesThe division into longitudinal and transverseframesgives design momentsin twodirectionsat right-angles.These momentsmust be provided for in full, as otherwiseequilibrium will not be satisfied.The loadsin the columnscan be assessedfrom eitherthelongitudinalor thetransverseframesandthe assumptionsof simplesupportasdefinedin 3.8.2.3may be usedif desiredfor internal columnswhereappropriate.

Where drops are usedaccountshould be takenof them in determiningthe sectionpropertiesof the slabif theyprojectmorethan0.151 into the span.

3.7.2.4Frame analysismethodsThe framemethodgivessatisfactoryresultsfor mostorthogonalgrids.Howeverwhereasthismethodis suitablefor analysisat theultimate limit state it doesnot provideaccuratepredictionsof deflectionsatserviceloads.

3.7.2.5 Framestiffness

3.7.2.6Limitation of negativedesignmomentsThis clauseprovidesa checkto ensurethat staticequilibrium is obtained.The value ofh~ for the purposesof this clauseshouldnot exceed1.5 timesthe size of the shortersideof a rectangularcolumn.

3.7.2.7 Simplified methodfor determining momentsThe coefficientsgiven in Table 3.19 have been preparedtaking due accountof thenecessary20% downwardredistributionof momentsrequiredunderthe single loadcase.The value of the effective shearforce (3.7.6) when using thesecoefficientsmay bedeterminedfrom thesimplified factors1.15 for internalcolumns.1.25 for cornercolumnsandedgecolumnsbentabout an axis parallelto the freeedgeand 1.4 for edgecolumnsbentabout an axis perpendicularto the free edge.

Handbook to BS8IIO.-1985

3.7.2.8Division of panels(exceptin the region of edgeand corner columns)The definition of column andmiddlestripsfor rectangularpanelshasbeenalteredwithrespectto CPI 10. This is as a result of work carriedout by Regan’3~ - For aspectratiosgreaterthan 2 the centresectionof the longerspantends to spanoneway andshouldbe reinforced with nominal steel only in the direction of the short span.The lateraldistribution of momentsis discussedin more detail by Regan~3’9~and ‘vVhittle13’~~.

3.7.2,9 Column strips betweenunlike panels

3.7.2.10Division of momentsbetweencolumn and middle stripsIt should be notedthat Table3.21 doesnot give a suitabledivision of momentsat edgecolumns.Theseareasrequire specialattentionas given in 3.7.4.2.

3.7.3 Designof internal panels

3.7.3.1 Column and middle stripsA distinction shouldbe madebetweenthe designof flat slabswith flat soffits and thosewith drops(or waffle slabswith solid areasaroundcolumns).The presenceof the dropor solid areacausesstressconcentrationat the outer corners.This affects the way inwhich the top reinforcementshould be distributedto resistthe negativemoment in thecolumn strip. The concentrationof reinforcementprovided by this clauseshould-notapplyin suchsituationsandthe reinforcementshouldbe placedevenly acrossthe columnstrip. Careshouldbe takento ensurethat the top reinforcementextendsover thecornerof the drop or solid areato control crackingin this area.

3.7.3.2Curtailment of bars

3.7.4 Design of edgepanels

3.7.4.1 Positivedesignmomentsin spanandnegativedesign momentsoverinterior edges

3.7.4.2Designmomentstransferable betweenslab and edgeor corner columnsEquation24 givesasimplified formulaprovidingareasonablebasisto assessthemaximummomentof resistanceof an edge joint. The reasonsfor restricting the effective widtharegiven by Regant319), - F

The valueof be for a circular column may be takenas that for a squareenclosingthecircle.

If the equivalentframemethodis usedfor determiningthe edgetransfermomentthisclauseallows up to 50% redistribution.The reasonfor this is that this methodis knownto give higher momentsof transferthan actually take place. Long discussesthis in ‘—

detail~320~.Wherethe simplecoefficientsgiven in Table 3.19or the singleloadcasein conjunction [

with the equivalentframe method of analysisare used.the momentsandshearforces Imay be redistributed a further 30%. This is equivalent to approximately50%redistributionof the elastic valuesfrom thesemethods. r

In circumstanceswhere.in spite of redistributionof the elasticmomentsto the limitsgiven, they still exceedM~max considerationshouldbe given to altering the structuralconfiguration. Otherwiseflexural cracking in the slab close to the edgecolumnscouldbecomeso excessivethat it affectsthe shearcapacity. [3.7.4.3 Limitation of momenttransferAlthough the limitation on moment transferis severeit is unlikely that much can be I

gainedby torsion reinforcementin slabswith a depth less than300mm. ~~g~fl(3i9~and UWhittle~318~discussthis in more detail.

3.7.4.4 Negativemomentsat free edgeIt shouldbe notedthat for normalsituationsthe total transtermoment(slab/cOlumn)IS

Li

U. .. . —

—.-...-. .

Paul SeLtun3

resistedby edSereinforcementin a narrowband(3.7.4.2).The remaininged2eof theslabshouldbe reinforcedwith nominal steel asdescribedin this clause.

3.7.4.5Panelswith marginal beamsor walls

3.7.5 Openings in panels

3.7.6 Effective shear forces in flat slabsThe equationsgiven in this sectionwhichmagnify the elasticsheardiffer from CP1IO.This resultsfrom work carriedout by Regant3’9~and Long 30), In fact,equation25 isa simplified version of thosegiven by Reganand can be shown to give resultswithin2% of Regan’sproposals.The enhancementfactor given in CP11O (1+12.SM/VL) isreasonablefor long spanswith smallcolumns.Howeverfor the morenormal situations(up to 8m span) the currentformulae which are basedon the column width are moreappropriate.

The use of the simplified factors (e.g. 1.15V1’ etc) isreasonable,where the spansdo~

- not differ by more than 25%. ~16wever.if,the live to deadload ratio exceeds1.00thenthe mor,e. Dg9rous ~l~u9q, be~tuad~

3.7.6.1 GeneralThe calculationsfor effective sheargiven in this sectionapply to elastic methodsofanalysis~Whenusingthe singleloadcaseof maximumdesignload on all spansor panelssimultaneouslythe valueof V~ffshouldnot be takenaslessthan1.1V1 for theslab/internalcolumnconnectionsand1.35V1 for theslab/edgecolumnsbentaboutanaxisperpendicularto the free edge.

3.7.6.2 Shearstressat stab/internal column connectionsin flat slabsTheremaybe occasionswhen the calculatedvalue~ the~simplifiedvaluegiven for approximatelyequalspans Considerationsh ~b&~2i~nto situationswherethe calculatedvalueis likely to be largerandshould be used.Thesewill include:

(a) unequalloading of slabpanels(b) slabswhere the differencein spansis morethan 10%(c)1slabs4where~ full ‘~elasticinoment.

This will causethe first interior column to havea higher moment.

3.7.6.3 Shearstressat otherslab-columnconnections

3.7.6.4 Maximum design shearstressat the columnface

3.7.7 Shearunder concentratedloads

3.7.7.1Modeof punchingfailureA punchingfailure of aslabwithout shearreinforcementoccurswhenthetensionin theconcretereachesits limit. Thenaturalfailure occurson inclined facesof truncatedconesor pyramidsat an angleof about350 to the horizontal.No theoryof punchingas yet isgenerallyacceptedandthe Coderecommendationsareempirical. expressedin termsofnominal shear stressesalong a control perimeter.The control perimeter is takenasrectangularin shape,~,5dfrom the columnor4oaded~area.The choiceof perimetersisbasedon the use of the shear resistancevalues for single-way slabs togetherwithconvenientandsimplegeometricvalues.

3.7.7.2 Maximum design shearcapacityThe Code recommendationsare empiricaland are expressedas a nominal shearstressat the face of the column or loaded area. The value of u0 is basedon the size of arectangletouchingthe loadedarea. For a circular column this is a squareof sideequalto the diameterof the column. The failure mode associatedwith the maximum shearcapacityis that of a diagonalcompressionstrut failure.Thismay beconsideredtoextendfrom the column face (or loadedarea)to a perimeter 1.5d from the column. 55

Handbookto BS8IIO:1985 Part I: Section3

3.7.7.3 Calculation of design shearstressfor a failure zoneEquation28 is anempiricalformulawhich differs from thatof CP110 whichwasexpresse~jin termsof the full depth of slab rather than the effective depth.The reasonfor thechangeis to bring it in line with the calculationof shearstressfor beamsandsingle-wayslabswhich are bothexpressedin termsof effectivedepth.

3.7.7.4 Shearcapacityof a failure zonewithout shearreinforcementThis clauseallows an increasein v~ for perimeterscloser than l.5d from the face ofcolumn or loadedarea.Howeverthis valueshouldnot exceedthe maximum permittedvaluegiven in 3.7.7.2.

Often a dilemmaexistsas to whetherthe bottom or top steelshouldbe takenas thetensile reinforcementand from where its anchorageshould be measured.If there isdoubtthen the bottomsteel shouldbe chosen.The requiredanchorageof the effectivesteelareais definedin Clause1.2.3.5.Thisexplainsthat the reinforcementshouldextendbeyondthe failure zoneconsideredby an effective depth or 12 times the diameterofthe bar.whicheveris greater~Eachfailure zone is LSd wide, correspondingto a notionalfailure line.

3.7.7.5 Provision of shearreinforcementin afailure zoneThis clauseincludesa numberof changesfrom CPl 10. The useof rectangularperimetersandzonesis consideredmore convenientfor placing shearreinforcement.Eachshearperimetercheckedis associatedwith a zone throughwhich the failure planeis assumedto pass.The objectiveof providing shearreinforcementfor a particularperimetercheckis to ensurethat the shearreinforcementis spreadevenlyover the associatedzoneandcrossesthe likely failure plane. In order to ensurethat punchingbetweenthe column(or loadedarea)andthe innermostshearperimeteris avoidedthis shearreinforcementshould be placednot further than0.5d from the faceof the column.

3.7.7.6 Designprocedure

3.7.7.7 Modification of effectiveperimeterto allowfor holesWhere holesoccuradjacentto one side of a column themay be more reasonablytaken as a parallel projectionperimeteras shown in Fizure H3.25.

-----Ishear perimeter .—

44.4

‘1 -

reductionof shearperimeterof the hole on to the shear

—————-I

ZI——

Figure H325: Reductionof shearperimeternear holes.

3.7.7.8Effectiveperimetercloseto a free edge

3.7.8 Deflection of panelsWhen usingTable 311 the valueof NI should be takencalculatedat mid-spanfor a panel.

Las the total bendingmoment

3.7.9 Crack control in panelsSee3.12.11.The only specialpoint to note is that, if drops are used,the total dep(h ~t

F

[f-i

t.

C,

L

LI

56

L12L

-~ - ~ i~ZL.ili... -. - -..--. -. -E. . . . . . ..

. . . - -0Part1-Section 3

the slabplusdropshouldbe consideredwhendecidinghowtherulesin 3.12.11.2.7applyNto the regionsof the slabwithin the drops.

13.7.10 Designof columnsin flat slab construction

I3.8

Columns

3.8.1 GeneralIn principle, columnswould appearto be considerablymore complexto design thanbeamsdueto thesubstantialnumberof extravariableswhich mayrequireconsideration.Theseare:

(a) Sectionsaresubjectedto combinedmomentandaxial loadratherthanjustmoment.(b) A considerablenumberof combinationsof momentandaxial loadmay be possible.

Thesewill frequentlyincludecasesgiving biaxial bending.(c) Bendingof thecolumn will leadto lateraldeflectionswhich, if significant,can affect

the capacityof the column.

Fortunately,therearealsoanumberof simplifying factorswhichreducethecomplexityin mostnormalsituationssuch as:

(a) k~iost columnsare of:ii’ec~angular~cz’oss-sectionand are designedas.jyn~metrically-reiziforced.-.There..~re~4inUeed.-very strongpractical and~behaviouralreasons- for~emp1oyingsymmericiaUyxeIn~rceci4sectionswherever~sible. -

(b) ~n~n y~4~zo~ip UL’Wndi1c~d’be~considrnd.Exceptfori1igbdy~oadcd~Aup,s e~~ii~1tfniioad~usl~aximi~immbmentwill

:deflectionsare;not significant..(d) aai4~ yM~~ws~.iisideatiozi,v

rift) ~Il jeii~iI~ot requireconsideration.(f) Since,in general,all sectionsof a column will containthe samereinforcement,the

distributionof momentsoverthe heightof thecolumn is rarely of importance,onlythemaximumvalues.

As a resultof the abovefactors.designof mostcolumnsis a very simplematter.Thefact that theCodehasto providerules for dealingwith the lesscommon,morecomplexsituationsshouldnot be allowedto obscurethis.

3.8.1.1 Symbols

3.8.1.2 Size of columnsSection3.3. andin particularTables3.4. 3.5 andFigure3.2 give informationwhich mayinfluencethe sectiondimension.

3.8.1.3 ShortandslendercolumnsSlendercolumns are those where the deflectionof the column under ultimate loadconditionsis sufficient significantly to influencethe ultimate strength.Shortor stockycolumnsarethosewherethe deflectionmaybeignored.Thelimit of 10 for theslendernessratio for unbracedstructuresis arbitraryas deflectionhassomeinfluenceon strengthatall slendernesses.Thesituationisdifferent forbracedstructuressince(aswill be discussedbelow), in bracedstructuresthe maximum momentsinducedby deflectionsoccur atroughly mid-heightof the column while the momentsarisingfrom normal frameactionaregreatestat top andbottom of the column. Studieshaveshown that for slendernessratios lessthan 15. the momentsarisingfrom the frameaction at the endsof the columnwill always give the critical designcondition. Above 15. it becomesincreasinglylikelythat the critical designconditionwill occur in the centralregion of the column.

3.8.1.4 PlainconcretecolumnsThis clausewas introducedin recognitionof the fact that, if it is acceptable(andindeednormal) to design masonrycolumns without reinforcement,then it must be equallyacceptableto designunreinforcedconcretecolumns.

57

I

Parr 1: Section 3

in FigureH3.28.It will beseenthat, for abracedcolumn,the effectivelengthwill alwaysbe lessthanor equalto the actual length,beingshorterwherethe membersconnectedtop and bottomarestiffer. By contrast,the effectivelengthof anunbracedcolumnwillalways exceedthe actuallength. The effectivelength of a bracedcolumn with pinnedendsor an unbracedcolumn with rigid connectionsat both endswould be equalto theactuallength.That for acantileverwith rigid connectionatoneendwould be twice theactualheight. Effective lengthsof cantilevercolumnswheresomebaserotationmightoccurwill exceedtwice the actual height.

N

‘e-

ffectivel

FEHandbook to BS8IIO 1985

Table H3.5 Assumed beam/column stiffnesses

Endcondition

Ratio 4X~ beam/columnstiffness

0.52 1.53 3.04 70

Usingthesefiguresand theequationsin Part 2 Section2.5gives the valuesfor effectivelength in Tables3.21 and 3.22.

An extensivediscussionof the effective length of bridge piers is given in reference3.21. Thismay be of valuein specialcases.

3.8.1.7 Slendernesslimits for columnsThis limit is simply consideredto be the limit beyondwhich currentknowledgeshouldnot be extrapolated.

3.8.1.8Slendernessof unbracedcolumnsThe additional limit applied herefor cantilevercolumnswill be seento be effectivelythe sameas the slendernesslimit appliedto cantileverbeamsin 3.4.1.6(b).

3.8.2 Moments and forces in columns

3.8.2.1 Columns in monolithicframes designedto resist lateralforces. (Unbracedframes)This is simply to indicatethat the simplified methodsof assessingmomentsand forceswhich are permissiblefor bracedframesshould not be used for unbracedframes.

3.8.2.2 Additional momentsinducedby deflectionat ULSThesewill be discussedin the sectionon 3.8.3.

3.8.2.3Columns in column-and-beam construction, or in monolithic bracedstructuralframesThis clausegives the simplified basison which axial forcesmay be assessed.Momentsshould be obtainedin accordancewith 3.2.1.2 (though the clauseomits statingthis)unless it is reasonableto assumeeffectivelyaxial loading.

Y,minimum moment

M,~minimum moment

Figure H3.29: Problem situation for treatmentof minimummoments.

FF

FF(7F[r

FULI[Lif,

U

-... -

..-. ~~L.2-’ . . - . . - -APart1: Section3

3.8.2.4Minimum eccentricityI It is impossibletoguaranteethatacolumnwill be absolutelyaxiallyloaded:somemomentwill inevitably occur evenif the calculationssuggestotherwise.Hence it is prudenttoallow for at leasta minimum moment.The valuechosenis arbitraryand,in fact, is lessstringentthan the rules imposedin manyothercountries.

The provision for biaxially bentsectionsactuallyintroducesan elementof ambiguityin somecases.The Codestatesthat the minimum eccentricityonly needbe appliedinonedirection at a time. The immediateassumptiononeis temptedto makeis that, forIthe casesketchedin Figure H3.29. one shoulddesign for M~ aboutthe major axis andtheminimumeccentricityaboutthe minor axis. However,commonsensewould suggestthat this column would actuallybe consideredto be uniaxially bent and the moment‘1 aboutthe minor axis ignored.If this was done.sinceM~ exceedsthe minimum moment,therewould be no necessityto considerthe minimum momentat all. It thus appearsthat the only logical way to interpretthis clauseis to checkthat the momentin at leastone direction exceedsthe minimum, otherwisethe Code would be implying that allcolumnsmustbe designedas biaxially bent.

3.8.3 Deflection inducedmomentsin solid slendercolumnsTheseclauseshavebeenderivedfrom work by Cranston~3~~.The logic behindthem isas follows:

Consider.for simplicity, a pinnedendedstrut. Due to inevitable imperfections,thiswill deflectlaterally underload. When the strut deflects,eachsection is subjectedto amoment given by the deflectionat that sectionmultiplied by the vertical load. Thismoment causesa further increasein deflectionand hence in moment. Under someconditions, this will lead to instability and a buckling failure: under lower loads, anequilibrium condition can be reached.For elastic materials.analysis‘of this situationleadsto the classicalEulerequationfor the loadcapacityof slenderstruts.Reinforcedconcretecolumns,especiallycloseto collapse.are not elasticandit hasbeennecessaryto developa moregeneralapproachto the problem.Furthermore,it is necessaryto beable-to deal with much moregeneralproblemsthan simply pinnedendedstruts. In itssimplestterms, the methodgiven in the Code can bederivedas follows.

The curvatureat the critical section in a strut at the momentof failure is obtainablefrom the basicassumptionsfor sectionbehaviour.It will be given by

1/r~=(e1—e~)/d (0.0035+e~)/d

Assuminga balancedsection.e~wilIbe given by 0.87f,}E1.For grade460 steelthiswillbe 0.002andhence:

1/r~0.0055/dIf the deflectedshapeof thecolumnis assumedtofollow a sinecurve,thenthedeflectionwill be given by:

Assumingthe overall sectiondepthis rouzhlv 1.1 timesthe effectivedepthandallowing

for theconservatismimplicit in thederivation,thismayberewrittenapproximatelyas:

a~=(1I2000)(4/h)2h

This will lead to a momentat the critical sectionof:

This analysisassumesa balancedsection.For sectionswith greateraxial loads thanthe balancedcondition,the strain in the reinforcementnearthe leastcompressedfacewill beless thanthe yield strain. It will lie in the range—O.O035~e

5~=0.002.seeFigureH3.30.This leadsto the ultimate curvaturebeingbelow that for abalancedsection.Thisis allowedfor by introducinga coefficientK suchthat the ultimatecurvatureis equal tothe ultimate curvaturefor a balancedsectionmultiplied by K. The expressionusedforK in the Codeis empirical.

If the sectionof the strut is designedso that it can withstandthis moment.then thestrut will be stable under this condition and buckling failure will not occurunderthisload. In most columnsoneis not dealing with a pinnedendedstrut and the additionalmoment due to the ultimate deflection is added to the momentsarising from normalframeaction.

61

Handbookto BS8IIO:198S

C

x

It is not necessarilypossibleto predictexactlyhowa column will deflectand.ideally.- oneshouldconsiderthepossibilityof buckling abouteitheraxis.Thiswilfclearlv involveconsideringbiaxialbending.Toavoidthisfor mostcommoncases,theadditionalmomentdueto deflectionis calculatedon the basisof thesmallerdimensionof the columns. For

DESIGN GENERALLY NOT PERMITTED60 (4j( ~)

20

MAJOR

AXIS

I—15

short ifshort if braced about braced

minor axis about bothaxes

10 / / / / / / / / /

010 15 20

m

cn

zz0-4

m2m

F-F-

m

-4m0

60(4v)

MINOR AXIS

.0035

——a

.0035

[IFF

[C

Figure H3.30: Variation ofultimate curvatureat differentaxial loads.

[

cCIr

II

I—if average — >20. check sup0orting structureh

for effecta of addItional moment in x dIrectIon

IF BENTABOUT MAJOR AXIS ONLY, DESIGN ASBIAXIALLY BENT WITH ZERO INITiAL MOMENT ABOUT MINOR AXIS

SLENDER

slenderness effects on column —

“a0 OO’j~

/0. a V0. ,..j

C,/0 0 ~-a/ !~.

/ 0./ ~ /0 o c/ ~a Ki

/SHORT / • /

/ ~. /slenderness effects / n

may always be ignored/ ~/ .~ /// - / L

BFigure H3.31: Interpretationof Clauses3.8.1.7, 3.8.1.3, 3.8.2.2, 3.8.3.4and3.3.3.9

PartJ:Secnon.,

uniaxially bent columnswhere 41h<20, this momentis applied in whicheverdirectionthe primarymomentsact, regardlessof whetherthis is aboutthe minor or major axis.This is aconservativeapproachandeconomiescan occasionallybe achievedby carningout a biaxial design.

3.8.3.1DesignSeediscussionabove

3.8.3.2 Design momentsin braced columnsbent abouta singleaxisFigure3.20 in the Codeillustratesthe provisionsof this clauseadequately.

3.8.3.3 Slendercolumns bent abouta singletzxis (major or minor)This simplified approachis dealt with at the end of the sectionon 3.8.3.

3.8.3.4 Columns where 41/i exceeds20, bentabouttheir major axisIn thiscase,thesimplification usedabovebecomesexcessivelyconservativeandabiaxialdesignwill be moreappropriate.

3.8.3.5Columns bent abouttheir major axisThe samecommentappliesas for 3.8.3.4

3.8.3.6Slendercolumnsbent aboutboth axesCareneedsto be taken herewhen the effective lengthsaboutthe two axesare verydifferent or where the column is bracedin one direction and unbracedin the other.Careful inspectionof Figures3.20 and3.21 should indicatewhich momentsshould becombinedwith which in particularcases.

3.8.3.7 Unbracedstructures

3.8.3.8Deflection of unbracedcolumnsEquation37 is a changefrom that given in CP11O:1972.The approachin BS 8110 is themore correct.

3.8.3.9 Additional momentson membersattachedto a slendercolumn

3.8.4 Designof column sectionsfor ULS

tel chart theoretically invalid

Figure H3.32: Validity of designchartsfor columnsconcentratedin corners.

tbl chart will be satisfactory

with reinforcementnot

w w

0

000

000

1~>

63

Handbookto B58110:1985 [K3.8.4.1 Analysisof sections F3.8.4.2 Designchartsfor symmetrically-reinforcedcolumnsThe designchartsare drawnon the assumptionthat the reinforcementis concentrated Fcloseto the facesof thesection.Thechartsare theoreticallyinvalid if otherarrangementsof reinforcementareused,for exampleas shownin Figure H3.32.

Variousmethodshavebeensuggestedfor enablingthe chartsto be used for this typeof sectionbut a reasonableapproachis to establishan effectivevaluefor d by calculating j

.

thepositionof thecentroidof thereinforcementin onehalfof thesection(FigureH3.33).This value can thenbe usedwith the designchartsin Part3.

This approachis conservativebut usuallynot excessivelyso.

5

r

- -~-~

centrL7 — - Ubars in

1/j section

Figure Ff3.33: Effectivedepthofa columnsection. 53.8.4.3Nominal eccentricityof short columnsresisting momentsandaxialforces IEquation38 will be particularlyappropriatewherethe column supportsa rigid structureor very deepbeams.A cross-sectionunderpure axial loadwill, from the assumptions

given in 3.4.4.1carry an axial loadof: CN=0.45fcoAc+0.87fyA~

The reduction of approximately10% built into equation38 allows for the minimumeccentricityof 0.05 h. [3.8.4.4Short braced columns supporting an approximately symmetricalarrangement ofbeams LEquation39 containsa further reductionfrom equation38 to cater for the momentswhich will arisefrom asymmetricalloadingon symmetricalbeams.Equation39 shouldnot be usedfor edgecolumns,unlessthe primarybeamsof the structurespan parallelto the edge. L3.8.4.5Biaxial bendingThe methodproposedin BS 8110 differs from that in CP11O.The reasonfor the change [is that, while the CP11O equationis probably technically superior.it cannot be useddirectly for designbut only for checkinga sectionwhich has alreadybeendetailed.TIlLmethodin BS 8110 is adequateprovided M~~/bh is not too different to M~/l?b2. Evenin thesecircumstancesa reasonableanswerwill be obtainedif care is taken in thedefinitionof h’ andb’. It is suggestedthat, wherethe reinforcementis not concentratedin the corners,effective values of h’ and b’ are computedas suggestedfor d in thecommentson 3.8.4.2(Figure H3.33). U3.8.4.6Shearin columns

64

--. .. .

— — — .~. .---~

~ ~ .‘.• - -:‘~~‘.~-:‘~‘ ~.

..-. I...-.-.-.....-.----

-. .. -aPart1: Section2

3.8.5 Deflection or columnsa This clauseis concernedwith the deflection under service loads, not the deflectionsdiscussedunder3.8.3which is concernedwith deflectionsat ultimate loads.

] 3.8.6 Crack control in columns

I3.9Walls

3.9.1 Symbols

3.9.2 Structural stability

3.9.2.1 Overall stability

3.9.2.2Overall stability of multi-storey buildingsThis clause is badly expressed.The intention is that lateral stability in any directionshould not be provided solely by walls bending about their minor axes. Walls must bearranged to provide a stiff structure to resist the lateral loads.

3.9.2.3Forces in lateral supportsThis clauseallows for possible~outof plumb’ ofthe structure or other unforeseeneffects.

3.9.2.4 Resistanceto rotation of lateral supports

3.9.3 DesIgn of reinforced wallsReinforced walls are effectively consideredas a special caseof reinforced columns. Areinforced wall has to contain at least0.4% of vertical reinforcement. otherwise it mustbe treated as unreinforced. Much of the commentary on the column section (3.8) appliesalso to this section.

3.9.3.1Axial forces

3.9.3.2Effective height

3.9.3.2.1 General. For walls constructedmonolithically with the surrounding structure.

see3.8.1.6and the commentary on that clause.3.9.3.2.2Simply-supported construction. See3.9.4.2.

3.9.3.3Design transverse moments

3.9.3.4 In-plane moments

3.9.3.5 Arrangement of reinforcementfor reinforced walls in tension

3.9.3.6Stocky reinforced wallsStocky is identical to short.

3.9.3.6.1 Stocky braced reinforced wails supporting approximately symmetricalarrangementsof slabs. As in the equivalent equation for columns. a notional allowanceis included in the equation for small moments.

3.9.3.6.2WalLs resisting transverse momentsand uniformly distributed axial forces

3.9.3.6.3 Walls resisting in-plane momentsandaxial forces65

[IHanabookto BSSI]O:]985

1.9.3.6.4 Wails with axial forces and signLficant transverseand in-plane moments.This Fclause gives a simplified approachto the complex problem of biaxial bendingunderultimateconditions.Theoretically,the problemcanbe tackledfrom first principlesusingthe assumptionsof 3.3.5.1but considerablecomplexity will be involved. p

Where tensionarisesalong the length of the wall, a simplestrip foundation will notbe sufficient unlessit is designedto resistthe resultantbendingmomentsin the planeof the wall. Becauseof this, a solution usingplain walls may be attractive. [3.9.3.7 Slenderreinforced walls

3.9.3.7.1Designprocedure [3.9.3.7.2 Limits ofslenderness r3.9.3.7.3 Transversemoments

3.9.4 Designof plain wallsNote the definition of plain walls given in 1.2.4.7. Effectively, a plain wall is onewithlessthan0.4% vertical reinforcement:it may well containsomereinforcementfor anti-crack or handlingpurposes.It may alsobe noted that wherewalls do not containany [reinforcement,the durabilityprovisionsof 6.2.4.2ratherthanthoseof 3.3.5wouldapply.Plain walls are intermediatebetweenreinforcedwalls and masonrywalls. Many of theprovisionsin BS 5628 might be consideredto give useful guidancein particularcases.The handbookto that codecould also be of value~3~.

3.9.4.1Axialforces CThis follows the rulesgiven for columnsin 3.8.2.3.

3.9.4.2Effective height of unbracedplain concretewalls

3.9.4.3Effective height of bracedplain wallsThe provisionsof this clause are consistentwith those in the Masonry Code (BS5628:1978). C3.9.4.4 Limits of slenderness [The limit of 30 given hereis slightly greaterthanthatpermittedin BS 5628 for masonrybut generallyless than that permittedfor reinforcedwalls.

3.9.4.5Minimum transverseeccentricityofforces [This clausefollows the provisionsfor columnsand reinforcedwalls.

3.9.4.6 In-planeeccentricitydue toforceson a single wall L3.9.4.7 In-plane eccentricitydue to horizontalforceson two or moreparallel walls

~.9.4.8Panels with shearconnections L3.9.4.9Eccentricityofloadsfrom concretefloor or roof [The provision regarding common bearing areas restricts the maximum calculatedeccentricitydueto imposedloadon the slabs to h16 in internal walls.

Eccentricityof load in a bracedwall by floor loading will ariseasindicatedin FigureH3.34(a).from which it will be seenthat eccentricitiesat the top andbottomof thewall L~vill be opposite in sign. Local cracking in the floor slabs will tend to reduce theeccentricitiesat both ends (Figure H3.34(b)). Preciseevaluationof the eccentricity IS

impossibleand so the simplification as sketchedin Figure H3.34(c) is introduced.The Ucross-sectionat the top of the wall must be designedto resist the total load N

3 actingat

the appropriateeccentricity.

i;i

:1— Wi~. ~

Part1: Section3

Unbracedplain walls shouldbe foundonly in single-storeyconstruction.Theprocedureinherentin FigureH3.35 is suggested:effectively,cantileveractionis beingassumedtogive the worst possibleeffect as regardsthe eccentricitycalculation.

herew II cause II

lleccentrctv/lllto move II

Ibi

Figure H3.34: Eccentricityin bracedwalls.

—a .—. bearing width

wI (lateralload toberesistedby wallI a.,

we,C

2—a.l+—-,— N

upper load, N,, assumedto act centrally

floor load, N2. acting~ through third point of

bearing area

N3. acting on wall is resultantof N, and N2

load N3 assumed to actcentrally here

(ci

Figure Ff3.35: Eccenzriciivin a single-storeyunbracedwall.

3.9.4.10Othereccentrically-appliedloads

3.9.4.11 In-plane and transverseeccentricityof resultantforce on an unbracedwall

3.9.4.12 Transverseeccentricityof resultantforce on a bracedwallSee3.9.4.9above. -

3.9.4.13 ConcentratedloadsA muchmoredetailedapproachto concentratedloadsis given in BS 5628 Clause34.In awkward situationsit could be worth usingtheseclauseswith Ym takenas 2.25.

3.9.4.14Calculation of designloadper unit lengthThe intention of this clauseis illustrated in FigureH3.36.

3.9.4.15 Maximum unit axial loadsfor stockybracedplain wallsThe principleson which equations43 to 46 arebasedareillustrated in FigureH3.37.

It tvill be seen that equation43 follows directly from this Figure by consideringequilibrium of vertical forces.

It is also worth noting that the stresson the concreteof 0.3f~ correspondsto a Ym

factorfor plain concreteof 1.5xO.45/0.3=2.25.This may be comparedwith the valuesgiven in BS 5628 of bet~veen2.5 and3.5. dependingon control. It is logical to use ahigher value of ‘y~ for plain concretethanfor reinforcedconcretesince, in the caseofreinforced concrete.both steel and concretehave to reach their designstrength toprecipitate failure whereas,with plain concrete,only the onematerial needsto reachits designstrength.This hasa higher probability of occurrence.

3.9.4.16Maximum design ultimate axial loadfor slenderbracedplain wallsThe additional eccentricity is assumedto occurnearmid-height of the wall. Using thesameassumptionas for bracedcolumns,the initial eccentricityat this level maybe takenas0.6e,. Substitutingthe totaleccentricity(0.6e,-l-ea)fore~ in equation43givesequation

67

151

‘/3 bearing width

Handbookto B58110:198S

line of action of load

U

~

CI

linear distribution maximumof load load per

length

unit Cno tension [

Figure Ff3.36: Load distribution along a wall.

C4

_ II-r

1o3 t~

x—h—2e4

Figure Ff3.37: Stressblock underultimate conditionsin plain wall. C44. Obviously, the condition at the top of the wall also needschecking;hencetherequirementto checkequation43 as well. L

3.9.4.17 Maximum unit axial loadfor unbracedplain walls UEquation46 should read:n~~0.3(h—2(e~.z+e3))f~

3.9.4.18Shearstrength IThis clauseappliesonly to walls cast in situ. Where large panelswith joints are used.the provisionsof 5.3.7 mustbe applied. F

Shearsin plain walls arisefrom the changein bendingmomentdown the wall. Where Lthe ratio of effective height to thicknessis 6 or more, the maximum shearthat canpossiblydevelopat right-anglesto the planeof the wall will correspondto a changeIII

eccentricityfrom h/2 at the top of the wall to —h/2at the bottom.Thus,automatically. rthe shearis lessthanor equalto one-sixthof the axial loadandfor this reasonneednoC 11be checked.

Where resistanceto lateral load is being assessedunder the stability requirements. fladequateperformance in shear may be assumedas long as the shearforce is againIlOC umore than one-quarter of the axial load.

58 c--. ,— .-——.—

—. . S .~. — . —.- . —: — - . - — :. - .~. . —

- -4.. -. . . . - ~ . -. ~

—

~ ...—-~.

Par: I: Sec:ion

3.9.4.19 Cracking of concrete

3.9.4.20 ‘Anticrack’ reinforcementin externalplain wallsReferto Table 3.4 for adequatecover~.

3.9.4.21 ‘Anticrack’ reinforcementin internalplain walls

3.9.4.22 Reinforcementaround openingsin plain walls

3.9.4.23 Reinforcementofplain wallsfor flexureThis reinforcementis also ‘anticrack’ steel andnot reinforcementfor strength.

3.9.4.24Deflection of plain concretewalls

3.9.4.24.1 General

3.9.4.24.2Shearwalls

Staircases

3.10.1 General

3.10.1.1 Loading

3.10.1.2Distribution of loading

3.10.1.3Effective spanof monolithic staircaseswithout stringer beams

3.10.1.4Effective spanof simply-supportedstaircaseswithout stringer beams

3.10.1.5Depthof section

3.10.2 Design of staircases

3.10.2.1 Strength, deflectionandcrack control

3.10.2.2Permissiblespan/effectivedepthratio for staircaseswithout stringer beamsThis clausemakesallowancefor somestiffening from the treads.

3.11 Bases

3.11.1 Symbols

3.11.2 Assumptions in the designof pad footings and pile caps

3.11.2.1 General3.11 gives simple but safe rules for the design of individual wall and column bases.Itdoesnot referto multiple columnbases,raftsandotherlargescalefoundationstructures,which should be designedby taking due accountof the ground conditions. For suchstructuresreferenceto Part2. Clause2.2 may assist.

The assumptionsof uniform and linearly varying soil pressureson basesare safebecausethe pressureswill in fact tend to reduce towards the extremities of the baseforcohesionlesssoils,andto be uniformly distributed in the caseof cohesivesoils.

3.11.2.2 Critical sectionin designof an isolatedpadfooting69

Handbook to BS8IlO:1985

3.11.2.3Pocketsfor precastmembers F3.11.3 Designof pad footings

3.11.3.1 Designmomenton a vertical sectiontaken completelyacrossa padfootingThe critical section is defined in 3.11.2.2.The bendingmomentson suchsectionsshouldbedeterminedby summationof themomentsarisingfrom all externalloadsandreactionson one side of eachcritical section. In the longer direction of span,the moment sodeducedmay be assumedto be constantover the critical section.but acrossthe shorterspan.the momentstendto begreateroppositethe column.Thecodeincludesanarbitran- rrule for detailing the reinforcementaccordingly(3.11.3.2). 1

Top reinforcementin padfootings is not normally required.Circumstancesin whichit shouldbe consideredinclude: C(a) when uplift is likely to occur(b) where two or morecolumnsuse a single padfooting(c) wherecontrol of suffacecrackingis essential. r3.11.3.2 Distribution of reinforcement

3.11.3.3Design shear [It shouldbe notedthat althoughthe critical sectionis assumedto be at the face of the -

column (3.11.2.2)an enhancementin designshearstrengthclose to supportsmayalsobeappliedin accordancewith 3.4.5.8and3.4.5.10.Fromtheseconsiderationsthecritical rsection for shearis more likely to be a distanced from the face of the column-.- L

3.11.3.4Designshearstrength nearconcentratedloadsThe following conditionsareconsidered:

(a) shearalonga verticalsectionextendingacrossthe full width of the base(b) punchingsheararoundthe loaded area. Providedthe column is placednearthe F

centreof thefooting V~ffmaybe takenas 1.15V2. Otherwiseit shouldbedetermined ~from 3.7.6.3.

3.11.4 Designof pile caps

3.11.4.1 General FIt should be noted that one major difference between the results of using bendingtheory [andtrussanalogyis the requirementfor anchorageof the main tensionreinforcement.Bending theory is likely to require only nominal anchorageof bars beyond the pilewhereasa full anchoragelength is requiredif trussanalogyis required. In order to Fovercomethis anomalythe following rulesmay be applied. L

(a) if the distancefrom the column face to the critical section(Figure 32.3 of BS 811(1.Part1) is lessthanor equalto 0.6dthencorbeldesignmethodsshouldbe considered [.— see 5.2.7. One requirement is that the tension reinforcementrequiresa full t~

anchoragebeyondthe loadedarea, in this casethe pile(b) if the distance from the column face to the critical section is 2d or more then .1 [

nominal anchoragebeyondthe pile of d or 12~ is all that is required(c) if the distancelies between(a) and(b) then linear interpolationmay be used

In assessingwhere the anchoragelengthshould be measuredfrom it maybe assumed [to be d/5 inside the outer limit of the pile. L

3.11.4.2 Truss method L3.11.4.3ShearforcesIt should be notedthat the critical section for shearmay be less than 2d from the faceof the column.The enhancementin shearresistancegivenin 3.4.3.8thenapplies.Wheremorethanone row of pilesexists thentwo critical sectionsshouldbechecked.One with

70 the shearenhancementandonewithout is shownin FigureH3.38.

. .

-- I., . — .

— — — . 4. 4.-.- ...,‘.....~.. --. ...- --

Part J— Section3

ii

v>0J\77 (N.... T>3 dia.

~‘dis.

-~ ~- ~1 ~-

0.2 dia. 0.2 dis.

Figure H3.38: Shearin pile caps.

3.11.4.4DesignshearresistanceThe requirementfor an anchorageof themain reinforcementequalto theeffectivedepthshouldbe measuredfrom the critical sectionas shown in Figure 3.23 of BS 8110, Part1. This criterion maycontrol the lengthof barsin situationswheresmalldiameterpilesare usedwith shallow pile caps.Otherwisethe criterion describedin the explanatorynotesfor 3.11.4.1will control.

3.11.4.5 Punching shearWhen it is necessaryto checkpunchingshearon a perimeteras shown in Figure3.23 itmax’ be found that the distancefrom the column to the perimetervariesfrom onefaceto another.In such situationsany enhancementfactor allowedby 1.7.7.4should onlyapply to the proportionof perimeterapplicable.

Where the spacingof piles is large. punchingshearon a perimeter1.5d from thecolumn may requirechecking.

3.12 Considerations affecting design detailsThis sectionof the Codeprovidesinformationon awide rangeof detailingaspectsgivingsensiblevaluesfor the practicalneedsof detailing.Howeverit doesnot set out methodsof detailing.The complexity in decidingwhat detailingmethodshouldbe usedinvolvescostcomparisonsbetweenmaterialsand the labourto bendand fix the reinforcement.Often simple methodsof bendingand fixing reinforcementinvolve morematerial. It isconsideredthat this is beyondthe scopeof the Codeandreferenceshouldbe madetootherpublications~34and 3.241

3.12.1 Permissibledeviations

3.12.1.1 General

3.12.1.2 Permissibledeviationson membersizesApplication of this clausewill be a matterfor judgementbut nominal dimensionswilleeneraHybe appropriatefor designingmembersat least 125mm thick.

3.12.1.3Positionof reinforcement

3.12.1.4Permissibledeviationon reinforcementfitting betweentwo concretefacesSpecifiedlimits for thepositionof reinforcementaresetout in 7.3.Thesearesummarizedbelow.

a, I

71

[IHandbookto BS8JIO:198S

Actualconcretecover < nominalcover — ~mm FActualconcretecoverwhere > nominalcover +5mmfor barsup toandreinforcementis locatedin including12mmdiameterrelationtoonlyonefaceof a + 10mmforbarsover12mm Fmember up toandincluding25mm

diameter+ 15mmfor barsover25mmdiameter [

Limits to the allowabledeviationof the location of the reinforcementare required Fbecause:

(a) too largea negativetolerancewill give durability problems C(b) too large a positive tolerancewill give strengthproblems.

Theselimits needto accommodatethe bendingtoleranceon reinforcement,which arc rspecifiedin BS 4466 andquotedin thecommentaryon 7.3: for bentbarsup to 1000mmlong, they are ±5mm.In addition when detailingreinforcing bars to fit betweentwoconcretefaces,the dimensionto be shown on the scheduleshould be less than that [derivedfrom the nominal dimensionby an amountdependenton the overall size of theconcretemember;for a totalconcretedimensionof up to 1000mm,the amountmay beassumedto be 10mm. Alternatively, the detailingmethodshould allow the barsto belappedandto slide to fit the dimensionas built.

Wherepossible,it shouldbeensuredthatreinforcementon atensionface is positionedaccuratelywith respect to that face; no reduction in effective depth need then beconsidered,nor will it be necessary,following the sameapproachas in 3.12.1.2. to Cconsiderany reductionif the memberthicknessis at least 125mm.3.12.1.5Accumulation of errors 53.12.2 Joints

3.12.2.1 Constructionjoints ISee6.12.

3.12..2.2Movementjoints - [See6.13 and Part2, Section8.

3.12.3 Designof ties [3.12.3.1 GeneralProvidedthe recommendationsof 3.1.4 havebeenfollowed, the requirementsof 3.12.3will normally ensurethat a structureis robust for in situ reinforcedconcretestructures(Figure 3.1). Where precastconcreteconstructionis involved care must be taken toensurecontinuityof ties as describedin 5.1.8. 1

Formany structuresit will be found that reinforcementprovided for the usualdead.imposedand wind loadswill, with only minor additionsandmodifications.fulfil thesctie requirements.In fact,normallydesignedin situreinforcedconcretestructuresdetailed rin accordancewith the requirementsof 3.12 other than for ties, will generallycomply Iwith thetie force requirements.Thusit is suggestedthatthestructurebefirstproportionedfor theseusualloadsandthena checkcarriedout for tie forces.It shouldbe notedthatin meetingthe requirementsthe tie reinforcementis assumedto act at its characteristic rstrength. L

3.12.3.2Proportioning of ties C3.12.3.3Continuity andanchorageof ties

72 [, ,• — . ....-..,.-. ..4~

U. - 4 - -.

I

Part1:Section3

3.12.3.4 Internal ties

3.12.3.4.1Distribution and location. Bars in internal ties may be assumedto be fullyanchoredto the peripheraltie if theyare anchoredaroundthe outermostbarsof theperipheraltie asfor a link (3.12.8.6).Otherwise,the anchorageshouldbeassessedfromaplanepassingthroughthecentroidof the peripheraltie; the availableanchoragelengthto anchoroff thebarmay be basedon the characteristicbondstressas given in 3.12.8.4.i.e. fbuXl.

4. Hence the requiredanchoragelength will be 1.15/1.4 times the normaldesignanchoragelengthgiven in Table 3.29.

3.12.3.4.2Strength. The strengthrequirementfor ties is relatedto thenumberof storeysin the building. The philosophy behindthis is that the consequencesof collapse aregenerallymoreseriousfor high buildings: furthermore,simply becauseof their greatersize, theprobabilityof misuseor the occurrenceof exceptionalaccidentalloadsisgreaterand the objectiveis to ensurethat the risk is approximatelythe samefor all heights.

3.12.3.5 PeripheraltiesIt is importantthat the peripheraltiesare adequatelyconnectedto both the verticalandhorizontal ties. In many cases,it will be helpful for the whole of the peripheraltie tobe arrangedto lie along or outside the centre-lineor medianplaneof the columnsorwalls respectively.In such cases,the internalties will serveautomaticallyas wall ties;wherecolumnsare involved, it may be assumedthat all internal ties anchoredto theperipheraltie within Im on eithersideof the column centre-lineform part of the columntie.

3.12.3.6Horizontal ties to columns andwalls

3.12.3.6.1 General. A further requirement for horizontal ties not included for internalor peripheral ties is that the tie force should not be less than 3% of the total designultimate vertical loadon the column or wall. This is likely to be the more critical formembersin the lower storeysof high-risebuildings.

3.12.3.6.2 Corner ties

3.12.3.7 Vertical ties (generallyrequired in buildings offive or more storeys)Since the steelformingvertical ties in columnsis tied togetherby stirrupsthereis littleprobleminconnectingit to thehorizontaltie.However,in thecaseof walls,reinforcementprovidedon the outerface will not usuallybe tied to the inner face. In suchcasesit maybe necessaryto provide links betweenthe two layersat eachfloor level.

3.12.4 Reinforcement

3.12.4.1 Groupsof bars

3.12.4.2 Bar scheduledimensionsFor smallprecastunits, the deductionsplus allowancesfor covermay lead to steel notextendinginto support regions. It is suggestedthat for such units the flexural steel becarriedright throughto the endface with small projectionsthroughthe stop end.

3.12.5 Minimum areas of reinforcement in members

3.12.5.1 General

3.12.5.2Symbols

3.12.5.3Minimumpercentagesof reinforcementTensionreinforcement:The values given in Table 3.27 are based on the requirement

73

Handbookto BS8IIO:]985

that the sectioncan carry a higher loadaftercrackingthanbefore.The maximumtensile Fstressassumedin the concreteis 3N/mm.

Compressionreinforcement:The minimum compressionsteel limit of 0.4% of thecompressionzoneis new andabriefdiscussionof this is in order.In CP114. theminimum pareaof compressionsteel in columnsis givenas: ‘~0.8% of the grosscross-sectionalarca jof the column required to transmitall the loading . This has traditionally beeninterpretedto mean:

mm ~ OO8N/PCb 0.0218N/f~~ FCPI1Ochangedthis to a minimum of 1% (without statingwhat it was 1% of) or, for

lightly loadedmembers. [AsminO.15NIf~Since any column requiring less than 1% of longitudinal steel could be classedas

~‘li~htlvloaded’~, it could be arguedthat the secondvalue alwaysgoverned.CP1iOgavealimit of 0.4%for walls andthis wasjustified on the basisof 5l r

that the presenceof reinforcementin walls reducesthe strength of the surroundingconcreteas placed. Under axial loading only, a plain concretewall can actually resist rmore loadthana correspondingwall with a smallamountof main reinforcement. L

As there seemedno consistencyin any existingminimum reinforcementprovisions.it was decidedto adopt the 0.4% limit for all situationson the groundsthat it was theonly proposal for which a logical reasonfor such a limit wasdiscoverable. -

Careshould be takenwith small sectioncolumnsas the minimum percentagerulescan leadto impractical reinforcementdetails if not sensiblyapplied.

In normal circumstances16mm diameterbars or larger should‘be used to ensurearobust cage.Smaller diametersshould only be consideredin short columns(less than2.5msay)orwherespecialmeasuresare takento ensurethat thebarsarefixed in position.

Lightly loadedcolumnswhere the section is largeenoughto resist the ultimate loadswithout the addition of reinforcementmay be consideredas plain concretecolumns(3.8.1.4).Thesemay bedesignedsimilarly to plain concretewalls (3.9.4).Theminimumreinforcementrequiredto control cracking(3.9.4.19) is 0.25% of the concretecross-sectionalareafor steelgrades460 andaboveand0.30% of the concretecross-sectionalareafor steelgrade250.

Transversereinforcementin flangesofflangedbeams:The valuesgiven in Table 3.27are half that requiredby CP11O.The reductionappearsreasonablewhencheckedagainstrecent researchdata. The statementin bracketsat the bottomof Table 3.27 could bemisleading.If this reinforcementwas requiredjust to resist horizontalshearthentherewould be little reasonwhy it should not be placed in both the top andbottom of theflange.Howeverthevaluehasbeenreducedsuchthat it would be wise for otherreasonsto placethis amountof reinforcementcloseto the topof theflange(e.g.minimumtensilereinforcement,control of shrinkagecracksetc).

3.12.5.4Minimum sizeof bars in sideface of beamsto control cracking

3.12.6 Maximum areasof reinforcementin membersTheseprovisionsgive limits to what is normally practical from the point of view of ‘—

placingconcreteand steel fixing.

3.12.6.1 Beams

3.12.6.2 Columns [3.12.6.3 Walls

3.12.7 Containment of compressionreinforcement LIt has generally been consideredprudent to tie compressionbars into the section bymeans of links to stop them buckling out. either when yield commencesor wherelongitudinal crackingmight developfrom any cause.Experimentalwork hasnot led tO L

any clear definition of what minimum tying is really necessaryso that rules given In

[

I~~~~4.44.4 .,.r..~•r. ... .

Part): Secuon3

3.12.7.1to 3.12.7.5are essentiallyarbitrary anddo no morethandefineacceptedgoodpractice.

3.12.7.1 Diameteroflinksfor containmentofbeamorcolumncompressionreinforcement

3.12.7.2 Arrangementoflinksfor containmentofbeamorcolumn compressionreinforcement

3.12.7.3Containmentof compressionreinforcementaroundperipheryof circular column

3.12.7.4Diameterofhorizontalbarsfor supportofsmallamountsofcompressionreinforcementin walls

3.12.7.5Arrangementoflinks for containmentoflarge amountsof compressionreinforcementin wallsThe necessityfor links in walls with morethan2% of steel will makefor considerablepractical difficulties. It is recommendedthat the cross-sectionalareaof the wall beincreasedto avoid this if at all possible.

3.12.8 Bond, anchorage, bearing, laps. joints, and bends in bars

3.12.8.1Avoidanceof bondfailure due to ultimate loadsA significant changein BS 8110 from theprovisionsof CPlIO andpreviouscodesis theremoval of the requirementfor a checkon local bond stresses.The local bond stresscheckwas not often a critical factor in design but occasionallyinfluencedthe detailingof sectionswherehighshearsoccurredin conjunctionwith low moments(simple supportsand points of contraflexure).Contactswith designerssuggestedthat many firms hadalwaysignoredthischeckwithoutapparentill effect.Furthermore,therewererealdoubtsaboutthe actualpurposeof the check.When CP11O was beingdraftedthe theory wasput forward that it wasa serviceabilitycheckagainstbond crackingbut it seemshighlyunlikely that bond cracking would develop under service loads. Furthermore,thebehaviourassumedfor sectionsis that theyare flexurally cracked.This is not true ofthe points where local bond would be checkedso the hypothesisunderlying the localbond checkis invalid in just thoseareaswherethe checkmight havean effect.Thus,inthe absenceof any clearpicture of what the checkwas intendedto achieve,it seemedpointlessto continueto requireit.

3.12.8.2 Anchoragebond stressThe assumptionsmadein this andmostothercodesis highly simplistic as bond strengthis a function of the ratio of cover to bar diameterandthe amountof transversesteelas~vellas concretestrengthandbar type.Reynoldst3261,from testson laps, gives a designformula for bond stressfor a type2 bar with no transverseshearas:

f6=0.2(0.5+cfcb)V(f~,D

For clek = 1, this gives avaluefor f3 of 0.3. An allowancefor nominal transversesteelwill bring this to the Codevalueof 0.5;in any case,thecoefficient of 0.2 in the equationgives a lower bound to all the dataused to derive theformula plus a furtherallowancefor asafety factor. It may alsobe notedthat bond strengthis markedlyincreased’bythepresenceof transversecompressionacrossthe barsuchas onewould expectto bepresentat a support.From the practical point of view, however, it seemedan unnecessaryrefinementto attempt a more realistic treatmentof anchoragebond since it wouldintroduceconsiderablecomplicationsinto designfor little economicgain.

3.12.8.3 Designanchoragebond stress

3.12.8.4 Valuesfor design ultimate anchoragebondstress

3.12.8.5 Designultimateanchoragebond stressesfor fabric 75

Handbook to BSS1IO.-1985 F3.12.8.6Anchorageof links FThesevaluesarisefrom experienceand cannotbe justified by calculation. It is believedthatthereis a misprintin (b) andthat thecontinuationbeyondthe bendshouldbe fourtimes the bar size andnot eight.

3.12.8.7Anchorageof weldedfabric usedas links

3.12.8.8 Anchorageof column starter bars in basesorpile caps [The ‘cover’ to startersin basesor pile capsis very largeandso. from the equationgivenabovefor bondstress,it will be seenthathigh bondstrengthscouldbe expected.While.for this case,therewould be expectedto be an upperlimit to the valueof ch,bwhich Cshouldbe usedin the equation,it was neverthelessbelievedthat anchoragelengthwouldnot be a limiting factor in the designof practicalbases.

3.12.8.9 Laps andjoints IA studycarriedout during the earlystagesof the revisionprocessled to considerableconcernabout the provisionsfor laps as they had existed in previousBritish codcs.Experimental evidence from Europe and America (e.g. references3.26 and 3.27)suggestedthat. in limiting circumstances.the lap lengthsgiven in CP11O could havefactorsof safetyof lessthan 1.0. Studiesof foreigncodessuggestedthat, in somecases. Fmanyof thesecodescould requirelap lengthsof up to twice UK values.Against this.theCommitteewas not awareof any caseswherelapshadfailed in practice.In drafting

the revisedclauses,the following factorswereconsidered:

(a) the bond strengthof bars castnearthe top of membersmorethan300mmdeeporso is significantly reduceddueto settlementof the plasticconcretearoundthe barwhich leaveswater filled lensesbelow the barandsometimescracksabovethe bar.The reductionis in the region of 20-40%for deformedbars

(b) closelyspacedlapscan lead to a planeof weaknesswithin asectionwhich can leadto areductionin strength:cornersarealsoasourceofweakness(seeFigureH3.39)

(c) bond increaseswith increasingratio of cover to bar diameter(seeabove).

£

,a) corner (bi cloas spacing (C) wide spacing [Figure H3.39: Modesof lap failure.

L-\ I

•2 of [‘Cear in

CO SI

LI force C

76 Figure H3.40: Effectofjoggledlap.

~ .. - ..~ - -

. ... -. -.

___________ -.4.4

Part]: Section3

3.12.8.10Joints whereimposedloading is predominantlycyclic

3.12.8.11Minimum laps

3.12.8.12Laps in beamsandcolumns with limited coverThe major considerationshereare:

(a) as mentionedunder 3.12.8.2above,someallowancefor transversesteel has beenincludedin the bond stressformula

(b) ‘joggles~. frequentlyused at laps, are a sourceof major weaknessand require thepresenceof stirrups for them to function. FigureH3.40 illustratesthe actionsof ajoggle.

3.12.8.13Designof tension lapsThe logic behindtheseprovisionshas beendiscussedunder3.12.8.9above.Table H3.6attemptsto clarify the intentionsof the clause.

Table H3.6 Multiplying factors for lap length

Tension lap lengths

~75mm 1’41b 1.O1~,

and (480) (480) (34o)•

• ~

—

~

Otherwise2•01b

(680)

1.412,

(480)

1.412,

(480)

Bars In topofsectionascastwithcover~20

Corner barsnotintopofsectionwIthcover<2e

Otherwise

lb=basicanchoragelength.0=barsize. Lap lengthsfor C30 concreteshownin brackets.

3.12.8.14Maximum amount of reinforcement in a layer including tension lapsThe logic behindthis is discussedunder3.12.8.9above.

3.12.8.15Designof compressionlaps

3.12.8.16Butt joints

3.12.8.16.1Bars in compression

3.12.8.16.2Bars in tension

3.12.8.17Weldedjoints in bars

3.12.8.18Strengthof welds

3.12.8.19Designof shearstrengthoffiller material in lap-joint welds

3.12.8.20Designof weldedlap joints

3.12.8.21 Limitations of length of weld in laps 77

Handbook to BS8IIO:198S

3.12.8.22Hooks andbends

3.12.8.23Effective anchoragelength ofa hookor bend

3.12.8.24 Minimum radius of bends

3.12.8.25Design bearingstressinsidebends [Whena bar carrying tensionis takenarounda bend,compressiveforcesaregeneratedin the concretewithin the bend.The concretewithin a bendis effectively subjectedto

Ia triaxial stressfield, beingrestrainedby theconcreteon eithersideof thebend.Concretein this situationcanwithstandvery high stresseslocally — much higher than thosewhichcanbecarriedby concretein flexure.This is trueprovidedthatthe averagestressremainslow. The provisionsof this clauseeffectively allow hither local bearingstressesas the Iaveragestressreducesin the planeperpendicularto the planeof the bend.The equationisa rearrangedversionof thatgivenin themostrecentrecommendationsof theCEB~’9U

3.12.9 Curtailmentandanchorageof bars

3.12.9.1 GeneralIt is necessaryfor anumberof reasonsto continuebarsbeyondwheretheyare theoreticallyno longer requiredto resist bending B(i) to allow for inaccuraciesin the analysis.For example.the loading maywell not be

absolutelyuniformly distributed, in which casethe shapeof the bendingmoment

diagramwill be different from that assumed A(ii) to allow for possiblemisplacementof the bars(iii) in thepresenceof shear.diagonalcracksmay form which. in the absenceof stirrups.

will causethe steelstressto bethatcorrespondingto the momentatasectionroughlyan effective depthcloserto the supports I(iv) cracks will occur at the points where the barsstop off and may well be of aboveaveragesize.This may locally reducetheshearstrength.

The minimum extensionbeyondthe theoreticalcut-off points of the greaterof the Ieffective depthor twelve timesthe bar size dealswith points (i) to (iii) while the extraprovisionsguardagainsta reductionin shearstrength.Provisions(c) and(e) control thesizeofthecrackatthecut-offpointand(d) ensuresthatthereisareserveof shearstrength.

Clearly, no bar canbe cut off lessthanan appropriateanchoragelength from the lastpoint whereit is assumedto be fully stressed.

Condition (c) will be the easistto applyandis recommendedforgeneraluse. Condition(d) will often apply in situations where low shear is present.e.g. in span regions ofbeams,wherethe trimming links automaticallysupply excessshearstrengthor in solidslabs. Condition (d) caa be complied with by adding extra links but this is notrecommendedsince apartfrom introducingextrashearcalculations.moresteelwill beinvolvedthanif themain reinforcementis extendedtocomplywith condition(c) or (e).

3.12.9.2Pointat which a bar is no longer required I3.12.9.3 Curtailmentofa large numberof bars

3.12.9.4Anchorageof barsat a simply-supportedendofa memberCondition (c), applicableto simply-supportedendsof members.is intendedto apply tosmall precastunits. As already indicatedin the commentaryon tolerances.such units Lcanhavereinforcementextendingright to the endface:asupportwidth of 60mm is thentheoreticallyfeasible. Since the effect of misplacingsteel can. in this instance,give .1

catastrophicreduction in strength. it is suggestedthat design is basedon the most

0unfavourableconfigurationwhich can arisefrom tolerances.Whenconsideringgroupsof barsextendingto simplesupports.theequivalentsize of

78 bar should be used. -j. . . . . . .

.1Part].- Section3

3.12.10 Curtailment of reinforcementThis sectionprovidessimplified rulesfor common situations.The result shouldgenerallybe conservativerelativeto applicationof the rules in 3.12.9.1.

3.12.10.1 General

3.12.10.2Simplifiedrulesfor beams

3.12.10.3Simplified rulesfor slabs

3.12.10.3.1General

3.12.10.3.2Curtailment of bars at end supportsof slabs (where simplesupport has beenassumedin assessmentof moments)

3.12.11 Spacing of reinforcement

3.12.11.1Minimum distancebetweenbarsThe requiredminimumbar spacingsare aimedprimarily at allowingadequateroom forthe concreteto flow around the barsand at obtainingadequatecompaction.With thisin mind, the reasonsbehindthe particularrequirements,as illustratedin Figure H3.41.should be self-explanatorv.Where an internal vibrator is to be used.room should beleft betweenthe top barsfor its insertion. Generally,spacingswider thanthe minimumshould be aimed at betweentop bars to allow the concreteto passthrough easily.Bundlingbarsmay be particularlyuseful in reducingcongestion.

SC

C

0 0C C

C C

.1

I—~~1

8 ~ ~ j <n.jIS I&k~~h.ff u~.u

Figure H3.41: Minimumbar spacings(h~ is the maximumsizeofaggregate).

The clausedoesnot absolutelybanthe useof spacingsless than thoserecommended.but it shouldbe notedthat suchspacingmay impair the developmentof adequatebondstrength.Very closelyspacedbarsmay producea planeof weaknessalong which bondfailure by splitting may initiate. In the light of our presentlimited understandingof thisform of bond failure. closerspacingsshouldbe avoidedunlesspositive evidenceof theefficiencyof the particulararrangementexists.

3.12.11.2Maximum distancebetweenbars in tensionThe maximumbar spacingsare specified in order to limit cracking.

Part2. Clause3.8 gives the following equationfor the calculationof the designsurfacecrackwidth:

3acrem

= 1-1-2 [ (acr—cmin)I(h—x)

~vhere a~, = distancefrom thepointconsideredto thesurfaceof the nearestlongitudinalbar:

cmin = minimumcover to the longitudinal bar:= averagestrain at the level considered:= design surfacecrackwidth.

S

S

0.U

79

[HHandbookto BS8IIO:198S

This equationhasthe propertythat,whenthecrackwidth is being considereddirectly Cover a bar (whena~ = ~ it reducesto:

Wa = 3 ~ Cm (34) Cwhile, with increasing distance from a longitudinal bar (aa—~), the crack widthapproaches:

= 1.5(h—x)Cm (3.5) [From inspectionof the equation,it canbe seenthat the crack width cannotexceed:

= 3aa 6m (3.6) CExceptdirectlyover barsor in beamswhere (h—x) is greaterthan 20 timesthe cover,the calculatedwidth will normally be muchlessthan that given by equation3.6.

Equation8 in BS 8110 gives: If. (5/8)A x l’13b

Clearly, this can be convertedto a strain by dividing by the modulusof elasticityof wthe steel.The maximumdesignsurfacecrack width is given in Clause3.2.4 in Part 2 as[0.3 mm. Substitutingthis for wa in equation3.6 and substitutingfor strain from the~relationshipgiven abovewe get.after rearrangement:

aC>32000/3b/fY [The maximumcalculatedwidth will occurmidwaybetweenbars. Therefore,as a~ willbe rathermorethanhalf the clearspacingbetweenbars, it is reasonableto write:

clearspacing~.7500O/3df~

Table 3.30 in the Codederivesfrom this formula.Clearly, wheremorereinforcementis providedin the sectionthan is requiredfor the!

ultimate limit state,awider spacingthanis given by theTable couldbeused.A suitable Uvaluecan be obtainedby multiplying the spacingsgiven in the Table by the ratio of thesteelareaprovidedto that requiredat ultimate. [3.12.11.2.1General

3.12.11.2.2Bars of mixed size. Where bars are beingarrangedin a section to control £cracks,good practicewould suggestthat the sizes of bar in the sectionshould not betoo disparate— hencethe limit on crackcontrolbarsto not lessthan0.45 timesthe sizeof the largestbar. Also, verysmall bars mixedwith largeronescould renderinvalid theassumptionson which Table3.30 is based.

3.12.11.2.3Clearhorizontal distancebetweenbars in tension. Seenotesabove. [3.12.11.2.4Cleardistancebetweenbars in tension.This is simply a slightly morerigorousapproachthan that in 3.12.11.2.3. L3.12.11.2.5Clear distancebetweenthe corner of a beam andnearestlongitudinal bar intension

3.12.11.2.6Bars nearsidefacesof beamsexceeding750 mm overall depth. This provisionis intendedto avoid the local yielding of bars in side faces. LThis can lead to large cracksin the websof such beamsevenwherethe crackwidthsat the level of the main steel are adequatelycontrolled.

Slabs. If a member is sufficiently shallow that equation3.6 gives a crack Lless than0.3 mm. thenflexural crackingcannotbe excessivewhateverthe bar

spacing.Further, the reductionin strainpermittedin Part2, Clause3.8.3 to allow fortensionstiffening hasso far beenignoredin formulatingthe bar-spacingrules. In slabs. U

where the steel percentageis usually small, this reductioncan be very significant. Forexample.with 0.5%of high-strengthsteel,the reductionin strainwill be roughly 50%.

I. . .4 — . — — —

4 4.4 .- -... . .

Part]. Section3

Thesefactorshaveled to the rules for spacingof barsin slabs.For convenience,theseprovisionshavebeenset out in the form of a flow chartin Figure H3.42.

In Table 3.30, the percentageredistribution to or from a section is the percentagedifferencebetweenthe ultimate momentusedin the designto the momentobtainedatthesectionfrom the elasticultimate momentenvelope.In termsof theparametersusedin theequationsabove,thepercentageredistributionin Table3.30isequalto iOO(Pb—1).

3.12A1.2.8Slabswhere amountof redistribution is unknown

3.12.11.2.9Spacingof shrinkage reinforcement

REFERENCES

3.1 BEEBY. AW. and TAYLOR. HPJ. The use of simplified methodsin CP11O— is rigour necessary?The StructuralEngineer.Vol.56A. No.8. August 1978.

3.2 BEEBY. AW. The analysisof beamsin plane framesaccordingto CPlIO. WexhamSprings,Cementand ConcreteAssociation,October1978. 34 pp.DevelopmentReport I (publication44.001).

3.3 WOOD- RH. The stability of tall buildings. Proceedingsof the Institution of Civil Engineers.Vol.11. September1958. pp.69-102.

3.4 meCoNCI~e~resocIEn’ aTIdTNEINSTrI~U-TIONOFSTRUCTIJRALENGINEERS. Standardmethodof detailingstructuralconcrete.London. The Institution of StructuralEngineers.1986.

3.5 CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION. The designof deepbeams inreinforcedconcrete.CIRIA Guide 2. 1977.

3.6 MARSHALL W.T. A survey of the problemof lateral instability in reinforcedconcretebeams.Proceedingsof the Institution of Civil Engineers.Vol.43. July 1969.

3.7 KRIZ. LB. and~ms. CH. Connectionsin precast concretestruct~eeB— strengthof corbels.Journalof PrestressedConcreteInstitute. Vol.10, No.1. February1965. ppA6-61.

3.8 MORROW. J. and vIEST. IM. Shearstrengthof reinforcedconcreteframe memberswithout webreinforcement.Journalof theAmericanConcreteInstitute.ProceedingsVol.53. March 1957.

3.9 KANI. G.M.J. Basic facts concerningshear failure. Journal of the AmericanConcreteInstitute.ProceedingsVol.63. No.6. June 1966. pp.675-692.

Figure H3. 42: Maximum bar spacingsin shallow members.

81

F’Handbook to BSSIIO:1985

r3.10 SWANN. RN Experimentalbasisfor a designmethod for rectangularreinforcedconcretebeamin torsion. London. Cementand ConcreteAssociation.December1970. 38pp. TechntcajReport452 (Publication42.452).

3.11 BEEBY. A w Modified proposalsfor controllingdetlectionsby meansof ratiosof spanto effectivedepth.London.Cementand ConcreteAssociation.April 1971. l9pp. Technical Report456(Publicacion42.456).

3.12 BEEBY. AW. Span/effectivedepthratios. Concrete.Vol.13. No2. February1979. pp29-31.

3.1.3 BEAL. AN Span/depthratiosfor concretebeamsandslabs.TheStructuralEngineer.Vol 6lA. FNo.4. April 1983. pp 121-123.3.14 JOHANSEN. K.w Yield-line formulaefor slabs.London.ViewpointPublications.1972.(12044)3.15 HILLERBORG. .‘.. Strip method of design.London. Viewpoint Publications,1975. (12067)3.16 BEEBY. AW A proposal for changesto the basis for the design of slabs. Wexham Springs.

Cementand ConcreteAssociation. April 1982. 38pp. Technical Report 547 (Publication [42.547).3.17 TAYLOR. R.. HAYES. B andMOHAMEOBHAI. OrG. Coefficientsfor thedesign of slabsby the yield inc

theory. Concrete.Vol.3. No.5. May 1969. PP 171-172. yr3.18 WHn4rLE. R.T. Designof reinforcedconcreteflat slabsto BS8110. London.ConstructionInduscr

ResearchandInformation Association.September1985. 4Spp. Report 110.3.19 REGAN. p E. Behaviourof reinforcedconcreteflat slabs. London. 1981. ConstructionIndustry

Researchand Information Association.89pp. Report89.3.20 LONG. AB.. KIRK. 0~ andCLELAND. oi. Moment transferandthe ultimatecapacityof slabcolumn [structures.The StructuralEngineer.Vol.56A. No.8. August 1978. pp.209-215.3.21 JACKSON. p.~. Thebucklingof slenderbridgepiersandtheeffectiveheightprovisionsof BS 544W);

Part4. Wexham Springs,CementandConcreteAssociation. 1985. l8pp. Technical Repori561. (Publication 42.561). F3.22 cRANSToN w B. Analysis and desiEn of reinforced concretecolumns. London. Cement~ndConcreteAssociation.1972. 54pp. Publication41.020.

3.23 ROBERTS. J J.. TOYEY. A.K.. CRANSTON. W B. and BEEBY. A.W Concrete masonry designer’s handbook.London. Viewpoint Publications.1983. Z72pp. (13.024/27).

3.24 WHITI~LE. R.T. Reinforcementdetailingmanual.London.Viewpoint Publications.1981. 1 l7pp-(12.085)

3.25 LAR5SON. LB. Bearing capacityof plain and reinforcedconcretewalls. Goteborg,ChalmersTechniskaHoegskola.1959. Doktorsavhandhingar.248pp. No.19. C3.26 REYNOLDS. o. Bond strengthof deformedbars in tension. Wexham Springs. CementandConcreteAssociation.May 1982. 23pp. TechnicalReport548.

3.27 BARTOS. p (ed). Bond in concrete.London. AppliedSciencePublishers.1982. 4-66pp.3.28 AMERICAN CONCRETE INs-nTu-rE. Symposiumon interactionbetweensteeland concrete.Journal

of theAmericanConcreteInstitute.ProceedingsVol.76. No.1. January1979, No.2.February1979.

3.29 CEB-RP. Model Code for concretestructures.Paris. Comite Euro-Internationaldu Beton.April 1978. Bulletin d’InformationN 124/125-E.

[

L

LLV

_L

-~ . -...--.:.A...-..-..--~

I. ——

SECTION FOUR. DESIGN ANDDETAILING:PRESTRESSEDCONCRETE

4.1 Design basis

4.1.1 GeneralSection4 containsmethodsfor assessingcompliancewith the ultimate andserviceabilitylimit state requirementsgiven in 2.2 for prestressedconcrete.The methodsgiven areprimarily concernedwith the prestressingrequirementsanddesign of flexural membersfor prestressedconcreteconstructionwherethe typesof reinforcingsteelandprestressingtendoncoveredin Sections7 and8areused.Thesemethodsaregeneralbut,forparticularapplications,otherwaysof analysinganddesigningthe structureor part of the structuremay be moreappropriate.

4.1.2 AlternativemethodsThis clausepermitsalternativemethodsto be used.firstly, becausethe Codeitself givesvery little guidanceon the analysisand design of certain types of structural member.e.g.prestressedcolumnsandslabs,andthe engineerwould haveto consultthe specialistliteraturefor hismethodof design;and,secondly,becausethetechnologyof prestressingis still at the stagewherenew developmentsin techniquesare taking place. and ne’vmaterialsareoccasionallybeingintroduced.Thesemaybeusedprovidedthattheengineercansatisfy himself that theyaresuitablefor the purposefor which they are intendedincomplying with the criteria in Section2.

4.1.3 ServiceabilIty classificationIt is the responsibilityof the engineerto decide.or possiblythe appropriateAuthorityor the client to specify,which classshouldbe usedfor a particularset of circumstances:in taking this decision, such factors as the nature of the loadine (static, dynamic.alternating,etc.), the relative magnitudeof permanentand imposedloading and theenvironmentalconditionsshould be considered.Equally. it is the responsibilityof theengineerto choosethe mostapproprateclasson economicand technicalgrounds.~ give the bestbalancebetweenultimate, cracking

~ and this would be the class adopted for many structures:tensilestressesare permitted under the desj~. service loads, theseare below

thedestgn~ten~ile-~irengthof d~ &n~i~i~7ind cr~kin~ ~i~alIv~occur. Whereit is essentialthatthestructureshouldnot crackunderserviceabilityconditions.a Class1designwould be adopted.

A Class 3 design could be used wherethere is no particular requirementfor thepreventionof crackingunder serviceabilityconditions. althoughgenerallythis wouldimply that the cracks would not open under permanentloads. Class 3. or partiallyprestressedconcrete,is an intermediateform of constructionbetweenfully prestressedconcreteandreinforcedconcrete.Therefore.providedthat the requirementsof Section2 aremet,it could be consideredfor usein manycaseswherereinforcedconcretemightbeused:for example,whereconstructiondepthis restricted.it canbeusedwith advantagewhere an otherwisesatisfactoryreinforcedconcrete design cannot comply with thedeflectionrequirements.

4.1.4 Critical limit stateAll prestressedconcretemembersmustsatisfy the requirementsof the ultimate andserviceabilitylimit states.Theshapeof the cross-sectionis animportantfactorin obtaininga good balancebetweenthe ultimate and the serviceabilityrequirements.An I sectionis normally suitable for Class I and 2 membersand a T section is ideal for a simplysupportedClass3 member.

In caseswhere the ultimate limit state in flexure is critical. ordinary untensionedreinforcement may be introduced to increase the moment of resistance. Somereinforcementis normally requiredfor other purposesin memberscontainingpost-

Handbookto 8S8110.-1985 illtensionedtendonsandthis may be utilised to increaseboth the resistancemoment and rthe design hypotheticalflexural tensilestressfor Class3 members.This approachis notpracticalfor memberscontainingpre-tensionedtendonsand the alternativeof tensioningan increasednumberof tendonsto a lower stresslevel is not economicallyviable.

The recommendedsequenceof calculationfor all membersis as follows:

(a) stresslimitations for serviceability(4.3.4) and at transfer(4.3.5)(b) prestressing(4.7) andlossesof prestress(4.8 and4.9) [(c) ultimatelimit state in flexure (4.3.7)(d) ultimatelimit state in shear(4.3.8)(e) deflectioncalculations(4.3.6) r(f) transmissionlengths(4.10), end blocks (4.11) and considerationsaffectingdesign L

details(4.12).

4.1.5 Durability and fire resistanceSee4.12.3.

The fire resistanceof a prestressedconcretestructureis treatedon an elementalbasisI.e. acolumn,beam,slab,wall etc. The tablesin this Codereferto widthsor thicknessesof sectionsandtheamountof covernecessarytotendonsandsecondaryreinforcement.

Part 1 of the Code containssimplified tabular data for general use in ordinaryprestressedconcrete construction. Where the requirement of the design is notencompassedby the rangeof values given in the tablesthen the designershould referto Part2. Section4 for a more detailed treatment.

4.1.6 Stability and other considerationsAlthough no specific recommendationsaremadeon stability in Section4. a prestressedconcretestructure.like any other, shouldcomply with the requirementsof 2.2.2.Thisin effect meansthat the tie forcesin 3.12.3shouldbe providedand,in precastconcretework. 5.1.8will also apply.

4.1.7 Loads [4.1.7.1 Load valuesWhereit is necessaryto considerthe prestressingforce asanappliedloadon thestructure.‘y~ shouldbe takenas 1.0.

4.1.7.2Design loadarrangements [..4.1.8 Strength of materials

[See3.1.7. 6.3. 7.1. and8.1.

4.1.8.1 Characteristicstrength of concrete L4.1.8.2 Characteristicstrength of steel

4.2 Structures and structural frames L4.2.1 Analysisof structures

[See3.2.1.As for reinforcedconcrete,the forces andmomentsactingon a prestressedconcrete

structure will generally be calculated by using linear elastic analysis for the loadsappropriateto both the ultimateandserviceabilitylimit states. [

If membersare restrainedor if the structure is statically indeterminate. rhendeformationsof the structuremay induceadditional forceswhich shouldbe consideredfor all limit states.The sequenceof constructionof the member,or of the structureasa whole.may also causeadditionalforces.e.g.dueto successiveprestressingin span-by- Uspanconstructionor to thebuilding of infill walls beforebeamshavebeenfully stressed.

[1

- — . - 4 . 4 .44.

] PartI: Section4

For the ultimate limit state.,~~y~or.~parusitice’~~ts. induced in a staticallyindeterminatestructurebecauseW~p~rtreactionschangewhenprestressis applied,canbe allowedfor in the mannerdescribedbelow.

(a) the bendingmomentsdueto 1.0CL shouldbe combinedwith the secondarymomentsIdueto prestress,dueaccountbeingtakenof theconstructionsequence

(b) the bendingmomentenvelopedue to 0.4Gk+1.6Q~andotherrelevantpatternsofloading shouldbe calculated

I (c) this envelopeshouldbe addedto the first bendingmomentdiagram(d) the resulting bending moment diagram may then be adjustedby redistributing

momentsas desiredin accordancewith 4.2.3.

IThis method,in effect, doesnot put a safety factor on the secondarymomentsdueto prestress.

4.2.2 RelativestiffnessSee2.5.2.

4.2.3 Redistributionof moments

4.2.3.1 General

F It ispossibleto designanddetailcritical sectionsin prestressedconcretesothat completeredistributionof moments as describe - ~ to 3.2.2.can occurjor the

ultimate limit state; . ution-Is i~r~dt6~b~Ivenpper~Iiniit

I ~Theconditionsunder which re istri ution is allowed are very similar to thoseforreinforcedconcreteand the reasonsfor eachof thesegiven in the commentaryto 3.2.2may alsobe takento apply

II!!! - ~ - ~pJ~,re~treiseI-

14.2.3.2Restriction in structuresoverfour storeys wherestructuralframeprovideslateral

] . stability

4.3 Beams

4.3.1 GeneralSee3.4.1.

4.3.2 SlenderbeamsBeamsbeinglifted during erectionmay be extremelyunstablecomparedwith their finalin-placecondition.Particularattentionshouldbegivento the choiceof thelifting method.Whereit is considerednecessaryto checkthestability of abeamduringlifting, arealisticestimateof the initial imperfectionshasto be madeto assessthelateralbendingmomentM1, which shouldbe limited to a valueprecludingcracking.

The two main imperfectionslikely to be presentare the distance.a1 (Figure H4.l).of the picking-uppoints from the minor axis, andthe lateral bow. a-,. The centroid ofthe beammusthangverticallybelowthe picking-uppointsso that, if the bowis parabolic.the angleof tilt dueto the imperfectionsin a beamlifted at its endsis

0 = (a1+0.67a~)Ie

whereeis the verticaldistancebetweenthe picking up points andthe centreof gravityof the beam(FigureH4.1).

The lateral bendingmomentat this stageis

Al2 MgG+NL(al+a2)

The first term is the productof the deadload moment. Al1, and the angleof tilt, andthesecondterm is the productof the longitudinal force in thebeamdueto the inclination

85

[IHandbook to BSSlIO:1985

rof the lifting cablesand the distance betweenthe line of action of this force and theminor axis.Even without the effects of buckling, this transversebending moment can be

embarrassinglyhigh.

EI

\ )I

IE

- A4~

The nextstageof the checkis to calculatethe factorof safety‘Yb of the beamasthough IIit had no imperfections.This is requiredto calculatethe buckling magnificationfactoron M2. An extremelysimplemethodof calculating‘~ hasbeensuggestedby Anderson

14’1for beamswith vertical lifting cables;thus U

-~ =elaceni

wherea~~1 is the verticalmovementof the centroidof the wholebeamdueto self-weight •

if it were supportedat the picking-up pointswith the minor axis horizontal.For beams Ulifted at their ends.~ can be takenasO.

64am,dwhere amid is the maximum(mid-span)deflectionof thebeamsimilarly supported.Thecalculationrequiredwheninclinedlifting pcablesare usedis rathermore complicated~421. k

Finally, the lateral bendingmomentmaybeassessed~4’1from

It has beenassumedabove that the beam doesnot twist as it buckles.However. the Leffect of neglectingtwist is slight for typicalstructuralconcretemembers.The importanceof the various dimensions,particularly e, is obviousfrom the equationsabove. It is Fessentialthat the lifting pointsbe rigidly fixed with respectto the beamso that thereis Lno possibility of accidentalside-slip. This would not only increaseat but would alsocausean effectivedecreasein e. r

In view of the sensitivity of M1 to small errors in y~, y~ shouldalways be assessed L

conservativelyanda value smallerthan 2.0 shouldnot be adopted.A considerableamount of guessworkis involved in adopting valuesfor a1 and a2.

Becauseof this. and the lack of warning of impendingcollapse.personnelshouldbekept clear. I.

4.3.3 Continuous beams LSee4.2.1 and4.2.3.

4.3.4 Serviceability limit state for beams L4.3.4.1 Section analysis a4.3.4.2Compressivestressesin concreteThe compressivestressespermittedin concreteat the serviceability limit statearebased

86 0. . .

- . - . - -,

1%A

liftingpoint

centre of gravityof whole beam

SECTION A . A

Figure H4. 1: Notation usedin calculationsfor slenderbeams.

-I— .. - .. -.. -4-

Part I: Section4

on acceptedpractice.Creepis approximatelylinearly relatedto stresswithin this range.but would not necessarilybe so for stresseswhich were a higher proportion of thecharacteristicstrength: if higher stresseswere adopted,therecould be an appreciableincreasein the loss of prestress.

4.3.4.3Flexural tensilestressesin concreteThe allowableflexural tensilestressesgiven in Table 4.1 for Class2 membersarethosethat pastpracticehas shown to be reasonable.Characterisxicflexural tensile strengthsfor the concretegradesarenot given in the Codebut a valueof approximately0.6Vf~is implied when a valueof Ym1.3 is takeninto account(2.4.6.2).

Class1 and2 membersbehaveelastically at this limit state:design basedon elastictheory usingthe stressesgiven herewill thereforebe satisfactory.Class3 memberswillbe crackedat this limit stateand should ideally be designedusing elastic theory andcrackedsections.In the absenceof a generallyacceptedcrack formula for prestressedconcrete,designfor this limit state is basedon hypotheticalflexural tensilestressesanduncrackedsections.Research~4-3-44~hasshownthat, in general.the limiting crackwidthcriterion will not be infringed if the memberis designedon the assumptionthat it isuncrackedandif the hypotheticalstressesgiven in Table 4.2 are not exceeded.

Research~4-5~hasalsoindicatedthat the crack widths in memberswith imperfectbondandin deepmembersare relatedto the depthof the section.As the hypotheticalflexuraltensilestressesare basedon a studyof structuresin serviceandon the testsreportedinreference4.3, it is necessaryto reduce the stressesin beamswith depthsgreaterthan400mm as thesewere outsidethe scopeof the test data. Table 4.3 gives depthfactorsthat shouldbe usedwhen designingbeamsof depthotherthan400mm.

Research~4-5’alsoindicatesthat it is feasibleto extendthe methodsgiven in Section3of Part2, to predict both crack widths anddeformationsin partially prestressed(Class3) members.

In consideringthe limiting tensilestressfor each classof member,the engineershouldrememberthat crackingdue to shrinkageand thermaleffects can occur, for example.becauseof inadequatecuringbeforethe memberisstressedor of lateremovalof moulds:this will impair the tensilestrengthof the concreteafterstressingand shouldbe eitherpreventedby propersupervisionor allowed for in the designby reducingthe tensilestresslevel.

4.3.5 Stress limitations at transfer for beams

4.3.5.1 DesigncompressivestressesIn accordancewith acceptedpractice.the concretestrengthattransfershouldbeassessedby testson cubescured, asfar as possible.underthe sameconditionsas the concreteinthe member.

4.3.5.2Designtensilestressesin flexureA smalltensile stressis permittedat transferin Class1 members.This may be usedtoadvantagein the endsof simplysupportedpre-tensionedmemberswith straight tendons:althoughthis stress(reducedsomewhatby loss of prestress)is still presentunderserviceload conditions,it is unlikely to be significant andmay be ignored in design.

Fairly high tensilestressesare allowed for Class2 and Class3 members:in the spanthesewill be cancelledby compressivestressesdueto deadandimposedloading. At theendsof members,considerationshould be given in certaincircumstancesto providingsome additional reinforcement,particularly if there is any risk of cracking due toshrinkage.4.3.5.2(c) suggeststhat the section should then be analysedas a crackedsection.An approximatemethodis as follows.

A Class 3 beam is shown in Figure H4.2. which shows the type of additionalreinforcementthat might be requiredto preventfailureat the endof the memberunderprestress.The design approachwould be to calculate the stressdistribution and inparticularthe tensile force (T), usingelastictheory and the concretesectionproperties.andto addenoughreinforcementat the top of the memberto carry this force: this isnot the same as a true cracked section analysis as no account is taken of straincompatibility. As this is to avoid failure, it is suggestedthat the force so calculatedbe

87

Handbook to BS8JIO:198S

A

additional steel FI I I I I I I I ~..

r

Kequivalent cracked tactic

[

U

F

A

Figure H4.2: Conditionsat endof a Class 3 beam.

multiplied by, say, 1.2 to allow for possibleaccidentaloverstressandthento designthereinforce~ienton the assumptionthat it is actingat 0.87f~. This reinforcementwouldhaveto be adequatelyanchored.

4.3.6 Deflection of beams

4.3.6.1 GeneralPrestressedconcretemembersareunlikely todisplayexcessivesagbut it maybe necessaryto limit the deflectionsthat occurafterthe installationof finishesandpartitionsin somecases.Guidanceon limiting values of deflection andsuitablelevels of permanentloadis given in 3.2 and3.3 of Part2.

4.3.6.2 Methodof calculationThe calculationof the detlectionsof ClassI and Class2 membersis simple comparedwith that for reinforcedconcretemembers.as such membersshould not be crackedunderservice loads. This beingso. it can be assumedthat the whole concretesectionbehaveselastically.Long-termeffectsmaybe estimatedby employinganeffectiveelasticmoduluswhich allows for creep.

The effectiveelastic modulusfor computing the deflection at the endof a particularperiod relative to that at the beginningcan be obtainedfrom the relationship

Eeff=Ecf(1+4)

whereEeff is the effectiveelastic modulusfor the period consideredE~ is theinstantaneouselasticmodulusatthe beginningof theperiodconsidered~ is the creepcoefficient for the period considered.

Values for the instantaneouselastic modulus and for the creep coefficient can beobtainedfrom 7.2 and7.3 respectivelyof Part2.

Deflectionsfor membersof constantsectioncan be calculatedby usingsimple elasticrelationshipssuchas

a=K12 MIEe~I

in which a = deflectionK = a constantdependingon the shapeof the bendingmomentdiagramI = effective spanAl = moment at appropriatepositionI = secondmomentof areaof section.

Valuesof K andAl for manycommonshapesof bendingmomentdiagramcanbeobtainedfrom Table 3.1 of Part 2.

When computing deflections. it must be rememberedthat the prestressCausesadeflectionbecauseit appliesa momentto the beamatanysectionequalto theprestressingforcemultiplied by theeccentricityof the forceat thatsection.Theprestressmay producea shapeof bendingmomentdiagramfor which deflectioncoefficientscannoteasily beobtained.In this case,the componentof the deflectiondueto the prestresswill have tobe computedby using some form of numericalintegrationtechnique.Suchtechniquesmay alsobe requiredwherenon-prismaticmembersare used.

It should be noted that. owing to the action of the prestress.the detlectionmay beeither upward or downward,dependingon the particular caseconsidered.A simpleexamplewill show ho~v. in contrastto reinforcedconcrete,the most critical conditionfor deflectionmay not be that obtainingafter all creephasoccurred.Considera beam

with prestresssuchthat the deflectionunderpermanentloads is upward.With time. this

K -

stress profile at A - A

—4.4~. — —4 ~ - . . - .~ - - . . . - -

Part1: Section4

a.upward deflection under permanent load

Z most cr tics, —

0upwaro deflection

TIMEdownward deflection unoer ~uciloadLh.

0

C

__ most critical~ downward deflection

Figure H4.3: Significanceof ~‘pe ofloading on the relation betweendeflectionand rime.

upwarddeflectionmay increase.This being so. any downwarddeflectionunderthe fullimposedloadwill decreasewith time.The mostcritical conditionfor downwarddeflectionwill thereforebe the application of full imposedload at the earliestpossiblemomentwhen the minimum amountof upwardcreephas occurred. (SeeFigure H4.3..)

Deflections can never be calculatedwith precision: the elastic modulus,creepandshrinkagebehaviourof theconcrete.loadinghistory.environment,constructiontimetableetc.,areall variableandlargely unpredictablein advance.If deflectionsare likely to becritical, it is perhapsmore realistic to assessupperand lower boundsdue to likelyextremesof the importantparametersand to allow in designfor this possiblerangeofbehaviour.This isparticularlyrelevantin precastprestressedunitswhicharesubsequentlyincorporatedinto a structureat indeterminateages.

Class3 membersare designedto be cracked under full imposed load. However,provided that the hypotheticalflexural tensile stressesgiven in 4.3.4.3are used,theprobability that the structurewill contain open cracks under its normal loading is low.Further, the possibledeviationfrom the uncrackedbehaviouris not large,andso littleerrorwill be involved in assumingthe wholesection to be actingandin calculatingthedeflectionas for Class1 andClass2 members1451.

4.3.7 Ultimate limit state for beamsin flexureIn most cases.Class 1 and Class 2 members already complying with the requirementsof the serviceabilitylimit statewill require only a checkat this limit stateandshouldcomply without modification. Class3 membersmay require the addition of ordinaryuntensionedreinforcementin order to increasethe resistancemomentof the section.

43.7.1 SectionanalysisThreemethodsareavailablefor the analysisof prestressedconcretesectionsin flexure.Theseare:

1. The sectionmay be analysedin accordancewith assumptions(a) to (f) by usingthestress-straincurvesgiven in Figures2.1, 2.2 and2.3 for concrete,reinforcementandtendonsrespectively.

2. Thesectionmaybe analysedin accordancewith assumptions(a) to (f) by usingtherectangularstressblock describedin assumption(b) forconcreteand the stress-straincurvesgiven in Figures 2.2 and2.3 for reinforcementand tendonsrespectively.

3. The sectionmay be designedin accordancewith 4.3.7.3and4.3.7.4 by usingdesignformulaeandTable 4.4.

Methods1 and2, for which a t\’pical section,stressdistributionandstrainprofile areshownin Figure H4.4, are applicable to sectionsof any shapewith any distribution oftendonsand reinforcement.The analysisnormally involvesa trial anderror approach.usingstrain compatibility, in order to determinethe neutral axis depth for which theforcesactingon the sectionarein equilibrium, as follows:

1. Assumea value for the neutral axis depthx and draw a strain profile by taking avalue of 0.0035 at the extremecompressionfibre.

2. Calculate the correspondingstrain at the level of the tendonsand add to this the89

Handbook to BS8JIO:1985

0.45 4..

7.4— As,. 4b

stress distribution

0.45 4.,

IiAf~

stress distribution

Io 0035

7

strain in concrete

Figure H4. 4: Designof prestressedconcretesectionsin flexure.

prestrain in the tendonsafter all losseshave occurred.Where appropriate.alsocalculatethe strain at the level of the reinforcement.

3. Determine the stressesin the reinforcementandthe tendonsfrom Figures2.2 and2.3 respectivelyandcalculatethe correspondingforces.

4. Calculatethe force in the concretein the compressionzoneand comparethis withthe total force in the reinforcementandtendons.

5. Modify the neutral axis depth andrepeatsteps1 to 4 until equilibrium of forcesisobtained.

6. Takemomentsabouta convenientposition. such as the neutral axis. for all theforcesactingon the section.

MethodsI and 2 will give very similaranswerswith method 2 beingeasierto apply.particularly for sectionswith non-rectangularcompresionzones.Method 3, which isdirectly applicable to sections with rectangularcompressionzones only, will giveapproximatelythe sameansweras methods1 and 2 in this case.

4.3.7.3DesignformulaeAn approximatemethodfor obtaining the resistancemomentof sections.in which thetendonsare effectivelyconcentratedat oneposition in thetensionzone,hasbeenderivedin accordancewith assumptions(a) to (f). The design stress-straincurve for a highstrength tendon (f~~=1860N/mm. E¶=I9SkN/m&) has been used and the non-dimensionalstresstermsareslightly conservativefor lowerstrengthtendons.Themethodusesthe rectangularstressblock for the concreteand is directlythat are rectangularfor a depth(from the compressionface)of - applicableto sectionsx is the neutralaxis depth. not lessthan0.9x.where

For a beamcontainingbondedtendons.equation51 may be rewritten in a form thatis suitable for plotting graphically,as follows:

[I

Method

0.0035I0

section

F

[

strain in concrete

r

0.Sx

Method 2

[

0

[

sectionCC

U

LLL

E9()

— — — — . . 4 . — .4~ . ~ .... ..444.4.4 4 -- ‘. - - .:. —- ~

— ~ —. .~ . .-K ‘ - ~— — . K . K — — — • — .4~. —-—-~-‘:-‘.‘.:-.-: -

Part1: Section4

0.25

0.20

0.15

4.,bd2 cia

0.05

0

0.Sx

fp

.

fp.

Figure H4.6: Designof T beamsfor flexure.

Using the valuesof fpb/O.87fp~andx/d given in Table4.4, a graphof M~/f~bd2againstf~A~If~bd may be producedas shownin Figure H4.5.

For a flangedsectionin which the flangethicknessh~~0.9x. theequationor thegraphmay be usedby taking thewidth of thesectionas the flangewidth b, asshown in FigureH4.6(a).Where h~<0.9x,as shown in Figure 4.6(b), the forcesacting on the sectionmay be equatedto provide:

Ap~fpb=0.45f~[(b—bW)h~±0.9bWxI

which may be rewrittenin the following form:

fpbIO.87fpU=0.52~ [(bIb~—1)hfld±0.9x/d]

This linearrelationshipbetweenfpb/O.87f~.,andx/d may be plottedgraphically,togetherwith the appropriaterelationshipgiven in Table 4.4, as shown in Figure H4.7. Theintersectionof the two relationshipsgives the valuesof fpb/0.87f~~andx/d to be used inequation51 with an appropriatevalueof 4. Alternatively,M~maybecalculatedfrom:

M~=0.45f~ [(b—bW)hf(d0.5hr)+0.9bWx(d0.45x)1

For a beam containingunbondedtendons,equation52 has beendevelopedfrom theresultsof testsin which the stressin the tendonsandthe length of the zoneof inelasticityin the concretewereboth determined146-4.7)

The beamis consideredto developboth elasticandinelasticzonesand the length ofthe inelasticzone is takento be lOx. The extensionof the concreteat the level of thetendonsis assumedto be negligible in the elasticzonesandthe extensionin the inelasticzoneis assumedto be takenup uniformly over the length I of the tendon.Thus, thetotal strain 8pb in the tendonsis given by:

Cpb = e~+0.0035[(d—x)/x] (lOx/I)

I

f

1,Ap

.

4~bd

Figure H4.S: Designchartfor prestressedrectangular beams(bondedtendon).

b

0.9x p

(a) - Ib)

91

Handbookto 858110:1985

1.0

0-s

0.870.8

0.7

0.6

Figure H4. 7: Treatmentofflangedsection it-here H~=O. 9x.

The correspondingstressin the tendonsis thengiven by

fpb = f~+(0.035E~) (d!I)(1—x/d) ~ 0.7f~

The neutralaxis depthmay be determinedby equatingthe forcesactingon the sectionwhich, for asectionwith a rectangularcompressionzone.provides:

Ap.fpb = 0.45f~b(0.9x)

This may be rewrittenin the form of equation53 andsubstitutedin the expressionforfpb to give:

fpb = f~~+(0.035EO(d/I) [1 ~2.47(~fPUAP5)/cfCIIbd))cfPb/fPU)] ~ O.

7fpuEquation52 is then obtainedby putting E, = 200kN/mm and, as an approximation.

= 0.7 in the last term.

4.3.7.4 .4llowancefor additional reinforcement in the tension :oneThis approximatemethodfor taking accountof additional reinforcementin the tensionzonewill generallyunder-estimatethecontributionof the reinforcement.A morerigorousstrain compatibility analysis~vouldallow the full designstrengthof a grade460 steel tobe utilised for all values of x ~ 0.636d. for example.

4.3.8 Designshear resistanceof beams

4.3.8.1 Symbols

4.3.8.2MaximumdesignshearstressA limit on the maximum shearforce carried by a memberis necessaryin order to stopthe diagonalthrustsin the web of the membercrushingthe concrete.When the web hasholes for ductsandtheseholesare not groutedafter stressing,or whendesigningin theconstructionalphasefor post-tensionedgroutedconstruction.the minimum ~vebwidthshould be used.Even after grouting. the groutedsection may causespalling of theconcreteon either sideof the duct.

Information on this problemmay be obtainedfrom reference4.8. The conclusionsfrom this work are that. for groutedducts.only the concreteandone-thirdof the ductwidth should be consideredfor the checkon maximum shear.as noted for symbol b~..

92 0— - — K — K — - K K - K — -

K

- - - - - K. ~ - ~ -. -- -. - -. ~

FC

CC

rrA

C

I

IC

LI

I;I

K U-. - -‘.. K.. -. - -. K - . - - -‘. . —

-- ~ -~ . . :i~ K- .. - K.: j K - - K K K K K K

Part 1-Section 4

I ~ I 1 I C

check V.,., check V.,, and V.,.,

Figure H4.8: Prestressedbeamshowing:onesfor shear.

4.3.8.3 Calculation of designshearresistanceAt the ultimate limit state,the memberwill havezonesthat are crackedin flexure andothersthat arenot; the behaviourin eachzonehascharacteristicfeaturesthat requiredifferent design methods.Figure H4.8 showsa prestressedconcretebeamunderload,togetherwith its bendingmomentdiagram:the zoneswherethe bendingmomentis lessthan the crackingmoment should be designedso that the shear force carried by thebeamis lessthan V~; the zoneswherethe bendingmomentis greaterthanthe crackingmomentshould be designedto ensurethat the shearforce carried by the beamis lessthan V~. The natureof equation55 ensuresthat.wherethe bendingmomentis lessthanthe crackingmoment, V~ is always greaterthan V. It is thereforenecessaryto checkonly for V~ in thesezones.Elsewhere,either V~0 or Vr~i, may be critical andthe sectionmustbe designedfor the worst case.This procedureappliesfor all members.In eachcase,two setsof calculationsmustbe made.A flow chartis shownin Figure H4.9.

A morecompletedescriptionof thederivationof theseclausesisgivenin reference4.9.

4.3.8.4Sectionsuncrackedin flexureIn the zoneof the beamthat is uncrackedin flexure,shearcracksmay occur in the webasshownin FigureH4.10. ~4, is calculatedto seethat theshearforce is not highenoughto causefailure by an extensionof thesecracksthroughthe beam.

Determine maximum shear forcalsi at critical sectionlal

~8~or5~d

?

Is V.~0.5 V.,? - Shear reinforcement need not be provided

NO YES

Is V<V., in the case of a slab, a member of minor importance orwhere tests have shown that shear reinforcement is unnecessary?

NO

YES A,, — 040, Imincnlum inesiIs VE(14+0.4b,dl? S., 0.874,

NOA,, = V-V., ________________________

s, O.874,d, s,~O.75d~, lO.Sd, if V>1.8V4, 4b,

.

NO

Determine V., at critical sections (uncracked and cracked inflexure) using the following expressions for Vp. and V.,,

(a) uncracked sections (M<M.,) V.,— V.,.,,Ib) cracked sections lM~M.,): V.,—lesser of Vp. and V.,,

V., 0.57b.,hV(f,24’0.8fp.~) where f,—0.24Vf,, -

where M.,=0.Sf.,.,, lI/y+Ae) and y—distance to extreme tensionfibre

Figure H4.9: Flow chartfor shear in prestressedconcrete. 93

Handbookto BS8IIO:1985

web shear crack flexural crack eventuallyeventually causing failurecausing failure

Figure H4. 10: Prestressedbeamwith possibleshearfailure.

The principal tensilestressat the centroidof the section is limited to 0.24Vf~.If the principal tensilestressis takenas positive,elastictheory gives:

= [(f~+f~)I2J — V[(f~—f~)2I4±f~]where f’- = V~

0 S/l&

S = thefirst momentof areaaboutthe centroidalaxis of the partof thesectiontoone side of the axis

I = secondmomentof areaof the sectionfe.. = compressivestressnormal to the longitudinal axis.

Theseequationsmay be rearrangedto give:

= ~

Assuming%. to be negligible gives

= (Ib~1S)V(p+f1f,~,)

ForarectangularsectionIb}S = 0.67b~h,andsubstitutingthis into thepreviousequation

gives:= 0.67b~hV(A

2+f~f~)

The prestressf~ hasa dominanteffect on the valueof V~0 and.as partial safetyfactors

havebeenincorporatedin the valuesof f, an additional factorof 0.8 hasbeenappliedto f~. This now gives equation54:

= 0.67&hVCf~2+0.8f~f~)

For I, TandL sectionsIb~/S>O.67b,,h,sothattheexpressionaboveisslightly conservative.However,for such sections.the point of maximum principal tensilestressis not at thecentroid but at the junctionbetweenthe web andflange:thesimplification of checkingonly at the centroidof the sectionis thereforeslightly unsafein thiscase.The twoeffectscancelout giving a reasonable,simple method for checkingall typesof section.

Table 4.5 is asolutionof equation54 forgivenvaluesof the prestressat the centroidalaxis of the beamandof f~ andmay be usedinsteadof equation54.

If a beamhasdeflectedtendons,the bendingmomentimposedon it by the tendonsis asshown in Figure1-14.11(a).This compareswith the momentappliedif the tendonswerestraight(Figure H4.11(b)).

For a beamwith deflectedor drapedtendons,a shearis appliedat the endsequaltothe rateof changeof momentor, moresimply, thetendonforceresolvedvertically.Thisshearforceshouldbesubtractedfrom, or addedto. the imposedshearwhereappropriate.before V~

0 is comparedwith the imposedshear.When additional tendonsare addedto provide a vertical prestress,then the web

crackingforce. V~0, will beincreased.Thiseffectmaybeconsideredby modifyingequation54 to include the verticalprestress.

~

wheref~ is the vertical prestress.Hence V~ = ~ and Table 4.5 may be used if+f~+0.8f~fJf~) is substitutedfor ~ If the prestressingtendonsapplyingthe vertical

prestressare vertical, thenno externalmomentis applied to the beamandthe verticalforce carriedby the tendonscannotbe subtractedfrom the shearforce. If the tendonsare inclined as in Figure H412 then.as in the caseof deflectedlongitudinal tendons.anexternalmomentandshearareappliedto thememberandthelattermaybesubtractedfrom or addedto the externalshearforce whereappropriate.

- .-~ K K•K - K- K. ~ .~ ~ ~K — ~ K K K — — K — — . - K. —K K — K — — K — — K K —— K K K — — — K ~ —— K K K - K K - . - K K . - K - K K ..4 - K- K K K K K KKKK - ‘4”~ K

-. -..- ‘,. ‘j.-...a’ — -—‘4“-‘-44.K..K .K..K •K ~K K - — - K - K~ K - — —. .4 - . .,. - - — - 4. — - . . K ~44 V -K K K K K — K — .~ K 4 K — —

[IF

I- K K K . K - - K K- - - - U . .. - :7:..-~: - -~ - ‘K--K>.- - - <K-K-....

K K..... . — K K — — —.:~---~‘~-~.-- ~ K

Part1: Section4

3<MEMBER

Pa

~==IiZIIZ==~

BENDING MOMENT

P sin a

SHEAR FORCE(a)

Ib)

-L ‘a4—’--MEMBER

Pa

BENDING MOMENT

Figure H4.11: Effectsof (a) deflectedand (b) straight tendonsin sectionsuncrackedinflexure.

P., P.,

/A.~ \ ~e

\~ ‘~

MEMBER P., P., P., P.,

PS

SENDING MOMENT FROM TENDONS

P., sin a

II5HEAR FORCE FROM TENDONS wFigure H4. 12: Theoreticaleffectof inclined tendonsin sectionsuncrackedin flexure.

Shear failure occurs along the plane on which the principal tension acts and theinclination of this plane to the beam axis varies with the prestress.A method fordeterminingthe positionof the critical sectionneartheendof apre-tensionedmember.with due allowancefor the developmentof the prestress.is given in reference410. Asimplification of this method is shown in Figure H4.13.

4.3.8.5 Sectionscrackedin flexureIn the zoneof a beamthat is crackedin flexure, a crack may becomeinclined andeventuallycausefailure (FigureH4.10). A checkon the valueof Vcr is neededto makesure that this doesnot occur. The position of this crackvaries, but testsindicate that acrack at a distanceof half the effective depth from the point of maximum moment isusuallytheonethatcausesfailure. A largenumberof testshavebeencarriedout’~”4’21

95

—S. P

P., P.,

,~ /~

Handbookto BS8lIO:1985 rI, I

f

p. r_________ I

— — - _______ _______cantroidal axis - - 5

I I

Figure H4. 13: Critical sectionfor shearat endof a pre-tensionedbeam.

and. from these,a lower boundto the ultimate shearcrackingloadmay be drawn.The II!expressionfor this lower bound is

= 0.037b~dV(f~)+Mg(MIV—d/2)

whereM~ is the crackingmoment.Thus. the shearforce at which the cracksextendis equal to the shearforce present

when the section is at its crackingmomentplus a constant.The valueof the constantI

dependson the tensilestrengthof theconcrete.The expressionmay,conservatively,besimplified to:

= 0.037b,.dV(f~~j+M~VfM 5This formula in itself would be an adequatecode design formula except that it iS

appropriateonly for memberssimilar to thosetestedin the experimentalinvestigationsmentionedpreviously. The testswerecarriedout on memberswhich hadhigh prestresSlevels(over0.5f~~)andthisformulais thereforenot directlyapplicableto memberswhichmayhavevery low prestresslevels. As therewas no appropriateexperimentalevidenceavailable,the formula was modifiedso thatit was dependenton prestress.The modified Fformula in the Codegives a linear relationshipbetweenthe reinforcedconcreteshear Liclauses,if the prestressis zero, to the formulaabove,if the prestressif ~

The modificationwas carriedout in the following way:

Va = A+B

whereA dependson materialstrengthandcould be likenedto the shearcalculatedfromthe v~ valuesin 3.4.5.

B is theshearforce to crackthe member.If A is changedto give the v~ values,modified by prestress.and B is zerowhen the

prestressis zero.the two sheardesignmethodswould fit, i.e.

V = [1—n(.f~.,/f~~)Jv~b~.d+M0VIM K INow, if bothf~, the effectiveprestress,andM0, the momentto causezerostressin theconcreteat the extremetensionfibre, arezero,the formulashouldgive the sameresult Eas the reinforcedconcretedesignmethod,

V = v~b~d aExpandingthe formula.

96 Vr 0.037b~.dV(f..~)+0.37(I/ya.,)V(J~~)+M0V/M

-K K. ~ K~ -‘-.....~. -- .4W~K — — 4~ K K U. - K ~ . — 44.-J :..~ ..— -. K— ~ - -4. K -K - ~ ~ ~.-.- - - - K K..4 .. K K —

- 4-. -. WI. K..~;K K K — K,K K 4 - 4~K4K4~.4~4.4

- — - - K - - - . .:-‘-‘-‘r.-<.....--’- K - -

Part I: Sectii-m4

This is the sameas the formulawhere0.37V(h~) is the tensilestrengthof the concreteand a, is the leverarm of the crackingmoment. Since the value of a~ in practiceis 4/iwhere shearis likely to be a problem.we obtain:

Valb,d = ~

By assumingthathid is unity — a conservativeassumption— and substitutingf~ = 50

V~,1b~4= 0.26+0.11+M0V/Mb~.d

Whenf~ = ~ n shouldbe such that (1—0.6n)t’~ = 0.37N/m&

If we take an averagevalue of v~ as 0.55 (for 0.5% steel),than

1—0.6n = 0.67

Thereforen = 0.33/0.6= 0.55

and so

= ~ (equation55)

BENDIHO MOMENTFROM TENDON

If a beam hasdeflectedtendonsin a region where it is crackedin flexure. the tendonwill apply an externalshearto the beam in a way that was describedin the previoussection. Figure H4.14 shows a typical case.There is someexperimentalevidence~

4’31which shows that the ultimatestrengthof beamsis reducedbecausethe main tendonsare not near the tension face and are unable to restrain cracking. Inclined crackingthereforeoccursearlier thanif the tendonwerestraight.giving a correspondingdecreasein the ultimate load. It is thereforenecessaryto addthe shearfrom the inclined tendonwhereit increasesthe imposedshearand to ignore it in othercases.

C

LJSHEAR FORCE FROM TENDON

Figure H4.14: Theoretical effectofdeflectedtendonsin sectionscrackedin flexure.

4.3.8.6 Casesnot requiring shearreinforcementWhen the shearforce V is less than0.5V~, thereis no needfor shearreinforcementasthe factor of safetyis very high. There is no evidenceof trouble dueto the omissionofminimum reinforcementin many joists. lintels andmembersof minor importance.

4.3.8.7 Shear reinforcementwhere V doesnot exceedVe~f~O.4b. dIn thiscase.minimumreinforcementis requiredas for reinforcedconcrete.Testevidenceshowsthat the provision of minimum shearreinforcementcan add considerablyto thestrengthof prestressedconcretebeamsand ensurea moreductile failure.

4.3.8.8 Shear reinforcementwhere V exceedsV~+O.4b,dIn this case.more than the minimum reinforcementis required.

4.3.8.9Arrangementof shearreinforcement 97

[IHandbook to BS& 10:19,~5

4.3.8.10 Spacingof shearreinforcement F4.3.9 Torsion FTorsion in prestressedconcretemaygenerallybetreatedin thesamewayasfor reinforcedconcrete.Thereare, however,certain differencesin behaviour.

Cracking may not be acceptableunder serviceabilityconditions. In this case,the ITprincipal tensile stressdue to prestress.flexure, shearand torsion calculatedfrom thecharacteristicloads and deformationsshould not exceed024V(f~~) at any location(including the flexural tensile fibres). Full elasticity should be assumedin the analysisboth of the structureandof the sectionsin thesecalculations. F

4.4 Slabs

4.4.1 General

4.4.2 Flat slabs [K

References4.14and 4.15 contain recommendationsfor the analysisand desi2n of flatslabscontainingbondedor unbondedtendons.The punchingresistanceof a prestressed

[concreteslab is greaterthan that of an ordinary reinforcedconcreteslab with a similarareaof flexural steelandthis is allowed for, in references4. 14 and4- 15. by consideringthe areaof the tendons~ to be replacedby an equivalentareaof ordinaryreinforcementA~.,f~~/4l0. A morelogical approach14’61is to calculatethe punchingresistanceas for an Fordinaryreinforcedconcreteslab, using the actualareaof the tendons,and to addthisto the decompressionload: i.e. the load requiredto annul the effect of the prestressintermsof the stressat the extremefibres put into tensionby the applied loading. Theseapproachesimplicitly assumethe tendonsto be bondedandthe responseto loadsbeyondthe decompressionlevel to be the same as that of ordinary reinforced concrete. Ifunbondedtendonsare used,the responseis less favourableandthe punchingresistanceis typically reducedby about 10%(4.16l.

4.5 Columns

For short columns underaxial load, thereis little justification for prestressin~.exceptfor handling. In this context, prestressingcould be used to advantagein concretepilesor possiblyin precastcolumns,designedfor axial load, to cover erectionstressesand tocaterfor any unexpectedeccentricloading. Generally.a low level of prestresswill dealwith theseproblemsandthisclauseallows prestressedcolumnshavinga meanstressdueto prestressof less than2.ON/mm to be designedas reinforcedcolumns.

The Code makesno specific recommendationsfor the design of prestressedcolumnswith a mean pre-compressiongreaterthan 2.ON/mm. In this case.it is suggestedthatthe additional momentapproachgiven in 3.8.3 be applieddirectly with the cross-sectionbeingdesignedasa prestressedconcretesection.Numerousmethodshavebeenproposedfor the analysis of prestressedcolumns14’7~’9~;generally.theseare similar in principleto the approachgiven in 3.8 for reinforced concretecolumns. Experimentalwork has

Lbeen carried out on prestressedcolumnswith slendernessratios of up to 30 undershort-term loading~4”- 4211 and undervarious levels of long-term loading~4~’.

I4.6 Tension members

4.7 Prestressing [4.7.1 Maximum initiai prestress 0

98 4.7.2 Deflecf pd tendons in pre-tensioning systems

--‘-K - -.

K K >~~K K K-K .~KKK KKK,:.--K.~.-.- I-~ i:..-~.~’- -->‘2.-~>-- - - ..-.>...7-K K,... —

Part]:Section44.8 Loss of prestress,otherthanfriction losses

4.8.1 GeneralAn assessmentof the loss of prestressthat can occur at variousstagesin the life of aprestressedmember is an essential part of the design calculations.particularly forsatisfyingtheserviceabilitylimit states:errorsin calculatinglossesareunlikely toseriouslyaffect the ultimateresistanceof the member.It is oftennecessaryto calculateonly thoselosseswhich occur at transferand to assessthe total loss,so that both the initial andfinal stressconditions in the membercan be calculated: however, for certain cases.particularly composite construction. it may be necessaryto calculate losses at anintermediatestage.The valuesgiven in this clausefor assessinglossof prestressdue to variouscausesarenecessarilygeneralandapproximate.This should be rememberedwhen making designcalculations;excessiveaccuracyis unnecessary,as theassumptionson which thevariousmethodsare basedwill not be completely realizedin practice.For example,it shouldbe borne in mind that, in practice.the prestressingforcemay bealtered by the imposedloads and by the varying climate to which the memberis subjected.Only rarely will itbe possibleto assessthis variation accuratelybut if, for example.the engineerknowsthat part of the imposed load will be permanentand will be appliedearly in the life ofthe member,he should modify the calculationsin the following clausesaccordingly.Wherenewmaterialsareusedorwheretherearespecialdesignconditions.theengineermay depart from the recommendedvalues,but he must ensurethat thesenew valuesare basedon adequateexperimentalevidence.Thermalcuring is often usedto acceleratethe hardeningof concrete,particularly forthe manufactureof precastelementscontainingpre-tensionedtendons.This can haveasignificant effect on the lossesof prestressdue to relaxationof the steeltendonsand theelastic deformationandsubsequentshrinkageandcreepof the concrete.Steamcuring.for example,often resultsin reducedshrinkageandcreepeffects14~In addition. therelaxationof the steelis acceleratedduring the period betweentensioningand transferwith a correspondingreduction in thestressapplied to the concreteat transfer1441.4.8.2 Relaxationof steel4.8.2.1 GeneralThe treatmentduring manufactureand the subsequentconditions of service have animportant influenceon the performanceof prestressingsteelin structures.The extentof thevariationin relaxationfor similarstraightenedwire is illustratedin FigureH4.15(a),which shows data for two types of cold-drawn wire of 5mm diameter.Wire H wasstraightenedby a roller-typestraightenerandsubsequentlyheat-treatedat a moderatelyelevatedtemperature(relaxationclass1). Wire E wasstraightenedby a final passthrougha die undertensionat a moderatelyelevatedtemperature(relaxation class2).The test period for obtainingdata in relaxation testsat constantstrain hasbeensetat lOOOh and the effect on relaxation of longer test periods is also shown in FigureuJ4 10 120. —

-~ / 0ioo/ ~80I-<60cn~ K w /~ZO~ O.40(I-wI-’ w~u. - 20 /F 0 0 K20(a) RELAXATiON AS PERCENTAGEOF INITIAL STRESSFigureH4. 15: Effectof(a)timeand(b) temperatureon therelaxationof5mmdiameterwire at variouslevels of stress.

20 30

(b1

99

[IHandbook to BS8lI0:19~S

H4.15(a).The strain in the prestressingtendonsdoesnot remainconstantduring thelife of a structurebut reduceswith time. owing to creepandshrinkageof the concreteIn the caseof pre-tensionin~.thereis also a reductionat transferowing to the elasticdeformationof the concrete.An allowancefor the effectsof thesestrain reductionshas F.beenmadein the Ion~-term relaxationfactorsgiven in Table4.6.

4.8.2.2 The lOOOh relaxation value F

The certifiedvaluesshouldbeobtainedfrom the manufacturers.However,in the absence 4

of this information, the maximum valuesspecifiedin the appropriateBritish Standardshouldbe used.SeeTable8.1.

4.8.2.3 Abnormal relaxation losses

Whenstructuresare likely to be at temperaturesabovenormal.the lossof prestressdue rto relaxationof the steel will be greater.and Figure H4.15(b)gives the resultsof 1000h testsat various temperatures.For steelswith a tensile strengthfrom 1540N/mm to

1700N/mm.the increasein the lossdueto relaxationabovethat at 200C max’ be takenas I.8N/mm per degC for an initial stressof 70% of the characteristicstrength.andr2.6N/mm per degC for an initial stressof 3flO/~ of the characteristicstrength.These Ivaluesappearto be less influencedby the manufacturingprocessthando the valuesfor

relaxationat 200C.

[Where large lateral forces may be appliedto the tendon.relaxationlosseswill againbe higher than normal. Theseloads may be imposedon the tendonby a changein thecable profile or becauseof the methodemployedfor gripping the tendon: lossesunderthesecircumstancescanonly be determinedby testswhich simulateaccuratelytheprecise [conditionsthat will occur during the actualconstruction.

References4.25—4.30shouldprove useful in assessingrelaxationloss underspecialconditions. [4.8.3 Elastic deformationor concrete

4.8.3.1 GeneralThe modulus of elasticit~- of concretedependssignificantly on the type and sourceofaggregatebut it will generallybe sufficient to use a mean valuetaken from Table 7.2 rof Part 2. In using the table.f~:s may be replacedby f~

1 to obtain a valuefor E~~. TheLmodulusof elasticity of the tendonsmay be obtainedfrom Figure 2.3 of Part 1. Whenthe tendonsare spreadover the cross-section.thestressesin the adjacentconcretecouldbe determinedseparatelyfor eachtendonbut it is generallysufficient to considerthemas concentratedat their centroid.

4.8.3.2 Pre-tetisioning [For pre-tensioning.the averagelossof prestressin the tendonsdue to elasticshorteningis given by ~ wheref0~ is the initial stressin the concrete.adjacentto thecentroidof the tendons,due to the prestressonly.

The actualstressin the concreteand the tendonswill be modified by the self-weightLstresseswhich will vary along the length of the member.Thesestresseswill dependto

someextenton practicalconsiderationssuchasthe methodof releaseand the adherence

Iof the concreteto themould and.subsequentlx-.on the methodsof handlingandstacking.Theself-’yeiTht stressesneedto be consideredwhencheckingthe initial stressconditionsin the concrete. L4.8.3.3Post-tensioningFor post-tensioning.the stressineof any tendonor group of tendonscausesa loss otprestressdue to elastic shorteningin any other tendonthat has alreadybeenstresseand anchored.The effect is progressiveand the overall lossdependsupon the numberLof stressingoperations. For a small numberof operations(there is no loss for a singleoperation).the losscanbe assessedat eachstageandsummed.Generally,however.theeoverall effect may be adequatelyassessedb’- akin~ the averageloss of prestressin thCUtendonsdue to elastic shorteningas ().5,E.,-’E~)i>,.. where J>~ is the mitt tI ~tre~ ifl

the concrete.adjacentto the centroidof the tendons.averagedalonethe len”th ot theIINI L

K K K K K K..K — K-. - . , - K - . K— K K K K K K K K K K K.... K — K ~ K K — K K —— — — — 4 — — K KKKKK K K KK K K K K K K — K —

Part1: Section4

tendons.In this caseit is appropriateto considerthe actualstressin the concretedueR to the combinedeffectsof prestressandself-weight(andany otherpermanentloads thatare appliedat transfer).

4.8.4 Shrinkage of concreteThe lossof prestressin the tendonsdue to shrinkageof the concreteis given by E,c.wheree~ is the shrinkagefor the periodconsidered.

The drying shrinkageof plain concretedependsmainly upon the relativehumidity oftheair surroundingtheconcrete,thesurfaceareafrom which moisturecanbe lost relativeto the volume of concreteand the mix proportions.The shrinkageof the concreteisincreasedif aggregateswith a high moisture movementor a low modulusof elasticityare used.Concretefor prestressingrequiresthe use of good quality aggregatesand amix ~vith a low water/cementratio and the shrinkagevaluesgiven in this clauseareadequatefor most designpurposes.

Where necessary,an estimateof the drying shrinkagemay be obtainedfrom Figure7.2 of Part2 for normal-weightconcretecontaininggood quality aggregates.Figure 7.2relates to concreteof normal workability, made without water reducingadmixtures.where the original watercontent is about 190 litres/in3. The shrinkagemay be modified

in proportion to the original water content for values in the range 150 litres/in3 to 230litresiin~. Forpost-tensioning,in caseswherea considerabledelayis anticipatedbetweenthe placingof the concreteandthe applicationof the prestress.allowancemax’ be madefor the shrinkagethatwill takeplacepriorto transferon the basisof the ambientrelativehumidity during that period.

4.8.5 Creepof concrete

4.8.5.1 GeneralOn the assumptionthat creepis proportional to the initial stressin the concrete,theaverageloss of prestressin the tendonsdue to creepof the concreteis given by

E5e~~, Es(4/Eci)fec,

where~ is the creepstrain in the concretefor the period consideredd is a creepcoefficient for theperiod consideredf~ is the initial stressin the concrete.adjacentto the centroid of the tendons.due to the combinedeffects of prestressand self-weight. For bondedtendons.the stressshould be taken at the section under consideration.For unbondedtendons,the stressshould be averagedalongthe length of the tendons.

4.8.5.2 SpecificcreepstrainThe creepof plain concreteunder sustainedstressconsistsfundamentallyof a ‘basiccreep’that developsunderconditionsof no moisturechange(sealedcondition) andanadditional ‘drying creep’ that respondsto environmentalinfluencesin the sameway asdrying shrinkage.The influenceof the type and sourceof aggregateis illustrated inFigure H4.16. Furtherinformation on the significant factorsaffecting creep. includingthe ageof loading, may be obtainedfrom references4.31—4.33.

The stress in the concreteat the centroidof the tendonsdoesnot remainconstantduring the life of the structurebut reduceswith time, owing to the combinedeffectsofrelaxationof thesteelandshrinkageandcreepof theconcrete.An approximateallowancefor theseeffectscould be madeby considerin2a stressthat is the meanof the initial andthe final values, due to the combinedeffectsof prestressand self-weight. However, inthe simplified approachadoptedin this clause,an allowancefor theeffect of the reducingstresshas beenincludedin the valuesgiven for the creepcoefficient.Thesevaluestakeaccountof the ageof loading but. somewhatillogically, the valuesgiven for UK outdoorexposureare allowedalso for indoor exposurefor Class1 and Class2 members.

Where necessary,the creepcoefficient may be estimatedfrom Figure 7. I of Part 2.In this case.it will be appropriateto allow- for the effect of the reducingstressin theconcreteowing to the lossof prestress.In somecases,it may also beappropriateto takcaccountof stressreductionsdue to the applicationof superimposeddead load and/orpart of the imposedload. an I

Handbook to 8S8110:1985 [I1600 F

CREEP x 106

30 years

Figure H4.16: Influenceof typeof aggregateon creep.

4.8.6 Draw-in during anchorageWith wedge-anchoragesystems.a loss of prestressoccursin the tendonsas a resultofthedraw-inof theanchoragecomponentsat lock-off. In thecaseof pre-tensionedtendons.the ‘loss’ is normally offset by ‘over-extending’ the tendonsduring the tensioningoperation.With post-tensionedtendonsit is possible.in somecases.to ‘recover the loss’by re-fitting the jack usinga specialbearingfoot to encircle the anchorage.Shims arethen insertedunder the anchoragewhilst holding the tendonat the requiredstressingforce. Normally,this is not aviableprocedureandthelossdueto draw-inof theanchoragecomponentsshould be allowed for in the design calculations.Typical values for thedraw-in for a particularanchoragesystemcanbe obtainedfrom the manufacturers.

The movementof the tendondue to draw-in during anchoragecausesa reversalofthe friction that develops in the duct during tensioning(4.9.3 and 4.9.4). For long [tendons,the movementdueto draw-in is takenup over a limited lengthof tendonwiththe greatestlossof prestressoccurringimmediatelybehindthe anchorage.

4.9 Loss of prestress due to friction

4.9.1 General IiiAttention is drawn to 8.7.5.4which indicatesthe requiredcorrelationbetweenmeasuredload andextension.Friction lossescanbe reducedundercertain conditionsby stressing ,...

from both ends.by over-stressingandthen reducingthe anchorageforce. by vibrating f~ -

thetendonorbeamduringtensioningor by usingawater-solubleoil in the duct(provided I...

that this does not affect bond after grouting and. hence.the ultimatestrengthof themember).

For long-spanprestressedbeams, low-friction saddlesor deviatorsmay be used at Lchangesin direction of the tendons;the tendonsare generally straight betweenthesaddlesand may be externalto the section.in which casethey passthroughductsonlynearthe anchorages.Underthesecircumstancesthe loss of prestressdueto friction maybe radicallydifferentfrom that indicatedin this clause.The coefficientof friction at the ~saddleswill dependon the type of tendon and its surfacecondition. on the type of -

deviator,and on whetheror not any lubricant hasbeenused.It has beenreported.for rexample. that the coefficientof friction when 28mm diameterstrandsweredeflected Lthrough an angleof I in 10 wasof the order of 0.08. Generally, the major part of thelossdueto friction occursin theductsnearthe endsof the tendons.Lossesdueto friction ~undertheseconditionsarebest determinedfrom tests. L4.9.2 Friction in jack and anchorage IIThe friction in thejack andanchoragewill varywith the prestressingsystembut is rarely Uamatterof concernfor thedesigner.sincethesupplierwill provideajack that iscalibrated

102

K K — K~KK — KK K — K K — — . — . KK . — 4 •KK.K

90 days 1 year 2

TIME UNDER LOADING

K IK K K . K K. —— 4~. •K4 — —4. K K K ---....~-: K K ;~••K ~ K-.K. KK — — ....~.K. K K K K

Part]: Section 4

to give a specified force at the duct side of the anchorage. However, it is important to

remember that this calibration will be based on the assumption that the equipment is

well-maintained and that the anchorages and ducts are properly aligned; otherwise. the

losses could be much higher.

4.9.3 Friction in the duct due to unintentional variation from the specified profile

4.9.3.1 General

4.9.3.2 Calculation of force

4.9.3.3 Profile coefficient

The value of K can vary considerably in practice depending mainly on the type of duct

and the method of support but also, to a lesser extent. on the ratio of duct size to tendon

size and on workmanship.

4.9.4 Friction due to curvature of tendons

4.9.4.1 General

4.9.4.2 Calculation of force

The duct profile normally comprises a series of parabolic curves, with common tangent

points where reversals of curvature occur, and short straight lengths leading to the

anchorages. It is useful to replace x/r~. by a. where a is the total angular change in

radians to the position at distance x from the jack. In this case, equations 58 and 59 may

be combined to give Px=Pae~~(Kx+~~Q

If (Kr+Ma) ~ 0.2, the equation may be simplified to the linear form

P~=P0[1—(JG+Ma)I where P0(Kr+j.~a) represents the combined loss due to friction in

the duct.

4.9.4.3 Coefficient of friction

The physical condition of the tendon and the duct is important. i.e. whether or not heavy

rusting or any sharp local distortions to the sheath are present. As indicated, it is possible

to reduce the stated values if special precautions are taken and if the values used are

based on tests: references 4.34-4.37 contain test data on both the coefficient of friction

and the profile coefficient for a wide range of practical cases.

4.9.5 Lubricants

4.10 Transmission lengths in pre-tensioned members

4.10.1 General

4.10.2 Factors affecting the transmission length

Transverse reinforcement will be necessary in the ends of pre-tensioned members to

satisfy the requirements of 4.12.7. In addition. if the beam is to be supported within the

transmission length, extra reinforcement will be required to carry shear; this should be

designed in accordance with 4.3.8. Generally, it is good design practice to be slightly

conservative in assessing the transmission length and the necessary transverse

reinforcement, particularly if the prestressed member is to be used in a composite

construction where the reinforcement will help in dealing with additional shear due to

differential shrinkage (5.4.6.4).

4.10.3 Assessment of transmission length

The recommended values for wire are based on both site and laboratory

measurements’

4381. For strand1439 441)), values based on laboratory measurements have

been increased to allow for variations that occur in practice.

103

Hanabook ~oBSSIIO:1985

4.11 End blocks in post-tensioned membersI-.

4.11.1 General rWhenconcentratedforcesare appliedto the endsof members.largeburstingforcesareinducedfor a distancealong the memberuntil the longitudinal stressprofile becomeslinear. Theseburstine forcesare usuallyconcentratedin the zonebetween0 -2Yo and2Y

0

of the endof the member.where2v0 is the side dimensionof the endblock.

ITThis force hasbeenconsideredin the pastboth theoreticallyandexperimentally.Thetheoriesconsiderpossiblew-avs in w-hich the longitudinal stressvariesalong the memberand deri~e the bursting stressas the accompanyingoutward stresscomponent.Theexperimentalmethodsusuallyinvolve the analysisof measuredsurfacestrainsandthese Chavesuggestedthat burstingstressesareslightly higher thanthe theoriespropose.Bothapproachesagreethat the major variable is the ratio of the side of the loadedareatothe side of the endblock. Table42. which gives the designbursting forces.is therefore

‘K

a compromisebetweenthe variousmethodsof end block design and gives valuesthatare betweenthe theoreticalandexperimentalconclusions.

References4.41 and 442 compare the various theories and the results of an

[experimentalinvestigationinto the problem. No detailed experimentalinvestigationshavebeencarriedout on the anchorageof very large tendons.When bearingplatesaregroupedtogether.a suitabledesignapproachis to treatthe

areaimmediatelybeneatheachendplate asa separateendblock andthen to link theseblocks together.This method of consideringthe end of th~ memberas a numberofsymmetricallyloadedprismsis presentedin references4.42 and4.43. Whenanchoragesare eccentricallylocatedon the end of the member. it is possibleto have high tensilestresseson the loadedareaof the memberand thesemay causespalling. This subject Lis discussedin reference4.44.

Distributions of both burstingand spalling stresses.determinedby a finite elementprocedure.for axial. eccentric and multiple anchorageson rectangularand I-sectionbeamsare show-n in references4.45 and4.46. Guidanceon the design of endblocks iscontainedin reference4.47.

4.11.2 Serviceability limit stateThe basis of thedesign is to use the tendonjacking load and to carry all burstingtensileforceson reinforcementactingat a designstressthat is limited to control cracking.

In the past.somedesign has beenbasedentirely on the results of the experimentalinvestigationsof the problem and. in conjunctionwith this. designershave used thehigherexperimentalforceswith the concretecarrying someof the burstingtensile forceto reducethe net amount of reinforcement.These methodscould still be used.buttensionshouldnot be carried in the concreteif the burstingforces are obtainedfromTable 47: nor should any theoreticalapproachto the problem that doesnot considerthe compatibility of transversestrains between the concrete and the end blockreinforcementbe used.

4.11.3 Ultimate limit stateWith unbondedtendons.over-loadingat an earlystagebefore any significant loss ofprestresshas occurredcould result in a force greaterthan the jacking load. The use ofthe characteristictendonforce ensuresan adequatepartial safety factor.

4.12 Considerations affecting design details

4.12.1 GeneralThe detailing rules for reinforcementare containedin 3.12 and,with the exceptionof3.12.5and3.12.11.theseshouldbeappliedwheneverreinforcementis usedin prestressedconcrete members. The rules i~iven here are additional and relate particularly toprestressingtendons.

4.12.2 Limitations on areaor prestressingtendonsWhen a beamcracks. tensionpreviously carried by both the steel and the concreteiS

I 04

L

K I— . — — K..— -44 ~ •..r. ~ -- . K . - K K -K . .... -K ~44— — — — 4 K 4 — — —4 — 4.4 — —— — . — .~ . . . K K — — K- .K~KKK - - -- K K K

71Part 1: Section4

now carriedby the steelalone. If the percentageof steelis very low, the steel may not

Fl be capableof carrying this additional forceandmay yield or rupture,causingimmediatefailure. A minimum amount of steel is thereforerequired to ensurethat the beam iscapableof carrying load aftercrackingandsoprovidea visualwarningof possiblefailureanssomemeasureof ductility.I If a beamhas a very high steel percentage.failure will also be less ductile as thestrengthof the beamwill dependon the concreteand failure will not be causedby yieldof the steel: failure could possiblyoccur before any cracking has takenplace.but this

I is unlikely becauseof the designmethodsadoptedin 4.3.4 and 4.3.7.This safeguardisparticularly relevantto compositeconstruction,where the prestressedunit might fail inthis way during erectionor constructionbeforethe in situ concreteis placed4

] The design ultimatemomentof resistanceshouldbe not less than:

= ~ -

wheref~.. is the prestressin the concreteat the extreme tensionfibre at a distancevfrom the centroidof the sectionof secondmomentof area1.

4.12.3 Cover to prestressingtendonsThe recommendationson cover in relationto durabilityandfire resistancerequirementsaresimilar to thosefor reinforcedconcretein 3.3.4 to 3.3.6: in practice. it will often berequirementsof fire resistanceratherthan durability that will control the cover to beprovided.

4.12.3.1 Bondedtendons

4.12.3.1.1 General

4.12.3.1.2Coveragainst corrosion. The exposureconditionsdefined in Table 3.2 areusedas the basisfor Table 4.8. which is essentiallyTable 3.4 with the minimum cementcontent takennot less than 300kg/in

3. This limit reflects the importanceof the cementcontent in protecting the steel, and the somewhatgreatersensitivity of prestressingtendonsto the effectsof corrosion,due to their generallysmall cross-sectionand highstresslevel. Seealso the commentaryon 6.2.

4.12.3.1.3Coverasfire protection. Table4.9 gives nominal coversto all steel to meetspecifiedperiodsof fire resistance.The format is similar to Table 3.5 for reinforcedconcreteandthe coversrelatespecifically to the minimumwidths and thicknessesgivenin Figure 3.2. See also the commentaryon 3.3.6 regardingcovers and anti-spallingmeasures.

4.12.3.2 Tendonsin ducts

4.12.3.3External tendonsIt shouldbe notedthat the cover addedto external tendonswill not in fact be put intocompressionby prestress.Somecompressionmay be inducedlaterowing to creepandshrinkagebut thismay not alwaysbe enoughto offsetthe tensilestrainsdue to imposedloading. It is essentialtherefore that cover provided in this context be thoroughlycompactedand that this concretebe anchoredto the prestressedmember(preferablyby reinforcement).The positioning of the tendonsand the shapeof the cross-sectionshould be so arrangedthat the influence of any transversecracking or longitudinalsplitting is kept to a minimum.

4.12.3.4 Curved tendons

4.12.4 Spacing of prestressingtendons

4.12.4.1 GeneralThe layout of prestressingtendonsshouldbe such thatthe concretecan be easilyplaced

I(15

Handbookto BS8IIO:1985 [71

prestressingtendons ntwo groups

(06

splitting

F

stress distrtbution

[due to prestress at theend of the transmissionlength

F

alternative rII .,~ type of splitting

if tendons aregroupedp

• horizontally

LW

[

splittingmost likelyto occurhere

.. . . . . ..• .. . .. . .

Figure H4.17: Splittingat endsofp re-tensionedbeams.

andthoroughly compacted.No generalrules can be formulatedbecausethe layout willdependverymuch on the type of sectionandon theamountof transversereinforcementprovided:it will alsodependto someextenton the methodusedfor vibrating theconcreteandon the type of tendonandanchoragesystemused.

4.12.4.2BondedtendonsWhere straight tendonsare groupedsome distanceapart in pre-tensionedmembers.

tensionmay develop at the end of the beamsbetweenthe groupsof tendonsas thepre-compressionspreadsout from being a seriesof point loadson the endof the beamto give a linear stressdistributionacrossthe sectionat the endof the transmissionzone.Figure H4.17 shows the areaswhere splitting is possible,the most likely spotbeing atany changeof cross-sectionin the depth of the member.Under thesecircumstances.stirrupsor helicesshouldbe usedto containthe tendonsat the endof the beamandtopreventsplitting from developing.This reinforcementshouldbe designedin accordancewith the specialistliteraturet4~and 4.49) andprovided over a distancealong the beamatleastequalto the total depthof the beam.

4.12.4.3 Tendonsin ducts

4.12.4.4 Curved tendons

4.12.5 Curvedtendons

4.12.5.1 General

The type of actionvisualizedin this clauseis illustrated in Figure H4.18(a).There mayalso be a risk of the side cover spalling in very narrowwebs and of the bottom coverspalling off where tendonsrun close and approximatelyparallel to the soffit of slabs.The manufacturers of most post-tensioning systems specify cover and spacingrequirementsfor their tendonsandductsandtheseshouldbe regardedas minima.

In general.where a numberof prestressingtendonsin the sameplanearecurvedinthat plane. the innermosttendonshouldbe stressedandgroutedfirst. Wherethis is notpossible.suchasin staticallyindeterminatestructures.then it may be necessaryto anchorthe tendonback into the compressionzone(as shown in Figure H4.l8(b)) for highlycurvedtendons.Considerationcouldalso be given to providinghelical reinforcementtocarry tensile stressesbetweenthe ducts.

The recommendationsin 4.12.5.2to 4.12.5.4are taken from reference4.50. Furtherresearchdataandsuggesteddesign rulesaregiven in reference4.51.

AK K .%•K -K..... K~K~K K

.4 .— KK K K .-.. K... K~— . .4 K — .~K — — —

--“4- K. ~KKK.K..KKKK1-.- KKK-K-KKE- - -- - -. ~K K KK K K K — K — K-~ . . K - — KK K — ~ KK K K K

‘-4---.-. K-K. -

Part I: Section 4

A

bursting crack

SEC flON A-A

Figure H4. 18: Bursting stresses from tendons with high curvature.

4.12.5.2 Cover

4.12.5.3 Spacing

4.12.5.4 Special measures to reduce spacing of ducts

4.12.6 Longitudinal reinforcement in prestressed concrete beams

4.12.7 Links in prestressed concrete beams

This clause catalogues the various situations in prestressed concrete design where

transverse reinforcement is required. The design and detailing~ of links for shear

considerations is governed by 4.3.8.7 to 4.3.8.10. If a pre-tensioned member is supported

near its ends, such that a considerable proportion of the transmission length (as

determined from 4.10.3) is within the span, the transmission length could be designed

as a reinforced concrete section in accordance with 3.4.5 as a conservative alternative

to the approach given in 4.3.8.4. The requirement for links to resist longitudinal splitting

forces at the ends of pre-tensioned members is dealt with in the commentary to 4.12.4.2.

4.12.8 Shock loading

As in reinforced concrete. the provision of transverse steel in major structural members

is considered to be good practice. irrespective of shear requirements and this is especially

so if the member has to resist shock loading. In general, for this situation, minimum

reinforcement requirements should be in accordance with 3.12.5.3. In post-tensioned

members, the ducts should be grouted.

REFERENCES

4.1 ANDERSON. A.R Lateral stability of lone prestressed concrete beams. Journal of the Prestressed

Concrete Institute. Vol.16. No.3. May-June 1971. pp-7-9. See also discussion by SWANNK R.A

Vol.16, No.6. November-December 1971. pp.85-87.

4.2 SWANN. RA. The lateral buckling of concrete beams lifted by cables. The Structural Engineer.

Vol 44, No.1. January 1966. pp-21-33.

4.3 BATES. s.c.c. Some experimental data relating to the design of prestressed concrete. Parts 1.

2 & 3. Civil Engineering and Public Works Review. Vol. 53. No627. September 1958.

pp.lOlO-lO12. Vol.53. No.628. October 1958. pp.1958-1961 and VoL53, No.629. November

1958. pp.1280-1284.

4.4 ABELEs. p.w Partial prestressing and its suitability for limit state design - The Structural Engineer.

Vol.49. No.2. February 1971. pp..67-86.

4.5 BEEBY. AW.. KEYDER. E. and TAYLOR. H.P.J. Cracking and deformation in partially prestressed

concrete beams. London. Cement and Concrete Association. January 1972. 26pp. Publication

42.465.

4.6 PANNELL. F.N The ultimate moment of resistance of unbonded prestressed concrete beams.

Magazine of Concrete Research. Vol.21. No.66. March 1969. pp-43-54.

4.7 PANNELL. FIN. and TAM. A. The ultimate moment of resistance of unbonded partially prestressed

reinforced concrete beams. Magazine of Concrete Research. Vol.28. No.97. December 1976.

pp 203-208.

4.8 LEONHAROT. F. Abininderung der Tragfahi~keit des Betons infolge stabforiniger. rechtwinklig

zur Drucknchtung angerdraehte Einlagen. pp7l-78. KNtTTEL. G and KUrFER. H eds.

Stahlbetonbau: Berichte aus Forschung und Praxis. Berlin. Wilhelm Ernst & Sohn. 1969.

Festschrift Ruesch.

lalA

IbI

107

Handbook to BS8IIQ.-i~S5 [~K K

4.9 REY\OLDS. oc. revisedby CLARKE. .L. andTAYLOR. H ~ Shearprovisionsfor prestressedconcrete [K

in the Unified Code CPI 10 1972. London. Cement and Concrete Association.Octobcr1974. l6pp. Publication42.500.

4.10 BLILDING REGLL~flO\S AD\ISORYCO\I\IrrTEE. Report by Sub-CommitteeP (high aluminacementconcrete).BRAC (75) P-U). Appendix K. 1975. F4.11 HAWKtN5. N.M. Theshearprovisionof AS CA 35—SAA Codefor prestressedconcrete.Institutionof EngineersAustralia. Civil EngineeringTransactions.VoLCE6. No.2.September1964.pp. 103-116.andUniversityof Sydney.Departmentof Civil Engineering1964.46pp.LS6681-

4.12 SOZEN. MA, andHA~K1N5. N.~. Shearanddiagonaltension.Discussionof a paperby ACI-ASCE

FCommittee326. Proceedingsof the AmericanConcrete Institute. VoLS9. No.9. Septeinher1962. pp1341-1S47.4.13 MACGREGOR. 1.0.. SOZEN MA. and SIESS. c.~. Effect of draped reinforcementon behavior of

prestressedconcretebeams.Proceedingsof the AmericanConcreteInstitute. Vol.57. No.6.December1960. pp649-678. r4.14 tHE CONCRETE 5octEt4~ Flat slabs in post-tensionedconcretewith particularregardto the useof unbondedtendons— designrecommendations.ConcreteSocietyTechnicalReportNo.17.1979. l6pp. r..4.15 THE CONCRETE soct~-r~. Post-tensionedtiat-slabdesign Handbook.ConcreteSociety TechnicalReportNo25. 1984. 44pp.

4.16 REGAN. ~E. The punchingresistanceof prestressedconcreteslabs.Proceedingsof the Institutionof Civil Engineers.Part 2. Vol.79. December1985. pp657-680.

4.17 BEN\E~I~C. E.\.. Thedesignof prestressedmemberssubjectedtoaxial forceandbending.Concrete Fand ConstructionalEngineering.Vol.61. No.8. August 1966. pp.267-274.4.18 zt~.~ andMOREADITH. F L. Ultimate loadcapacityof prestressedconcretecolumns.Proceedings

of the American ConcreteInstitute. Vol.63. No.7. July 1966. pp767-788.4.19 BROWN. Ki. The ultimate load-carryingcapacityof prestressedconcretecolumnsunderdirect [and eccentricloading. Civil Engineeringand Public Works Review. Vol.60. No705 Aprtl1965. pp539-541.Vol.60. No.706.May 1965.pp683-687.Vol60. No.71)7.June1965. pp.84l-

4.20 HR. andHALL. As. Testson slenderprestressedconcretecolumns. Detroit. American FConcreteInstitute. 1965. pp.192.204.SP-l3.

4.21 ARONI. s. Slenderprestressedconcretecolumns.Proceedingsof the AmericanSocietyof CivilEngineers.Vol.94. No.5T4. April 1968. pp.875-904.

4.22 CEDERwALL. K.. ELFGREN. L. and LOSBERO. A. Prestressedconcretecolumnsundershort-timeandlong-time loading. Goteborg.ChalmersUniversity of Technology.1970. l6pp. Publication70:3.

4.23 KIRRBRIDE. T.w, Review of acceleratedcuring procedures.PrecastConcretc. Vol.2. No.2.February 1971. pp.93-106.

4.24 FEDERATION INrERNA-rIONALE DE LA PRECONThAINTE. Accelerationof concretehardeningby thermalcuring. FTP Guide to Good Practice.1982. I6pp.

4.25 BANNISTER. IL. Steel reinforcementand tendons for structuralconcrete.Part 2: tendonsforprestressedconcrete.Concrete.Vol.2. No.8. pp.333-342.August 1968.

4.26 BATE. 5cc..CORSON. RH. and JEFFS. AT. Prestressingnuclearpressurevessels.Engineering.Vol.197.No.5111.3.April 1964. pp.492.495.Also Building ResearchStationCurrentPaper.Engineeringseries 12. 1964. 6pp.

4.27 BATE. s.c.c. and CORSON. RH. Effect of temperatureon prestressingwires. Conferenceonprestressedconcretepressurevessels.London. March 1967. London. Institution of CivilEngineers.1968. pp.237-24~.PaperNo.21.

4.28 CAHILL. r. andBRANCH. GD. Long-termrelaxationbehaviourof stabilizedprestressingwiresandstrands.Conferenceon prestressedconcretepressurevessels.London.March 19fi7. London.Institutionof Civil Engineers.1968. pp.219-228.PaperNo.19.

4.29 ~BRAM5. ~ts. andCRLZ. C.R. The behaviourat high temperatureof steel strandfor prestressedconcrete.Journal of the PCA Researchand Development Laboratories. Vol.3. No:3.September1961. pp.8-19. L4.30 BANNISTER. IL. Steel reinforcementand tendonsfor structural concrete. The ConsultingEngineer.Vol.35. No.2. February1971. pp.81)-90.

4.31 NEVILLEAM. Creepof concrete:plain, reinforcedandprestressed.Amsterdam.North-HollandPublishingCompany. 1970. 622pp.

U4.32 E\ ANS. R.H. andKONG. F K. Estimationof creepof concretein reinforcedconcreteandprestressedconcretedesign.Civil Engineeringand Public Works Review. Vol61. No.7)8. May 191,6pp593-596.

4.33 THE CONCRETE SOCIETh - The creepof structuralconcrete.ConcreteSociety Tcchnical Paper. LNo.101. 1973. 47pp.4.34 COOLEY. E.H. Friction in post-tensionedprestressingsystems.London. Cementand Concrete

Association.1953. S7pp. Publication41.001.4.35 ‘vv~i-r. K.J. Measurement of friction in corrugatedcurved prestressingducts. SvdnL~.

LCommonwealthExperimentalBuilding Station. 1964. l7pp. Technical Record52:75:3224.36 CO’IMIsSIE ‘.OOR LITNOERING vAN RESEARCH INGESTELD DOOR DE BETONvERENI(;INc,. Frictional los~cs Ifl

prestressingtendons.(in Dutch). The Hague.1968. 6lpp. ReportNo.30.4.37 ~si-t~io% o~ CI\ IL ENGINEERS. Conferenceon prestressedconcretepressurevessels.London.

March 1967. London. Institution of Civil Engineers.1968. Group E: Propertiesof materials(Prestressingtendon~l. Papers22-27. pp.251.3(x).

4.38 BASE. GD. An in’estigation of the transmissionlength in pre-tensionedconcrete.London.

L

K K — — K K K K

K.. - K I~ -. - -K K~.KK - - -- - . - K K -

Parr I: Section-4

Cementand ConcreteAssociation.195S. 29pp.Publication41.005.4.39 BASE. G.D. An investigationof the useof strandin pre-tensionedprestressedconcretebeams.

London.CementandConcreteAssociation.1961. l2pp. Publication4L01 1.4.40 MAYFIELD. a.. DAVIES. 0. andKONG. F.K. Sometestson thetransmissionlengthandultimatestrength

of pre-tensionedconcrete beams incorporatine Dvform strand. Magazine of ConcreteResearch.Vol.22. No.73. December1970. pp.219-226.

4.41 ZIELINSKI. I. and ROWE. RE. An investigationof the stressdistribution in the anchoragezonesof post-tensionedconcretemembers.London.CementandConcreteAssociation.1960.32pp.Publication41.009.

4.42 ZIELIN5Ki. i. and ROWE. R.E. The stress distribution associatedwith groups of anchoragesinpost-tensionedconcretemembers.London.Cementand ConcreteAssociation.1962. 39pp.Publication41.013.

4.43 GLYONY. Prestressedconcrete.NewYork. JohnWilev&SonsInc. 1960, VoLI. S59pp.Vol2.74lpp.

4.44 LENsCHOW. RI. andSOZEN. M.A Practicalanalysisof the anchoragezoneproblem in prestressedbeams.Journalof theAmerican ConcreteInstitute. Vol62. No.11. November1965. pp.1421-1439.

4.45 YE-I-rRAM. AL. andROBBINS. K. Anchoragezonestressesin post-tensionedmembersof uniformrectangularsection.Magazineof ConcreteResearch.Vol.21. No.67.June1969. pp.103-112.

4.46 YEfl~RAM. A.L. andROBBINS. K. Anchoragezonestressesin post-tensioneduniform memberswitheccentricandmultipleanchorages.Magazineof ConcreteResearch.Vol.22. No.73.December1970. pp.209-218.

4.47 CONSTRUrrION INDLSTRY RESEARCH AND INFORMATION ASSOCIATION. A guideto the design of anchorblocksforpost-tensionedprestressedconcretemembers.CIRIA Guide 1. June1976.34pp.

4A8 ARTHUR. PD. and GANGULI. s. Testson end-zonestressesin pre-tensionedconcreteI beams.Magazineof ConcreteResearch.Vol.17. No.51. June 1965. pp.85-96.

4.49 KRISHNAMURTHY. o. Designof end zonereinforcementto control horizontal cracking in pre-tensionedconcretemembersat transfer. Indian ConcreteJournal. VoL47. SeptemberandOctober 1973.pp.34-6-351and 379-385.

4.50 DEPARThIENT OFTHE ENvIRONMENT. Prestressedconcrete curved tendons.London. Departmentof the Environment.August 1969. 3pp. Interim Memorandum(Bridges) 1M2.

4.51 MCLEISH. A. Burstingstressesdue to prestressingtendonsin curved ducts.Proceedingsof theInstitution of Civil Engineers.Part.2. Vol.79. September1985. pp.605-615.

I 1)9

F’SECTION FIVE. DESIGN AND DETAILING:PRECAST AND COMPOSITE CONSTRUCTION r5.1 Design basis and stability provisions F

5.1.1 General

5.12 Basis for design

5.1.3 Handling stresses [5.1.4 Compatibility

5.1.5 Anchorageat supports

5.1.6 Joints for movement

5.1.7 StabilityThis section is emphasisedbecause.often, precastunits designedby oneengineerareincorporatedinto a structuredesignedby another.It is most likely that the engineerinthe chainof authoritycloserto the eventualclient will haveresponsibilityfor the overallstability of the structure.

The requirementsof 3.1.4 with regardto the provisionof tie forces,the importanceof the layout of the structurein plan.andthe possibleprotectionof thosemembersvitalto stability apply equally to precastandcompositeconcreteconstruction.The detailing rrulesof 3.12shouldbe usedwheneverappropriate;5.3.4gives someadditionalrules for [anchoringandlapping barsmorerelevantto the specialproblemspresentedby precastconstruction.

Bars which areusedto provide the tie forcesrequiredin 3.12.3shouldbe positioned [anddetailedso that they have the necessarycover to enabletheir full strengthto bedeveloped.If tiesare to be provided by lappedbarsin narrow spacesbetweenprecastunits. attention should be paid to the requirementsof 5.1.8.2. In complyingwith thevertical tie requirements.lifting and levelling bolts may be used to form part of thiseffectively uninterruptedtie.

For buildings supported by plain concretewalls, the vertical tie requirementsare

[K.~~satisfiedif the tie is ableto carry the deadandlive load from the floor above.Wherethis is notso. orwherein any structureof five or morestoreysthe requirementSof 3.12.3.7are not met.Section2.6 of Part 2 permitsan alternativeapproachto design.This is the ~alternativepath’ approach,where,for eachstorey in turn. the notional (removal of any single vertical load-bearingmemberis considered,and the structurecheckedto ensurethat the loads can still be carried by catenary,cantileveror someotherformof structuralaction.Any buildingcomponentthatisnormally notload-bearing Lmay be taken into accountand the Ym valuesshouldbe takenas 1.3 for concreteand1.0 for reinforcement.There are limitless possibilitieshere over the rangeof typesofstructure(and their usage)coveredby this Code andthe valueof the loadingis left tothe discretionof the engineer;in general,all permanentloadswould be consideredand Lsome fraction of imposedloading— this will dependon usageandspecialconsiderationmay haveto be given to warehouses,plant rooms.etc. 2.4.3.2gives someadviceon thissubject.Only rarelywill it be necessaryto considerdebrisloading,becauseof therelative Imagnitudesof the safety factors for normal and exceptionalloads andalso becauseofthe tie force requirements.

The ‘alternativepath’ methodoutlinedabovewill be the mostappropriatefor precastconcretestructuresmadeof load-bearingconcretepanels.For this reason,a definition Lis given of what constitutesa single load-bearingelement.This involves the furthernecessityto definea ‘lateral support’ in 2.6.3.2.2of Part2: this maybeeitherasubstantialpartition at right-anglesto the wall being consideredandtied into it or. alternatively,a Lnarrow width of the wall itself which hasbeenlocally stiffenedandis capableof resistinga specifiedhorizontalforce.

26 of Part 2 introducesa secondalternativedesign approach.againonly for thesituationwhere3.12.3.7is not compliedwith. Insteadof a vital structuralmemberbeing

I It) LK K~ —— . — - . .

K IK — — K K. — K K — K- - K K -- - - 4 - -

Parr I:Section5

consideredto be renderedineffective, it is proposedthat the designis satifactoryif the-membercan resist a pressureof 34kN/m, the y factors being as describedfor thealternativepathapproach.This approachis thereforeattemptingto quantifythe effectsdue to exceptionalloading, but at the sametime a minimumtying togetherhasstill tobe provided since 3.12.3 mustbe compliedwith. In practice,this will generallybe the

mostappropriatealternativeapproachfor columnsin framed structures.It shouldbe realisedthat only in exceptionalcaseswill structuresrequire other than

the provision of ties. Key elementsdefined in clause2.2.2.2will be identified from astudyof the structuralschemeand2.2.2.2(d)makesit clear that in most circumstancesvertical tying of the structurewill be the normal design solution.

Key elementswill only be met in thosestructureswhere thereis an exceptionalandunavoidabletendencyfor more than the local areaaround the elementto collapseinthe eventof accident.

5.1.8 Stability tiesThesemaybelocatedin anypartof thestructureprovidingthat theyinteractproperly.

5.1.8.1 Tiesgenerally

5.1.8.2 Connnuitv of tiesThe categories(a) to (d) give guidanceon how tiesare to be provided. Othermethodsmay be developedbut the importantprinciple is that the tie should be provided in anidentifiableposirive way.

5.1.8.3Anchoragein tall structuresThis requires that in buildings of five or more storeysall precastmembersmust beanchoredto the tied part of the structure.This is to avoid excessivedebrisloading inthe eventof an accident.

5.1.8.4AvoidanceofeccentricityThis is to ensurethat, in the caseof accident,tieswhich haveto straightendo not allowunits to fall from bearings.

5.1.9 DurabilityIn this respect.connectionsshouldbe robustandshouldbe filled with good qualitygroutor well compactedconnectionconcrete.The provision of caps or sealsto sensitiveconnectionson the peripheryof a structureshouldalso be considered.

In preparingthe design.detailing andspecificationfor connections,the difficulty ofachievingon site the intended quality in relation to dimensionsand in situ concreteshouldbe taken into account.

5.2 Precast concrete construction

5.2.1 Framed structures and continuous beamsIt is in generalmore difficult to provide full momentcontinuity in precastconcreteconstructionthan in in situ structuresbut, where this is to be the basisof design.thenthe proceduresgiven in Sections3 and 4 in the Code may be adopted.including theredistributionof moments.Redistributionmay be particularly useful in reducingdesignmomentsat connections.

5.2.2 Slabs

5.2.2.1 Designof slabsAgain the basisfor analysisanddesignof precastslabsshouldbe thatgiven in Section3 or 4 as appropriate.

5.2.2.2 Concentratedloadson slabswithout reinforcedtoppingThis clausemakesempiricalrecommendationson the width of a slab (perhapsmadeup

111

[IHandbook (0 BS8IIO:19&5

of anumberof precastunits) whichcanbeconsideredto be helpingto resistconcentratedloads, including line loads from partitions in the direction of the span.The type ofpartition will haveaconsiderableinfluenceon the waythe loadisdistributedtransverselyacrossthe slab: moreover,the type andwidth of the precastunits and the connectionbetxveenthem canhavea considerableinfluence.A limited amountof testinghasbeencarriedout on a rangeof standardfloor unitsandgenerallythis hasshownthat the actualtransversedistribution can be defined accuratelyby meansof the load distribution orgrillage analysis for bridge decks. which are in common use in this country. If. in aparticular case, a more accurateassessmentis required than is given by this clause.references5.1., 5.2 and5.3 should-beconsulted.

IMany manufacturerswill haveresultsfrom load testson their units in structuresandtheseshouldbe available for guidance.

5.2.2.3 Concentratedloadson slabswith reinforcedtoppingThe commentsto 5.2.2.2apply herealso.

5.2.2.4Slabscarrying concentratedloads C5.2.3 Bearings for precast membersThe definitions in 1.2.5 are importantand havespecific meaningswhen used in thefollowing clauses.

5.2.3.1 General NThis section comes from the work of a Committee of the Institution of StructuralEngineers(S.4t.The clausesdo not requirethat thereis a definite checkon theprovisionof overlapof reinforcement(a); it is clearly impossiblein bearingson brickwork etc. CTheuseof theclauseswill howevergive overlapwhereit is appropriateto beprovided.5.2.3.2 Calculation of net bearing width of non-isolatedmembers

IWhenassessingtheeffectof potentialrotation.therestraintandsupportof thesupportingmembershouldbe consideredwhenassessingits likely rotation.

5.2.3.3Effective bearing lengthFigure 5.4 is in error in that the vertical dimensionin the lower part of the figure isshown asbearingwidth whenit shouldshow bearinglength.seedefinition 1.2.5.5.

5.2.3.4 Designultimate bearingstress IThe requirementto rely upon the weakerof the bearingsurfacesis clearlyimportantifthe bearinglength of the supportedmemberis similar to the available length for bearing

[of the supportingmemberand vice-versa.Where for exampleone memberis narrowwith respect to the other,higherbearingstressesin the wider member.subjectto testor the provisionsof reinforcementto preventbursting,basedon clause4.11 in Part 1.

Liwill be appropriate.Higher bearingstressesthan0.8f~ may be usedwhen justified by tests.References5.5 — 5.9 providedataon bearingstresses.

5.2.3.5 iVet bearing width of isolated members I5.2.3.6 Detailing for simple bearingThis refersfonvard to Clauses5.2.3.7and5.2.4.

5.2.3.7Allowancesfor effectsof spallingat supportsPlasticload sheddingpacksare now availablewhich reducethe effectsof the problett3.describedin 5.2.3.7.4.

5.2.4 Allowance for construction inaccuracies U

112 5.2.5 Bearings transmitting compressiveforces from above u— . — — 4 K

IK4K ~4K4K4 •4~ 4K ..K. — K — — . .— 4— .— ..KKKK K.. K K K

Pan]: SectionS

5.2.6 Other f~rces at bearings

52.6.1Horizontal forcesat bearingParticularattentionshouldbe placedon the detailingof bothsupportingandsupportedmember. Continuity reinforcementmust be anchoredto both membersin such a wayas to avoid planesof weaknessaway from the support.It should be realisedthat theprovision of tensilerestraintwill renderboth supportingandsupportedmemberproneto tensioncracking.Reinforcementshouldbe providedto minimise crackwidths in thisregardsinceit canbea seriouscauseof failure if properprovisiontocontrolanddistributecracksis not made.

It will often be sufficient, insteadof providinga full sliding bearing(a), to provideaflexible bearingwhich allows sufficient capability to move laterally.

5.2.6.2Rotation at bearing offlexural membersThe useof suitableelasticbearingmaterialswill do much to distributeandsmoothoutthe bearingstresses.

5.2.7 Concrete corbels

5.2.7.1 General

5.2.7.2 DesignThe essenceof the designmethodrecommendedfor a corbel is the assumptionthat itbehavesasasimplestrut-and-tiesystem,asindicatedin FigureH5. 1 for loadsappropriateto the ultimate limit state.So that it can function in this way, it is first necessarytoeliminatethe possibility of a shearfailure and5.2.7suggeststhat the total depthof thecorbel (h) be determinedfrom shear considerationsin accordancewith 3.4.5.8.Thecorbelwidth (b) will normally be determinedfrom practicalconsiderationsandthe sizeof the bearingplate transmitting the ultimate load (Va) to the corbel should then becalculatedby using a bearingstressnot greaterthan 0.8f~, as suggestedin 5.2.3.4,providedthat it may be shownthat the horizontalforceat the bearingis low (lessthan0.1 Va).

V.

-1

1A

I

0.9x0.45 4~ cos (3

0.9x cm tcl force diagram

The requirementsof 5.2.7for the proportioningof the corbelandthe detailingof thereinforcementareillustrated in FigureHS.2. Of the threemethodsshownfor anchoringthe main tensionsteel (A~1) at the outer face of the corbel, that in diagram (a) is themostefficient technically. It alsohas somepracticaladvantagesin that the ratio a~/d ishigher than for the other two methodswhere the requirementsof 3.12.8.24regardingthe minimum radii of bendshaye to be met for the main tensionsteel.

For higher a~/d ratios, design will be controlled principally by flexure at sectionA-A (Figure H5.l). Particularattentionhasto be paidto the occurrenceof horizontalforcesatthe bearing,sincethesecan considerablyreducethecorbelstrength;thisproblemis discussedanddealtwith in reference5.10.

5.2.7.2.1 Simplifying assumptions

d

Figure HS.1: Designbasisfor corbels (5.2.7.2).

113

Ha,tdbookto BS8I/O:]985

L-‘ ba

II (A.,)

I, I!Cal ~ —

.....~..L....... outside edge of bearing I-I - tobe kept clear of bend in

I main reinforcement (minimum~ V~, ctearance=1 bar diametBr)

A..Ii _____________________________________

__ I

DETAILING RULESh d

Ii) h~<O.5 h

K’ (4) Otherdetails as ~er diagrams

—~ ~-

A., fl~ma Cn re nforcement in theform of hot zontal loops

II . bars provided to anchorII(cL... L~ II horizontal stirrups IFigure ff5.2: Possiblemethodsofanchoringmain tensionreinforcementin corbels.

I

5.2.7.2.2 Reinforcementanchorage

3.2.7.2.3 Shearreinforcement

5.2.7.2.4Resistanceto appliedhorizontalforce [5.2.8 Continuous concretenibsReference5.11 has a considerableamountof informationon the reinforcementof nibs.The designeroften hasto considerthe strengthof continuousnibs supportingregularly Lor irregularly spaceddiscreteloads. e.g. from doubletees.In this condition. a globalcase.supposingfailure of the completenib in bendingandshear.as well aslocal bending

Land shearfailure modesbeneaththe discreteloads,shouldbe considered.

5.2.8.1 GeneralSeeabove. I5.2.8.2Area of tension reinforcementSeeabove. L5 ‘8 3 Positionof tension reinforcementSeeabove.

1 3 4 DesignshearresistanceII! C

K. ~ K--K K -K ~ K4~~K K K — K K K K — — KK4

4~ — — K — —K-K...... K K K K K K : K :‘~ :~- K :4”’~.. ~

--K ~ - •KK~4~4-- - - K K K ~K- “K - -. -. - .. - -—— K — K• .:‘~. — .~ . . —

Ib)

K.—~ -‘K----. K: K K •~ - ‘. . ~•: ~ —. .:- - •.-~ ~ ~ ;~ - -K - -. K K K.—

Parr I: SeCtion .~

5.2.8.5Links in the memberfrom which the nib projectsThis Thang up steeV is importantand shouldalso be consideredin any design whereprimary beamsare loadedaway from supportsby secondarybeams.

5.3 Structural connections between precast units

5.3.1 GeneralIt is necessar~-to considerall aspectsof joint requirementsat an earlystagein design.Note the emphasisplacedon the needto considerboth strengthandstability during theconstructionand erection stages.Severalfailures have occurred in precastconcreteconstruction in the past. becauseof lack of considerationof the erection procedureprevailingon site.

5.3.2. Design of connection

5.3.2.1 MethodsThe useof testswill bequite common.It is difficult to justify all the designrequirementsof joints. anchorage.bursting,bearing.etc.,with the designdatacurrently available.Inmany cases.compliancewith thesedatawill result in a cumbersomeand heavydesign.Although it is not commonlythought necessary.the same degreeof attention given tothe analysisin detail of in situ connections,e.g. beamto column joints, will give similardifficulties of justification. A testjustification will be of greatassistancein proving thecorrectinteractionof the internal memberforce lines.

5.3.2.2Manufacturing and constructionA designer familiar with the processesand techniquesof precasting will consider thepoints mentionedin the clause. Advice should, in other cases,be sought from anexperiencedmanufactureror. at the tender stage. his input should be welcomed.Reference5.12 gives manysimple tips for good detailing.

5.3.2.3 Designfor protectionSee5.1.9.

5.3.3 Instructionsto siteThe strengthandstability of precastconcretestructuresgenerallydependverymuchonthe connectionsand on the way they are made;mentionhas alreadybeenmadeof theinfluence that workmanshipand tolerancescan haveon strength.It is vital, therefore,that clear and detailed instructions,preferablyon the drawings,are passedfrom thedesignerto the manufactureranderectorto ensurethat the joints are adequatelymade.Instructionsshouldalsobepassedon with regardto thevariousfactorsmentionedin 5.3.2.

5.3.4 Continuity of reinforcement

5.3.4.1 GeneralThe engineer’sattentionis drawn to the necessityto establishclearly what function isexpectedfrom eachjoint generallyand,in this particularcase,what is the function otthe reinforcementthroughthe joint: i.e. is it to providefull momentcontinuity, or partialcontinuity in helping to control crackingand deflection,or simply part of the tie forcerequirementsof 5.L8. Variousacceptablewaysof achievingcontinuity are listed: theseare coveredin detail in subsequentclauses,but in generalit shouldbe noted that thedetailing rulescontainedin 3.12shouldbe obeyed.

5.3.4.2Lapping of barsFor flexural memberswhere continuity of reinforcementis required through a jointbet~veenprecastfloor slabsor beams,the mostcommondetailsare illustratedin FigureHS.3. For details (a) and (b), 5.3.4.2statesthat the lap requirementsof 3.12.8shouldbe satisfied The biggestproblemhereis thatvertical links will be requiredin the support

115

(IHandbook to BS8IIO:19&

4 rbars projecting from

extra bar precast units arelapped by means of

tap length lap length extra bars— I —

M M____________________ r

(a) ~-~< \ -. — precast- unit

in situ concrete bars projecting fromprecast units are laoped

lao length with one anotherM M

(bi

lateralreinforcementM M

(c)

-~ •.<~flF~TT

vertical loops proiect,ngfrom precast units

—~

in situ concrete

horizontal loops projectingfrom precast units

(dl

6

LV

= 144

‘b 1 Bib

M M

1~-->~~

vertical stirrups cantreline of suoportrequired section subiected to

moment M

Figure HS.3.’ Methodsofprovidingcontinuityofrein forcemenrforprecasz’floorsandbeams.

zonegenerallyand the main reinforcementmust be containedwithin these:this cancreatepracticalproblemson site in achievingthe requiredlap lengths.astheselinks willbe cast into the precastunits.

Details (c) and (d) show in generalterms the more popularalternativesolution ofachievingcontinuityof reinforcementthroughloops. In general.detail (c) is preferableto (d) both on technicalandpracticalgrounds(seereference5.13).The bearingstressesinside the loops have to be limited in accordancewith 3.12.8.25.To comply with thesebearingstressesunderthe ultimate loads,empirical rules basedon test data aregivenin Figure H5.4 for proportioningvertical loopsin flextural zonessuch as thoseshownin Figure HS.3. If h

0 is lessthan 14 b. straightportionsshouldbe addedto the loopstotransfersomeof the tensionforce in the barsby bond.Lateral reinforcementof the typeidealizedin Figure HS.3(c)shouldbe provided throughthe loops,anda loop from oneprecastunit shouldgenerallybe adjacentto a similar loop from the otherunit. i.e. theloopsshouldnotbe toostaggeredacrossthe width of thejoint. Thereis no clearevidencethat the presenceof lateral reinforcementin the form of dowels,as shown in FigureH5.3(c),enhancesbearingcapacityby morethanabout15—20%.butit shouldbeincludedinanycase,partly tocomplywith5.1.7andtoensuremoreductilebehaviournearfailure.

It isnecessaryto roughenthesidesof substantialgroutpocketsintowhich reinforcementis to be embedded:this is to guard againstthe tendencyof the grout to shrink awayfrom the sides of the pocket: an exampleof where this would be necessaryis shownin

‘—no~~vnere n< 141 L

l~—6 130—ni fl

Figure HS. 4.’ Empirical detailing rules for achievingcontinuiri ofreinforcementwith

116 vertical loop bars.

-.K.. :‘-.‘ i--K-’- ..,~ -.-:.‘“.. ~.4444. -K K K K — —-K-K.... ~

- K. - ‘•~:. - ~ . . - . K

Parr]: SectionS

longitudinalcolumnresnforcement

sides of recessto be roughened

mortar joint

Figure HS.S: Exampleof wherea grout recesswouldneedits sidesroughened(a jointbetween two large columnssubjectto mainlyaxial load).

reinforcing barn compression

lateral tensilestress

bar bucklesOutwards

high compressive stressunder bar because of end

(al bearing (bi

upward reactionon bend

Figure HS.6: Problemsat endsof compressionbars (a) stressdisribution in concrete,(b) effect of bendingor hooking compressionbars.

FigureHS.5. A point to notefrom Figure1-15.5 is the way in which the longitudinal barshavebeendetailedat the endof the column.There is sometest evidenceto indicatethat the load transfermechanismbetweenbars in compressionand the surroundingconcreteis partly by bondandpartly by endbearingtS’4).This causeshigh compressivestressesimmediatelyunderthe barandlateral tensilestresseswhich can lead to verticalsplitting (FigureH5.6). The problemis madeworse if thecompressionbar is hookedorbent(Figure HS.6) becausethis increasesthe tendencyfor the barto buckleoutwards.The bestsolutionis thatadoptedin FigureHS.8;thisproblemis dealtwith in considerabledetail in reference5.14.

5.3.4.3 Reinforcementgroutedinto apertureFigure HS.5 gives an examplewherea tensioncapacitymayneedto be considered,tocomply for examplewith the tie requirementsof 3.12.3.7.

5.3.4.4 OverlappingreinforcementloopsSee5.3.4.2.

5.3.4.5 SleevingThe two typesof sleevingpermittedby 5.3.45areshownschematicallyin FigureH5.7.Researchhasbeencarriedout on the type (1) sleeve,whoseprincipal advantageis the

117

grout hole

Handbookro BS8IIU:I9.~§

A

0-

4@

- bar heldbywedging

anchoragelength foundfrom testdata

sleeve

deformedreinforcingbar subiectto compressiononly

[IC

sleeve

resin or mortargrout

_____ deformedreinforcing bar

VA(b(

a,

Figure H5 7: T~’pesof connectionreferredto in 5.3.4.5— (a) compressionsleeve.(b) co,npressionand tensionsleeve.

threadedanchor

reinforcing bar

Figure HS.8: Examplesoftypesoft/treading referredto in 5.3.4.6.

low anchoragelength achievedby someresin mortars(see references5.16 and 5.17);the designof this type of connectioncan be carriedout only by referring to the testdata 3)3 ~ind5.13.-S [SI However,the fire resistancerequirementmay precludethe useofresin mortars.Commercialmechanicalsplicesof type (2) arecurrentlyavailable.all ofthem backedby test data.The manufacturer~sinstructionsandthe recommendationsof5.3.4.5shouldbe followed carefully in designingboth typesof connection.

5.3.4.6 Threading of reinforcementExamplesof the three methodsreferredto in this clause for using threadedbars areshown in Figure HS.8. The first two methods(parts(a) and (b) respectivelyof FigureHS.8) are generallyused for joining precastcolumns, wherethey have the practicaladvantageof cetting the units off the cranequickly. Testshavebeencarriedout to studythestrengthof this typeof connectionwhenthecolumnsaresubjectedto axial IoadslSKbl.

These are by no means the only methodsof joining precast columns. Threadedanchoragescanobviouslybe usedin a wide rangeof situationsin precastconcretework:Figure H5.S merely shows one possibleapplication. In general.the strengthof suchaspecialconnectionshouldbedeterminedby testsandby referenceto Clause3. 12.8.16.2.

5.3.4.7 Strength of threaded couplingsSeeabove.

5.3.4.8 Welding of barsSomeuseful informationon particularusesof weldingin precastconcretework generallycan be found in references519—5.21

I

C[

Li

L

I

I

UK. ~KKKKK K K4K~K.~K~~ -K--K-K-K.K.--K-KK— I - K KK KKK

K — K K — K — K K K K K — K K K K — K —— — — 4 K K — — K K — K K — K

— K — — K —4 K K— K K... —. — — — — —

bars may be ______

just touching r

in situconcrete

(a)

IIII

IIIIii

II

K K I- - -

---- ... . -- - ~K•K K. K.. K K -

I Part!: SectionS

NB Load fromsupported memberis generallyapplied throughbolts, locatedover depth of

Ia) AELIVAT~ insert

columnI conventional I~ I

I IINB Load (I )~ 7column l~from the ~““.‘—‘4~ reinforcementsupported r steel isertmember is iLl I [7””~”T (billet. r.s.j.,generally L etc.) Whichapplieo ontop ot nsert i; Ii . is wide in

relation to theI insert may column

K? Ii~r.nnnr~ protrude from tr~

Jill) one or bothcolumn faces —

(bi END &IJVArcN WOE IL5VA11~

Figure HS.9: Basic typesof connectionusingstructural steelinserts.

5.3.5 Connectionsother than those involving continuity of reinforcement

5.3.5.1Joints with structuralsteelinsertsA cleardistinction has to be drawn at the designstagebetweenthe two basic typesofpossibleconnection.Theseare illustrated in Figure HS.9 which showsa narrowplateembeddedin a column, to which asmallerplate in the endof the supportedbeamwillbe boltedorotherwiseconnected.In consideringthedesignof this connection,it is bestto refer to the specialistliterature(notablyreference5.22); the behaviouris somewhatanalogousto the behaviour of end blocks with low Ypo/Yo ratios, in that it is dominatedby the tendency of the narrow steel plate to split the column. However, it has beenshown15’~1that an endblock approachis not very successfulfor designas the strengthis also influenced by shrinkageeffectsat the bottom of the plate and by the preciselocationof transverselinks in the column.

The behaviourof the more commontype of connectionshownin Figure H5.9 (b) iscontrolled by distribution of bearingstressesunderneaththe steelinsert. An excellentgeneraltreatmentof the designproblemsassociatedwith this type of connectionis givenin reference5.23which is basedon Americandesignpracticeandon certainsimplifyingassumptions.If the insert protrudesfrom two oppositesides of the column and theimposedloadson eachside areequal,design is relatively simple. since5.2.3.4permitsa uniform bearingstressof up to 0.8J~ underthe ultimate loads.Considerationshouldbe given to the loadsimposedon the concretein the column from the structureabovethe joint and, if designing to the upper stresslimitation, the provision of additionalcolumn links immediatelybeneaththe steelinsert shouldalso be considered.

The rigorousdesignof a single steelinsertof the type shown in Figure H5.9 (b) canbe complexbecausethe distribution of bearingstressin the concreteunderthe insert isnot known. However,a simpledesignsolutioncanbe obtainedfrom theassumedforcesystemshown in FigureHS.10 wherethe bearingstressesaretakento be uniform alongthe lengths l~ and 13. The values of V~, l~, (from 5.3.5.1) and b (from practical

considerations)will be known initially and l~ may be calculatedfrom:= vii

b1x0.8f~

SlOE ELF~~TAOS

This enablesthe steel insert to be designed(i.e. h, to be determined) to resist a119

Handbook to BSSJIO:I%S

centre-lineof maincolumn steel

structuralsteelinsert

~, ~ aoditional steel sect:on

provide more ~eanng

~ effective beanng wioth

area if reouireo: :riethen Decomes i2b.—x.l

sac-7~oN A-A

Figure H5. 10: Force systemfor design ofsinglesteelinsertsfor columns.

moment equal to V~(lt-~-l2): shearat the column face would also requirechecking.It isthen necessaryto calculateV-~ andL. This may be done by taking momentsabouttheline of actionof V., thus:

This gives a quadraticequationin terms of 13. The design max’ then be consideredsatisfactoryif l~<0.6(l~—1I2 13). i.e. if the bearingstressareasdo not overlap.Note that.in lightly loadedcolumns. it may be necessaryto provide the tying-down force V~ bywelding vertical reinforcing barsto the steel insert and anchoringthem in the columnunderneath.The width b will generallybe governedby practicalconsiderations:i.e. itmust fit inside the column reinforcementcageand allow the concreteto be placedandcompactedandthe width shouldnot exceedone-thirdof the column ‘vidth. If thereisthen insufficient bearingareato satisfy the inequality given above. additional bearingmay be provided by welding on additional steel sectionsas sho~vn in Figure HS.10(SectionA-A).

In practice,many typesof structuralsteel insert can be used and eachshould beconsideredcarefully in the light of the abovegeneralcomments.It is consideredthat.with many of these,higherbearingstressescould be carriedsuccessfully,especiallyifadditional column links wereprovided.

A commondesignproblemis that of a billet protrudingfrom eachsideof a columncarryingdifferent loads on eachside. This would occur if spansare not equalon eachside of the column.In this casethe stiffnessof the insert is of importanceandthe design

or slab

IF

1,

C

C

II2

.column or wail U

LL

effective joint area in bedding of mortar for precastcompression 6.3.6) floor unitsassumed to be not greaterthan 90% of the cross-sectional area of the wallor column

Figure 1-15.11: Effectivejoint area for compressionjoints f5.3.6~i.

LF

Ul1)

K - KK. . .~. . . - - ~. .. - K - . - - - — K

Part 1: Section5

problembecomesstaticallyindeterminate.Verystiff insertswill carrythe load in bearingwith avariablebut linearstressacrossthe column.-like a padfoundation.Flexible billetsmay imposeverticalbearingstressesin the upwarddirection in thecentreof the column.

More researchis requiredin this areaand designshould haveexperimentalbacking.

5.3.5.2Resin adhesives

5.3.5.3 Othertypes

5.3.6 Jointstransmittingmainly compressionThe strengthof this typeof connectionmay be assessedby the methodsgiven in 3.8 or3.9 asappropriate,providedthat a reducedeffectiveareais assumedfor the joint. Thereducedarea.assessedin accordancewith sub-clause(a), is illustrated in FigureHS.11.Where the precastfloor units intrude well into the joint and sub-clause(a) becomesrestrictive, thensub-clause(b) comesinto operation.but only where the floor or beamunits are solid over the bearingarea.

A detailedstudy of the factorsinfluencing the strengthand behaviourof horizontaljoints in compressionis given in references5.14and5.24.

In designinghorizontal joints at the top and bottom of walls or columns.3.9.4.18makesno specific recommendationswith regard to forcesacting at right-anglesto thewall. It is suggestedthat thereis no needto checkthe resistanceof suchjoints to lateralforces provided that the calculated horizontal shearis less than 25% of the normalcompressiveforceactingon thejoint. Wherethis is not so,it will benecessaryto considerthe designof the joint in accordancewith the provision of 5.3.7 (d).

5.3.7 Joints transmitting shearIn precastconcreteconstructionusually involving the use of fairly wide floor or wallslabs.the design conceptfor the structureas a whole may be such that a numberoftheseslabsmay be requiredto act togetherto transmit forcesin the planeof the unitsin the direction of their span:oneof the examplesquotedin the clauseis that of floorunitsactingasa wind girder in transmitting lateral loadsactingon the sidesof abuildingbackto an in situ concretecoredesignedspecificallyto carry these.

To achievethis full diaphragmaction it is necessaryto designthejoints betweentheunits to carry shearforcesactingin the planeof the units themselves.Five alternativemethodsof designare given in 5.3.7,oneof whichmust be followed. The methodsarepresentedin orderof increasingshearresistancerequirements:precastunitswith smoothsurfacesand no reinforcementacrossthe joint betweenthem are allowed to transmitonly very small shearforces [sub-clause(a)]. In sub-clause(c), steel provided fromconsiderationsof stability may be consideredto be sufficient to preventseparationofthe units. If reinforcementis providedso that it can developits designstrength.highshearscan be carried [sub-clause(d)]; in this case.too, accountmay be takenof an~’normal compressiveforce acting on the joint, thus giving a modified value for Fb. Thedesignmethod presentedin sub-clause(d) is basedon the ‘shear-friction hypothesis’developedby Mast’5~~ which is a design methodcapableof providing a conservativesolutionto a numberof apparentlydifferentproblemsin designingstructuraljoints forprecastconcreteconstruction.For example.Mast has developedit for usein designincconcretecorbelsand also for the horizontal shear connectionproblem in composite

T-beams.

5.4 Composite concrete construction

5.4.1 GeneralThis section applies to flexural membersconsistingof precastconcreteunits actingtogetherwith in situ concreteto carry the imposedloads. The essentialrequirement.therefore.is that the two concretesshould act as one effective section: if this can beachieved.then the resultingcompositesectionmay be designedin accordancewith therequirementsof Section3 or4 asappropriate.5.4thereforegivesthe necessaryadditionalrequirementsto ensurecompositeactionandalso dealswith thosespecialproblemsthat

121

Handbook to 858] 10. 198S

can arisebecausethe two concretesarecast at different times. However, only generalrecommendationscan be made,as the typesof compositesectioncanvary a greatdealin practice. -5.4.2 Analysis and design of compositeconcrete structures and members

5.4.3 Effects of construction methodsIt is importantto bearin mindthe needto passappropriateinstructionstosite(5.3.3).

5.4.4 Relativestiffnessof membersThiscan be of significancein situationswhereconcreteis pouredasa structuralinfill ortopping betweenandon precastunits. r5.4.5 Assessmentof strengths of sections of precast pre-tensioned units designed as

continuousmembers rOn many occasionsthe supportsection will be designedas reinforced to carry hoggingmoments. In this situation.prestressmay be ignored in the compressionzoneat the

ultimate limit state.If no redistributionof momentis carriedout, it is likely that at theserviceabilitylimit state the compressivestressesare low. spailing will not occur andrcreepwill be low. Sucha sectionwill notneedaconcretestresscheckat the serviceability Llimit state. If compressivestressesare high. i.e. the transmissionlength may be shortcomparedwith the span.some debondingof prestressingtendonsmay be necessary.In ranyeventit is importanttorecognisetheproblemsdiscussedin thecommentaryto 5.2.6.1. [

5.4.6 Serviceabilitylimit states

54.6.1 Serviceability

5.4.6J.1 General II5.4.6.1.2Prestressed precast units. The startingpoint in designinga prestressedconcrete rmemberwill generallybe the serviceabilitylimit state involving the useof permissiblecompressivestressesas given in 4.3.4.2. Providedit can be shown that failurewould be

of the under-reinforcedtype,5.4.6.1.2allowsan increasein thesestressesof up to 50%;this appliesparticularly to the situationwhere the prestressedunit is unpropped.Thismeansthat the maximumcompressivestressunderall loadscan be permittedto reach Liia value of l).5f~~. As a substantialproportion of this stresswill be due to permanentloads, it is suggestedthat this enhancedstressshould be used with caution and for [particularsituationswhere the detrimentaleffectsdueto possibleexcessivecreep~il I Lbe less severe. It has beencommon practicein bridgedesign in recentyears to applythis enhancementallowanceas shown in Figure H5 12. For thosesituationsshown in LFigure HS.12(a). the full stressvalueof 0.5f~~could be used;in Figure HS.12(b) a 25%

in situin situ concrete concrete

-K - LA A

L8 B precast

precast beam

Libeamai maximum compressive siresa IbI maximum compressive Stress

in precast beam, at A-A in precast beam at A-A ciis 0.5 f.,~ is 042 f~Figure 1-15.12: Suggestedenhancedcompressivestressvaluesin compositebeamsfor

12 differentformsof construction.

— . .. ~ — — — .~ — .

-- f—- --K?

___ ‘-~--~-:f--~~ -. 4.- ~•••~~ - K ~ K- --..... ... K - -. - . — ~44 ~ K~ K—K.KKK— K K — ~ — K — — K.. — . . K~ ~:--. - —K - •~~~:- ~ ~ . K .K-K..K--K- - KK K K K — K K K K K-. • - - - - - - K- -. - - — - - - K K - - -. ~.

________ --K- -~ --K.->. -K ~ -4 K

PartI: SectionS

increaseof up to 0.42f~ is suggested.For other intermediatecasesthe level of stressallowed should dependmainly on the amount of lateral restraint provided to thecompressionflangeof the precastbeamby the in situ concrete.

5.4.6.2 Tension in in situ concrete

5.4.6.2.1Prestressedprecastunits in direct contactwith in situ concrete.The presenceofa prestressedflange on the tensionside of addedconcrete(as at B-B in Figure H5.12(a)) considerablyretardsthe formation of crackingof that concrete.The stresslevelsgivenin Table5.4 shouldensurethat no significant flexural crackingdue to the imposedloads will haveoccurred in the in situ concreteat level B-B in Figure HS.12 (a) andthereforethe t~vo concretesshouldcontinueto actas onecompositesection.The tensilestressesgiven in Table5.4 may be increasedby up to 50% (providedthatthe permissibletensilestressin the prestressedconcretebeamis reducedby the samenumericalamount)and thereforemore prestressis required; this recognisesthe fact that the greaterthelevel of prestressat the contactsurface.the greateris the apparentenhancementto thetensile strain capacityof the addedconcrete.

5.4.6.2.2 Prestressedprecastunitsnot in directcontactwith in situ concrete.For thesituationillustratedin Figure H5.12 (b) the in situ concretetop flangeshould be treatedstrictlyas a reinforcedconcretesectionsubjectedto any local transversebendinganddesignedin accordancewith 3.4.7.

5.4.6.3 Tensionin prestressedprecastunitsThis is of relevanceif the prestressedunit hasthin exposedwebswherecrackscould bea visual or durability problem. So long as reinforcementis provided in the units or inthe compositeinfill. tensionin prestressedunits is less of a problem in constructionofthe type shown in Figure HS.12 (a).

Where continuity for live loading is achievedby placingreinforcementin the in situconcretetop flangeanddesigningthe supportsectionto be reinforced,this will certainlyinducetensilestressesin the top of the prestressedbeams(A-A in Figureff5.13) underthe serviceloadsand 5.4.6.3suggeststhat thesestressesbe limited in accordancewith4.3.4.Evenif a full allowanceis madefor transmissionlengthandloss of prestress.thismay be ratherrestrictive in design if straight tendonsare used. It would not seemunreasonable.for the typeof sectionin FigureH5.12 (a) only, to treatthe endsof theseprestressedunits as being Class3 underfull service loading, as any crackingwill belimited by the requirementsof 3.4.7 for the supportsectionandwill be remotefrom theprestressingtendons;if this suggestionis adopted.it should apply only to a length oneither side of the support centre-lineapproximatelyequal to twice the overall sectiondepthatthe supports.Anothersolution to thisproblemwould beto taperthe top surfaceof the prestressedunit towardsthe support.

5.4.6.4 Differential shrinkage

5.4.6.4.1 General. If an in situ concretefloor slab is cast on an older precastunit. thetwo concretestendto shrink atdifferent rates,becausemuchof thecreepandshrinkagestrain in the precastmemberwill havetakenplace beforethe connectionis made.The~Kff~6~.Z~f Uuislsto_inducesecotidarystressesin the compositesectionas a whole, the

~ K

rn~~nt,~~i4iicb is~1i~Wi~iih~iensdestressinducedii~thebottomof theprecast

centre-linein situ~ concrete of continuity steel

support -

-- - ~precast beams

\4. positive connection required in thiszone to guard against long-termeffects

Figure H5.13: continuity in compositeconstruction.i—-K,

Handbook to BS8I 101985

unit, which couldbe importantif thesectionis designedright up to the limiting stresses rin 4.3.4.3.As suggestedin 5.4.6.4.1theseeffectsarenot generallyof greatsignificancein most

practical casesfor simply-supportedmembers.Theyare likely to beworse if the precast Cunit is prestressed.where the stressin the top fibre (i.e. at the interface)due to prestressis nearzero.becausethe greatestdifferential strainmovementbetweenthe two concreteswill occur in this circumstance.It is suggestedthat. even then, theseeffects require

investigation only if thereis a differenceof more than onestrengthgradebetweenthe rtwo concretesand if the time interval betweencastingthe precastunit andplacing thein situ concreteis more than aboutS weeks.The t~-pe of compositecross-sectionmostsusceptibleto theseeffects is the compositeT-beamillustratedin Figure 1-15.12 (bj. r5.4.6.4.1Calculation of tensilestress. A method for evaluatingdifferential shrinkageeffectsis given in reference5.26 which alsogivessome indicationof how the differential rshrinkagecoefficient, referred to in 5.4.6.4.2.can be evaluatedfor designpurposesforvanoustypes ot sectton.FurtherIntormation is given in reterence~.27.

5.4.64.3 Approximate value of differential shrinkage coefficientfor building in a normal Fenvironment.Seeabove.

5.4.7 Ultimate limit state [This section is a major departurein BS 8110 ~vhencomparedwith the previousmethodof shearconnectiondesign.The methodof CPIIO wasbasedon elasticsectionanalysisandwas thouizht appropriatethereforefor the serviceabilirvloadings.

This new methodinvolves the forcesacting at the ultimate limit state as this is theappropriatelimit state for the mechanism.The intention was not to makeany changewith respectto safety factors and this and the old CF 110 methodwere intendedtoproducesimilar results.

Reference5.28providesfull detailsof the sourceof the newdesign method.

5.4.7.1 Horizontal shearforce due to designultimate loads [IThe analyticalmethod requiresa different approachdependingon the position of thecompressionzone relative to the plane underconsideration.There may be occasions

C-wherethe planeunderconsiderationis so low in the tensionzonethat 5.4.7.1 (a) is veryconservative.In thosecases,only the tensioncarriedacrossthe planein shearneedstobe considered. c5.4.7.2Averagehorizontal designshearstressThe definitions in Table 5.5 are intendedto representreal practicalsurfacesratherthanidealisedand unrepresentativesurfacesmentionedin previouscodes.The as castor asextruded finishes are deliberatelyintroducedto cover the finish producedby slip formor extrusionmachinesnow commonly used to produceprestressedslab units.

In bridge constructionthe term “rough as cast~ is often used to describesurfacesat Lthe top of the conventionalcast where the vibrator is removedleaving a very roughsurfacewith largeaggregateparticleson the surface.Wherebridgebeamswith this finishare incorporatedinto buildings. the horizontal stressesfrom the roughestcategory inTable5.5 are appropriate. I5.4.7.3 Nominal links

5.4.7.4Links in excessofminimum Li5.4.7.5 Vertical shear I5.4.7.5.1General. Reinforcedconcretecompositemembersmay be designedby using3.4.5 and. as long as there is adequatelongitudinal shearconnection betweenthe Uconcretes.the grosssectionmay be usedin the design.

The design of prestressedconcretecompDsite membersis a little more complicated124 [

K K K K K — KK K jK~~K - KK -

.—-.—.....—-—K.—... -- -K. K.. - K

K K K K K K K KKK. KKK KKKK K UK K -~ .~ K...

- .- - . -.--‘Ac- .- - - -. - - - - -. . KK - K KK — ‘• K — — K —~ — K K K K K ,. K K K.4.- - K.K.c-K-~-..K->~KK~.KKKKK--4K-K-K- ~ ~ - K..’ .K .

Part]:Section.5

(ci (c) C S C

Figure HS.14: Compositesectionsconsideredin designingfor shear (5.4.7)— (a)original member, (b) with compositeinfill. (c) with compositetopping.

than the design of non-compositeprestressedconcrete membersand the simplifiedmethodsin 4.3.8 do not necessarilyapply.

The assessmentof V~0, the shearcapacityof a memberuncrackedin flexure. in 4.3.8assumesthat the membercarriesall the shearin its web and that the critical point ofmaximumprincipal tensilestressis at the centroid.In compositeconstruction,it is ideallynecessaryto checkall possiblecritical sectionsand to ensurethat the principal tensilestressof all the structuralconcretein the memberis less than the permittedvalue of0.24~ Whenin situ concreteis placedbetweenprecastprestressedconcretemembers,the precastconcreteprovidesrestraint to the infill and increasesits capacity to carrytension. In thesecases,therefore,it is generallyconsideredsatisfactoryto check onlythe principal tensilestressin the precastconcrete.For mostpracticalcases,it will befound that the precastpartof thecompositesectionis capableof carryingall the ultimateshearload, and this is all that the Code requires.Further complete checks,on thecompositesectionas a whole, will be requiredonly if the ratio of imposedloading todeadloading is exceptionallyhigh. - -

The shear force at which flexure—shear cracks form. V~. may also be calculatedbyusing4.3.8.It is necessaryto considereachcompositesectionon its meritswhendecidinghow muchof it is resistingshear.FigureH5. 14 (a) showsan original precastprestressedmemberthat is incorporatedinto a compositememberin two ways: in FigureH5.14 (b)it hascompositeinfill andin FigureH5.14 (c) it hascompositetopping.In (b), the infillconcretemay crackbeforethe original memberandthe post-crackingshearstrengthofthe infill maynot necessarilyaddto the grossshearstrengthof the member.

It is thereforewise to makesomereductionin the infill concreteshearin calculatingflexure—shearstrength,the amountof the reductiondependingon whetherthe infill isrestrainedbetweenprecastmembersor restrainedby reinforcementcastinto the precastmember.

When the compositememberhas a structural topping. as in Figure 1-15.14 (c), theflexure-’shear capacity, V~, may be calculated by usingthe grosssectiondepthandtheweb width of the original memberbecause.in thiscase.the additionalconcretehasbeenplacedin an areawhere it can addto the shearstrengthof the member.

5.4.7.5.1In situ concretebetweenprecastprestressedunits. Seeabove.

5.4.8 Differential shrinkage betweenadded concreteand precast members

5.4.9 Thickness of structural topping

5.4.10 Workmanship

REFERENCES

5.1 SOMERVILLE. o Some loading testson double-Tfloor units. London. Cementand ConcreteAssociation.July 1965. TechnicalReport391. 15 pp.

5.2 SPARKE. A.~4. Distribution tests on hollow box precastfloors. Civil Engineeringand PublicWorks Review. Vol.62. No.726. January1966. pp.83-86.

5.3 LAGLE, o.i Loaddistributiontestson precastprestressedhollow-coreslabconstruction.Journalof the PrestressedConcrete Institute. Vol.16. No6. November-December1971. pp 10-18.

5.4 INSTITUTIONOFSTRUCTtJRALENGINEERS. Structuraljoints in precastconcrete.London.56pp 1978.5.5 WILLIAMs A. Thebearingcapacityof concreteloadedover a limited area.CementandConcrete

Association.1979. TechnicalReport526 7Opp.125

Handbook ro BSSIIO:1%.5

5.6 HAWKINS. N M. The bearingstrengthof concrete loadedthrough rigid plates. MagazineofConcreteResearch.Vol.20. No,62. March 1968. pp3l-44~.

5.7 HAWKINS. N \I. The bearingstrengthof concreteloadedthrough flexible plates. MagazineofConcreteResearch.Vol.20.. No63. June1968. pp95-1O2.

5.8 Kltiz. L.B andRAThS. C.H. Connectionsin precastconcretestructures— bearingstrengthof columnheadsJournalofthePrestressedConcreteInstitute.Vol.8.No.6.December1963.pp4S-75

5.9 GERGELEY. ~. andSOZE~.. \I.A. Design of anchoragezonereinforcementin prestressedconcretebeams Journalof the PrestressedConcreteInstitute. Vol.12. No.2. April 1967 pp63-75.

5.10 SOMERVILLE. G. The behaviouranddesignof reinforcedconcretecorbels. London.CementandConcreteAssociation.August 1972. Technical Report .4.72. l2pp.

5.11 CLARKE, IL. Behaviourand designof small nibs. Cementand ConcreteAssociation.1976TechnicalReport512. 8 pp

5.12 RICHARDSON, jo. Precastconcreteproduction. Viewpoint Publications.1973. 232pp5.13 FRANZ. G. The connexionof precastelementswith loops. Proceedingsof a symposiumon

designphilosophy and its applicationto precastconcrete.London. 1967. London. CementandConcreteAssociation. 1968. pp.63-66.

5.14 SOMERVILLE. o. Horizontal compressionjoints in precastconcreteframe structures.Thesissubmittedto the City University for the degreeof PhD, December1971. 196pp

5.15 SOMERVILLE, G. and BLRHoL5E. p Test on joints betweenprecastconcretemembers.Garston.Building ResearchStation. 1966. Current papersEngineeringseries45. 18 pp

5.16 Ic,oNiN. L.A. Gluedjoints for reinforcingbarsandprecastreinforcedconcreteunits London.Civil EngineeringResearchAssociation.1965. CERA TranslationNo. 1. 16 pp

5.17 MARKESTAD. .~. and JOHANSEN, K, Jointing reinforcingsteel with resin mortars.Nordisk BetongVol. 14. No. 1. 1970. pp-79-93

5.18 IvEY. oLFatigueof groutedsleevereinforcingbarsplices.Proceedingsof theAmericanSocietyof Civil Engineers.Vol.94. No.STl. January1968. pp 199-210.

5.19 TOPRAC. A.A, and THO%IPSON IN. Welding between precastconcrete units. Journal of thePrestressedConcreteInstitute. Vol.8. No.3, June1963. pp.14-29.

5.20 LEBEL. L.M. andKNYAZI-IE\ IC-H. MG. Investigationsof joints of precastreinforcedconcreteslabs.Beton i Zhelezobeton.No.2. 1969 pp.82-85.TranslatedfromtheRussian.Garston.BuildingResearchStation.May 1970. Library Communication983. 7 pp.

5.21 HANSON N W. and coN\ER. I4,W Seismicresistanceof reinforcedconcretebeam-columnjoints.Proceedingsof the American Society of Civil Engineers.Vol.93. No.5T5. October1967.pp-533-560.

5.22 HOLMES. ~i. and POSNER. co. Factorsaffecting the strengthof steel plate connectionsbetweenprecastconcreteelements.The Structural Engineer.Vol.48. No.10. October1970. pp.399-406

5.23 PRESI~RESSED CONCRETE INs~rnt-ra.Designhandbook— precastandprestressedconcrete.Chicago.Third Edition. 1985. 528 pp.

5.24 INTERNATIONAL COUNCIL FOR BUILDING RESEARCH STUDIES AND DOCUMENTATION (C-Is). Proceedingson aninternationalsymposiumon bearingwalls.Warsaw1969. Oslo.NorwegianBuilding ResearchInstitute. 1970. 15 pp.

5.2.5 MAST, R.F. Auxiliary reinforcementin concrete connections.Proceedingsof the AmericanSociety of Civil Engineers.Vol. 94. No.ST6. June1968. pp.14-85-1504.

5.26 KAJFASZ. s.. SOMERVILLE.o andROWE. RE. An investigationof the behaviourof compositeconcretebeams.London.CementandConcreteAssociation.November1963. ResearchReport 15.44 pp.

5.27 BIRKELAND. H.W Differential shrinkagein composite beams. Proceedingsof the AmericanConcreteInstitute. Vol.56. No.11. May 1960. pp.1123-Il36.

5.28 FEDERAtION INTERNATION ALE DC LA PREcON-rRALN-rE, Shear at the interface of precastand in situconcrete.FIP. WexhamSprings.Slough. 1982. 31 pp.

I 2(~

74

- -4 .- - -

~1SECTION SIX. CONCRETE:MATERIALS,SPECIFICATION ANDCONSTRUCTION

J

6.1 Constituent materials of concrete

6.1.1 Choice and approval of materialsAs ~vithCP110. oneof the basicprinciplesof thisCodecontinuesto bethat the Engineermust decidethe essentialfactorsto be specified.ideally in termsof readily measurableparametersor attributesrequiredfor the work (e.g.pumpability,freedomfrom bleedingetc(.The concreteproduceris then left with the greatestfreedompossibleto designtheconcretemix to satisfytheserequirements.Theconcreteproducerwill usuallyhavemoreknowledgeof the local materials.quality of cement,typeandgradingsof the aggregate.etc and henceshould be in a good position to provide concretehaving the desiredperformancecharacteristics.

Whilst making a clear preferencefor materialscomplying with. or selectedfrom. aBritish Standard,the Code doesallow the use of non-standardmaterialsparticularlywherethere arepossible technicalandcost benefits. However, with all materials.theCodeemphasisesthe needfor satisfactorydataon their suitability andfor assuranceofquality control. The performanceof concretemade with non-standard0; ‘infamiliarmaterialsand their suitabilirv may be establishedon the basis of previoI~- uata. pastexperienceor specific tests. Whereverpossible.certificatesof compliancl ~ith Britishor otherclearlydefinedstandardsshouldbe provided by the materialsupplier.

Unfamiliar materials or combinationsof materialsmay produce concretewhosepropertiesdiffer considerably from thosewith conventionalmaterials. For exampleconcretescontainingground granulatedblastfurnaceslag (ggbfs)or pulverized-fuelash(pfa) have longerfinishing times. which may be an advantageor a disadvantage.

6.1.1.1 Design

6.1.1.2Materials

6.1.2 Cements,ground granulated blastfurnaceslagsand pulverized-fuel ashesThe British StandardGlossaryof buildingand civil engineeringterms, BS 6100:1984,definesPortlandcementas ~activehydraulic binder basedon groundPortlandcementclinker and indicatesthat Portlandcementis a generalterm for the variousforms ofPortlandcement. In particularBS 12 coversthe main ones,OPCand RHPC, BS 4027coversSRPCandBS 1370 coversLow heatPC. In all these,no addition otherthan ofgypsumor oneagreedgrindingaid (i.e. propyleneglycol) ispermitted.The nextcategoryof PortlandcementscoversPortland-blastfurnacecement(BS 146)andthecorrespondingLow heatPortland-blasrfurnacecement(BS 4246) andPortlandpfa cement(BS 6588).Thesecementsare -blendedhydrauliccements~accordingto the definitionsgiven in therespectiveBritish Standardsandin the Glossary.BS 6100.However,it shouldbe notedthattheymaybemanufacturedeitherby blendingof thecomponentsorby intergrinding.

The materialsthemselves(i.e. pfa and ggbfs) arecoveredby British Standardsviz:BS 3892: Part 1 for pfa andBS 6699:1986for ggbfs. The only other cementpermittedin BS 8110 is Supersulphatedcementto BS 4248. which is not availablein the UK.

In recentyears.the potentialadvantagesin somecircumstancesof combiningPortlandcementwith either ggbfs or pfa havecome to be realised.The recentamendmentsofBS 5328 havedefinedcementas a hydraulicbinder which can be(a) hydraulic cementthat is an active hydraulic binder formedby grinding clinker to

BS 12. B51370or BS 4027.

or

(b) hydraulic binder,manufacturedby a controlledprocessin which PortlandcementclinkerorPortlandcementis combinedin specificproportionswith a latenthydraulicbinder consistingof pfa or ggbfs. to BS 6588. BS 146 or BS 4246, accordingto thelatent hydraulic binderused.

127

Handbook to BSSiIO:198S [I

or r(c) hydraulicbinder. manutacturedin the concretemixer by combiningPortlandcement

to BS 12 with a latent hydraulicbinderconsistingof pfa to BS 3892:Part1 or ~bfsto BS 6699.complyingwith the generalrequirementsfor proportionsandpropertiesgiven in BS6588.BS 146orBS4246accordingto thelatenthydraulicbinderused.

In line withthis, thecementsin BS 8110aredefinedin termsof threecategoriesas:

(1) Portlandcementincludingordinary,rapidhardening,low heatandsulphate-resisting(6.1.2.1(a)):

(2) CementscontainingggbfsBS 146 andBS 4246 andcementcontainingpfaBS 6588

EL(6.1.2.1(b)).The availability of thesecementsvariesthroughoutthe UK:(3) Combinationsof Portlandcementandggbfs or pfa (6.1.2.1(d)).

The third categorywhich permits combinationsof Portlandcementsandggbfs and

Cpfa at the mixer (mixer-blends)is a new departureand care mustbe taken to ensurethat such mixer-blends produce equivalent concretes to those made with thecorrespondingblendedcements.Whenusingmixer-blendsthefollowingprinciplesapply’:

[(i) The relevantBritish Standardfor blendedcementshouldbe usedas the basis forcomparison:

(ii) The mixer-blendcombinationsshouldgenerallybe basedon BS 12 cement:(iii) The ggbfs and pfa shouldcomply with appropriateBritish Standards:

BS 3892 Pulverized-fuelash Part 1: Specificationfor pulverized-fuelashfor useas a cementitiouscomponentin structuralconcrete.

BS 6699:1986Specification for ground granulated blastfurnace slagfor usewithPortlandcement.

(iv) It is permitted in 6.2.4.3to replacePortlandcementwith at least an equalweightof ggbfs or pfa. The cementingefficiencesof thesematerialsmay be lower and3.3.5.5statesthat the total massof Portlandcementplus ggbfs or pfa may needto be increasedto achievea specifiedstrength:

(v) Confirmation must be provided that combinationsof cementsand ggbfs or pfaconform with the propertiesof the correspondingblendedcement.

Satisfactoryperformancecan be judgedeither by tests of the combinationsagainstthe relevantblendedcementstandardor otherpefformancetestsin concrete.

Thereare many test dataavailable on the use of ggbfs or pfa in concretebut caremust be taken that where previous data are relied upon, the same materials andproportionsarecurrently beingused.

Certification proceduresfor the percentageof ggbfs and pfa which are now beingprovided by the suppliersof thesematerialsalong the following linesmay provideanacceptablemechanismof confirmation:

“When blendedin the combination(100—X)% BS 12 Portlandcementand X%ggbfs(or pfa) complying with BS (. - .), the resultsconfirmthat for the period(. .

the proportions and propertiesof this combination were in compliancewith the

physicalandchemicalrequirementsof BS (. - ), asdeterminedin accordancewith Lthis procedure.”The performance.andparticularlythe durability,of concretemadewith thesematerials

canbe consideredas beingequal to that of Portlandcementconcreteprovidedthat the

Iggbfs or pfa concretecomplieswith the samegradeaswould be achievedby the Portlandcementconcrete(3.3.5.5).In order to obtain concreteof equalstrengthat 28 days. It

maybe necessaryto increasethe total massof Portlandcement+ggbfsor pfa comparedwith the massof Portlandcementin the concrete.

The propertiesof fresh and greenblendedhydraulic cementconcretesare differentfrom Portlandcementconcretesandconstructionpracticesmay haveto be modified totake thesedifferencesinto account16’~.

2The use of the appropriatetype of cementor ggbfs or pfa can assist in producingconcretewith specialpropertiesrelated to durability as shown in TableH6.1.

U128 . [

— K — K— —4K~ K — K K K

K -K.- ~ - - - ~ -— .. ~. •... . ••4. - K44 — — ~ — — — K KK K K K K .K K —— —--7 K.. - K- K - K K K K - K K. , K- - - .. K K K K - - ~ ---, -.— — 4 — . K K K — — K — — K K — K

4... -. -~ - - K. - - K - K

PartJ:Sect,on6

Table H6.1 Concrete characteristics requiring the use ofspecial cements, or ggbfs, or pfa

Property ofconcrete

Considerthe useorcementtoBritish Standard or theuseof ggbfsor pfa

Furtherinformation

Ear1~’ strengthdevelopment

RHPC to BS 12Ultra-high early strengthPortland cement

Low heatevolution

BS 1370 BS424-6BS 14-6 BS6588Combinationsof OPCto BS 12andggbfsorpfa

Improvedresistancetosulphateattack

BS-1027 B54248

Combinationsof OPCto BS 12andggbfs.70%—90%orpfa.25%—40%

see6.2.3.3BREDigest250

Improvedresistancetoalka!i-~ilicareaction

Uselow alkali cement(lessthan0.6%equivalentNa2O)

Combinationsof OPCtoBS 12andggbfs.atleast50%orpfa.atleast30%

see6.2.5.4BREDigest258

seereferences6.1.6.2.6.3.6.4

6.1.2.1 General

6.12.2 Propertiesof concretemade with cementscontainingggbfs orpfa

6.1.2.3 Combinationsof cementsandggbft orpfa

6.1.1.3.1Proportionsandproduction

6.1.2.3.2 Performanceand suitabilityfor purpose

6.12.4 Cementsfor sulphate-resistingconcreteThis clausereflectsthe fact that if aproportionof SRPCis replacedwith an equalweightof pfa. the sulphateresistanceof theresultingconcretemay be reduced.Howeverif thehigher minimum cementand maximum water/cementratio of the OPC (BS 12)/pfacombinationis adopted.an SRPC(BS4027)/pfamix shouldgive adequatedurability.

6.1.2.5 Cementsfor low heat concrete

6.13 Aggregates

6.13.1 GeneralThe aggregatescoveredby the Codecompriseall types of materialsclassifiedin termsof their densityas:

Normal-weight(particledensity2000.—3000k~/m3)

Lightweight (particledensityless than2000kg/in3)Heavyweight(particle densitygreaterthan3000kg/in3)

6.1.3.2AggregatespecificationsWhereverpossiblethe Code prefersaggregatescomplyingwith the appropriateBritishStandardbut other materalsmay be used provided thereare satisfactorydataon thepropertiesof concretemadewith them.

Hereagain,theemphasisis on ‘performancein concret&.Forexamplesuitablegradings129

I

[Ir

HandbooK £0 BSSIIO:1985

are not laid down: the requirementbeing that theoverall gradingshouldbe suchas toproduceconcreteof the requiredworkability and fi nishability which can be placedandproperlycompactedinto positionwithout the useof excessivemixing waterandresultant‘bleeding’.

Wherenecessary,specialaggregatecharacteristicscanbe definedby referenceto theappropriateBritish Standardor otherauthoritativedocumentsasshownin TableH6.2.

Table H6.2 Choice or limitation of aggregate characteristics

Aggregatecharacteristics Choiceor limitations

Furtheriiiforiitation

Nominal maximumsize

20mm Suitablefor mostuses40mm Thickor1ightl~

reinforcedsections10. t4mm Thinorheavilv

reinforcedsections

Grading Variationsfrom relevantBSaccommodatedby concretemixdesiitnSeparatefine andcoarseaggregatefor strengthgradeC20andabove

BS5328

Shellcontent Accommodatedin concretemixdesign

BS882

flakiness Dependentonaggregatetypeandconcretegrade

BS882

Dimensionalchange Higherinitial dryingshrinkagewith high moisturemovementaggregates(e.g.Scottishdoloritesorwhinstones)

BRE Digest35

Fire resistance Forhighdegreesof fireresistance.limestonesorlightweightaggregatesmay beneeded

BS8110Pt.2Section4

Wearresistance Heavydutygradeaggregateforindustrialfloors

BS882

Density Specialaggregatesrequiredforhighor low densityconcretes

BS81IOPt.2SectionS

High strcngth-

Eachaggregatehasceiling

strengthfor a givenparticlesize.Crushedrockaggregatesmay benecessaryfor concretegradesabove60N/mm.

6.1.4 WaterBS 3148 includes requirementsfor the testing of water for its suitability for use inconcrete. However it does not give any limits with which the water should complyalthough some suggestionsfor the interpretationon the test resultsare given in anAppendix.

Watersuitablefor drinking is suitable for concrete.Where.however,untreatedwateris obainedfrom the groundsurfaceafter having passedthroughorganicmaterialssuchas peat. it would be advisableto test it before using it in concrete.Water from deepboreholesis generallysatisfactoryin the UK.

6.1.5 Admixtures

6.1.5.1 Ge,,eralThe Codefully recognisesthecontributionwhich admixturescanmaketo improvecertainpropertiesof concreteby their chemical and/or physical effects. The British Srnndard

13(1 CK K K — K K... KK KKKKKKK KK — K

K KKKKKKK K K K. — ,K.KK, ~ — K K.~K KK. K ~ KK , — KKKKKKK 4 K K~.K, 4 .,KKKK K K— - - - -:-K K K .KK. K K — — K K — — — — — K

C

C

B

I

[

Li

ii

[

LI

K I-.--..: . K.KK~4~•~ K K , K. K--K-...- K•... ~ K K ~ K K ~ K K KK - - - K K KK~ K K 7 .. .. .. . ~ ... .. ... K— K~ - 4~ -. K,.. K - - - K - - KK-K K4~K - K.. K~KK - -----KPart I: St’L’n Oil 0

for admixtures BS 5075 gives specific requirements for accelerating, retarding, waterreducing. air-entraining admixtures and superplasticizers.It is important to appreciate that the behaviour of one or more admixtures in a concretemix depends upon their interaction with the particular cement and aggregate materialsbeing used. If these materials are changed the behaviour of the admixtures may be verydifferent.The frost resistance of concrete depends to a large extent on its permeability, and theprovision of adequate curing and the degree of saturation of the concrete when exposedto frost. Concretes with a high degree of saturation and subject to de-icing salts haveincreased risk of frost damage.In such cases air entrainment should be specified in terms of the average air contentof concrete in accordance with 6.2.3.2.The limitations on the total chloride content in concrete given in 6.2.5.2 are supportedby limitations on the chloride ion content of admixtures which should not exceed 2%by mass of the admixture or 003% by mass of the cement when used in prestressed orreinforced concrete with any type of cement. or unreinforced concrete with cementscomplying with BS 4027 and BS 4248.6.1.5.2 Admixture spec~flcations6.1.5.3 Approval and performance of admixtures6.1.5.4 Concrete durabilityAny chloride ion in the admixture should be included in the calculation of the totalchloride contents in Table 6.4.The Na:O equivalent in any admixture should be included in the calculation of thetotal alkali content (6.2.5.4).6.1.5.5 Air-entraining agents

6.2 Durability of structural concreteThis section treats all aspects of design to achieve durability and therefore gives a broaderperspective than, for example. Sections 3.3 and 4.12 which are concerned specificallywith the requirements for cover and concrete quality. particularly as they influence sizingof sections at the design stage. Design also means identifying the structural form andconstituent materials appropriate to the life-time and environment.In addition to the general environment (6.2.3.1 and as defined in more detail in 3.3.4).~freezingand thawing and de-icing salts’ (6.2.3.2 land ~-exposureto aggressive chemicals”(6.2.3.3) are identified as other broad types of exposure condition.6.2.1 General6.2.2 Design for durabilityAs indicated in the Code in Clause 2.1.1. durability is an aspect of the structure thathas to be considered carefully and consciously taken account of in design. This impliesa consideration of facets of design. materials and construction. and a convenient checklist for these in the different stages Is:Desi~n — assessment of environmental conditions during expected life— geometry of structure and sections to improve weathering properties.i.e. control of flow of ~vater— cover to reinforcement and its adequacy— depending on severity of environment. surface protection to concreteMaterials — constituents and their quality— mix proportionsConstruction — mixing. placing and compaction of concrete— curing— accuracy of form’.~’ork

131

Ha,Iabook w B58110:1985

— achievingthe specifiedcover— appropriatequality assuranceprocedures.

6.2.2.1 Shapeand bulk of concrete rTheemphasisshouldbeon ensuringgooddrainageof waterandtheavoidanceof standingpoolsandrundownwater.Equally the cracksreferredto arenot thosecontrolledby the

clausesin Section3 but thosewhich may occur whenthe chosengeometryandbulk ofthe sectionmake them virtually unavoidable— in other words,badly designed!

The particularaspectsrequiring attentionas regardsthe cover havebeentakenintoaccountin deriving Table 3.4 andno further adjustmentsarenecessary.

6.2.2.2 Depth of concrete cover and concrete qualityThe alkaline environmentprovided by fresh concreteprotects reinforcement.Fromexperience.appropriatecombinationsof cover andconcretequalityensurethat in wellrdefinedenvironmentsthe effectsof carbonationof the concreteandof penetrationof I:chloridesdo not leadto unacceptablecorrosionof thereinforcementduring the expectedlife of the structureor component(2.1.1). Within limits, a trade-offis possiblebetween rfree water/cementratio. cementcontentandthicknessof concretecover to achievethesamenominal protection,exceptthat for moresevereexposureconditionsthe availablecombinationsbecomemore restricted.

The cover for a given strength,using the reducedconcretegradesgiven in 3.3.51. [andexposurecondition.e.g.Table3.4, is broadly in line with that in Table19 of CP11Oexcept for increasesof 5mm for mild andmoderateexposureof lower concretegrades.Theseincreasesreflect concernthat the durabilityof somebuildings is proving less thanhad beenanticipated. Variability of strengthtends to be greaterat lower gradesandtypical variations in cover haveproportionatelygreatereffect at lower covers. Theseinfluencesare significant togetheronly for mild and moderateexposure.Inaddition.

Chowever,restrictionsare placedon minimumcementcontentandmaximum freewater!cementratio to provide adequateimpermeability for the particularthicknessof cover.Although compliancewith compressivestrengthcan be demonstrated.compliancewithlimits for water/cementratio andcementcontent is difficult to demonstrate,especially~in hardenedconcrete.Basedon analysisof a substantialnumberof recordsfrom readvLmixed concreteplants (6.5) it is possibleto specifya lowest gradeof concretewhich. ifachieved,will ensurethe limits on cementandwater/cementratio for 95% of materialsin currentuse.The inclusionof these‘lowest grades’ in Tables3.4 and4.8 representsapractical approachto achieving compliancewith the necessaryquality of concrete.although it shouldnot be takento imply that durability is a function only of compressivestrength.It follows that the reducedvaluesof grade in Clause3.3.5.2do not represent [relaxationsas such but are valuesthat it will seldombe possibleto use becauseof thedifficulty of demonstratingcompliancewith the otherlimitations.

Clause3.3.5.2doesnot permit thesevaluesof grade to be usedfor mixescontaining

[pfa or ggbfs even though3.3.5.5indicatesthat the protectionto reinforcementshouldbe equal to that of Portlandcementconcreteif the 28 day strengthsare equal. Therestriction arisesbecausedatafor maximum water/cementratio andminimum cementcontent in relation to durability are not available for concretescontainingpfa or ggbfs [in the sameway that theyare for Portlandcementconcrete.Althoughsome pfaor ggbmixes may conform to the limiting valuesin Table 3.4 and 3.3.5.2 the wide range inpercentageadditionspermissiblemeansthat this is not generallytrue. For all but the I.lowest percentageadditionsa proportionatelygreatermassof pfa or ggbfs will almostcertainly be required. Put anotherway. the strength equivalencedataare basedonassumptionsaboutminimumcementcontentin Table3.4 for Portlandcementconcrete Lwhich are not necessarilytrue for- pfa or ggbfsconcrete.

Becausethe equivalenceconceptis basedon broadcomparisonswith limited dataitis reasonableto exercisesomecaution,giventheconcernto avoidprematuredeterioration,~in seryice.Thisconcernextendsto sulphateresistingPortlandcementin 3.3.5.6for yeryfsevereor extremeexposureconditionseventhoughincreasedcoveris recommendedfo rL~achievingequivalentprotectionto reinforcement.

U6.2.3 Exposureconditions

I 32

L

-K KKKK;KK K K K K I• ..‘::,~.-- ~ . - - . - KK — ~ — — — ~. — K — — K .K — — — , .~ K,K . •K — ~ — —~ — K K — KK K K K — K —— K K —— K KKK ~

- ~ K K KK - K K K .. K

Part!: Section6

6.2.3.1 Generalenvironment

6.2.3.2Freezingand thawing and tie-icing salts

Air-entraining agentsThe resistanceof concreteto freezing and thawing dependsto a large extenton itspermeability,the provisionof adequatecuringandthedegreeof saturationof theconcretewhen exposedto freezing,concretewith a higherdegreeof saturationbeing the moreliable to damage.The useof salt for de-icing roadsgreatlyincreasesthe risk of damagefrom freezingand thawing.

The useof de-icing chemicalscan causeconcretedeteriorationthroughtwo differentmechanisms.Firstly, the melting of ice and snow producespoolsof water available tobe absorbedby the concrete.This can raise the level of saturationin the concreteandthe salt solution remainsliquid at lower temperaturesthan purewater. Thus concretemay be subjectedto many cycles of freezingand thawing at a much higherdegreeofsaturationthan if the de-icingsalt hadnot beenused.Secondly,de-icingsaltsincreasethe presenceof chlorideswhich, in reinforcedconcrete,can posea corrosionrisk.

Air-entrainingagentsentraincontrolledamountsof airin concrete,andgreatlyimproveits durability and in particular its resistanceto damageon freezing.Air-entrainmentcausessome loss in strengthbut, as a designedmix is required. this will be offsetautomatically.The engineershould specifyair-entrainmentwhere the concretewill bein contactwith de-icing saltand shouldspecifythe averageair contentof the concretein accordancewith 6.2.3.2.Sitecontrolof aircontentiscoveredby BS 1881:Part106.

Careis requiredin the selectionof air-entrainingadinixtures.It is recommendedthatproductsbe obtainedfromreliablefirms havingatechnicaldepartmentcapableof advisingon the use of the product. The admixture mustnot only causethe entrainmentof theair in the requiredamount,despitevarying mixing andagitating times, but must alsolead to the correct size and spacingof the air bubblesin the freshly mixed concrete.When thoserequirementsare met, the air is reasonablystablein the fresh concrete.which can then be handledand compactedby vibration without serious loss of air. Itshouldbenotedthatdifficulties maybemetin entrainingairinto mixescontainingpfa.

6.2.3.3 Exposureto aggressivechemicalsThis Clauseis concernedwith aggressivechemicalsexternalto the concrete.The samechemicalsmay be introducedin the mix constituents(61.5) and havean aggressiveinternaleffect.Concreteusedinagriculturalsituationsmay besubjectto acidicsolutions,e.g. food processing,silageeffluentt66’6.7), Engineersshouldbe particularlywaryof oldindustrialtips andthe chemicalstheymay containt68~.

The omissionof valuesfor cementcontentandfreewater/cementratio againstclass 1in Table 6.1 is coveredby the footnoteto the Tableandarisesbecausedifferentvaluesmay be appropriateandarestatedelsewherein the Code.For concretein contactwithnon-aggressivesoil (i.e. class 1 of Table 6.1). Table 3.2 definesthe environmentas‘moderate’;for amoderateenvironmentTable3.4giving coverto reinforcementrequiresa minimum cementcontentof 300kg/in3and a maximumwater/cementratio of 0.60:thesevaluesthenapply to class1 of Table6.1. However,unreinforcedconcreteis treatedin 6.2.4.2. and for a moderateenvironment.Table 6.2 requiresa minimum cementcontentof 275kg/in3anda maximum free water/cementratio of 0.65.

Basedon longstandingpractice and absenceof durability problemsin class 1 non-aggressivesoil conditions.concretemadewith normal-weightaggregateand used forfoundations(strip andtrench-fill) to low-rise structures(6.2.4)may havea lower cementcontentnot less than220kg/in3if the gradeis not less thanC20. Undertheseconditionsthe recommendationsfor increasedcoverto any reinforcementin 3.3.1.4. for concretecastagainstunevensurfaces.will usually’ apply.

The presenceof water is necessaryfor sulphateattackto occur: attemptsto dry onesurfaceof concretecan exacerbateflow of moistureandthe rateof attack.

6.2.4 Mix proportions

6.2.4.1 Generalthis Clausepicks up the generalprinciplesfor achievingdurablereinforcedconcrete.

133

riHandbook to BSSJIf.J:I98~

given in 6.2.1. and focuseson mix proportionsby referenceto Tables3.4. 4.8. 6.1 and r6.2. It emphasisesthe importance of achieving the lowest free water/cementratio ~compatiblewith producingplacedconcreteof uniform consistencyandof ensuringthespecifiedminimumcementcontents.

If it is necessaryto use admixturesit shouldbe ensuredthat the limiting valuesarestill metbecausethe valuesin Tables3.4,48. 6.1 and6.2 are basedon dataon concretesmadewithout admixtures.

It is equally important to be aware of the behaviourduring curing of concretescontaininghigh cementcontents.particularly in excessof 550kg/rn2,when high dryingshrinkat~eor thermalstressesmay be induced.

6.2.4.2 Unreinforced concreteTable6.2 isanalogoustoTable3.4exceptofcoursethereareno requirementsfor cover.

6.2.4.3 Mix adjustments in Tables 6.1 and 6.2The changesor adjustmentswhich maybemadeto valuesinTable62 are againanalogousto those relating to Table 3.2. However, recognizing that in somecasesit may be rappropriateto specifyprescribedmixes,recommendationsare given for mixesdescribedin BS 5328 which will providethe necessarycementcontentsandmeetthe free water/cementratio limits. [

6.2.5 Mix constituents

6.2.5.1 General [The importanceof properselectionandcontrol of materialsis emphasised.

6.2.5.2Chlorides in concrete [It with by the

Control of the risk of corrosionof embeddedmetal by’ chloridesis dealimits in Table 6.4. which representa small modification to the stricter limit of 0.06%introduced in 1977 which excludedsome inland aggregatespreviously regardedascompletelysatisfactory.Although averylow limit for chlorideisrequiredin this categoryit is consideredthat the risk of corrosionwould not be increasedby raising the limitfrom 0.06% to 0.1%. To achievethe revisedlimits, washingof sea-dredgedaggregatesis essential.

It is consideredthatthereis sufficientinformationandexperienceof theuseof cementscomplyingwith BS 4027 or BS 4248.for the chloride limit to be set at 0.2%,subjecttocontinuingreview. The 0.2% limit applies to both plain andreinforcedconcrete.Itis [

neededin plain concretefor sulphateresistancepurposesand in reinforcedconcretefor ‘-‘

both sulphateandcorrosionresistance.Wherethe type or useof concretelies in morethanonecategory.e.g.steamcuredconcreteusinga sulphateresistingcement.the more [onerous limit should be applied. The value of 0.4% for most reinforced concreterepresentsa simplification for the previousmethod of expression.

6.2.5.3 Sulphates in concrete Li6.2.5.4 Alkali-silica reaction [A revisededition of the Guidancenoteson tninimising the risk of alkali-silica reaction.togetherwith a set of Model SpecificationClauseswas publishedfor public commentinOctober I985£~.4£. K

It. mustbeemphasisedthat therecommendationsrelateto conditionsfoundin mainland [Britain. and before using them Engineersworking outside that area should satisfy Lthemselvesthat local conditionsare comparable.

The recommendationsin the Codeare in line with thosegiven in the September1983 rGuidanceNotes.The revisededition includessomeimportantchangeswith the current Ladvice being as follows:

As the threeelementsof moisture,high alkali levelsandreactivesilica aggregatesallhaveto be presentfor damageto occur. it is only necessaryto eliminate oneof themto minimise the risk of ASR. The GuidanceNotes recommendvarious ways in whichthis ma~ be achieved,but stress the importanceof giving as wide a choiceof methods

34 [-~.-K-~KK.

K - -- ~ K K - K -- K — K. ~ ~ K— — — ~ . K — , ‘K — , - — — — — K K•~K K - — K -— KK K K K K -. - - K .K~KKK K K

fl Part!: Sectiono

as possibleto the contractorto minimisecosts.Taking the four sub-paragraphsin Clause6.2.5.4in turn:

(1) Controlling moisturewill only be successfulif the equilibrium relative humidity inthe concreteis lessthan 75%.This can be the casein dry, well-ventilatedparts ofI buildings.It will not applyto foundationsevenif waterproofed.to externalmembers.or to thosesubjectedto condensation.

(2) Guaranteedlow alkali cementto BS 4027 has less than0.6% alkali content.This

I requirementhas to be specifiedat the time of ordering.Providedthat their water-solublealkali contentis takeninto account,either ggbfs or pfa can be usedas apartial replacementfor BS 12 Portlandcementto reducethe alkali contentof thecementitiousmaterialsbelow0.6%.ui

(3) When avoidanceof ASR is basedon limiting the alkali contentof the concretetoamaximumof 3k~in~. all sourcesof alkali haveto betakeninto account.In particularthe contributionof sodiumchloridewhetherfrom aggregatesor from mixing water

7 mustbe included.(4) If ggbfsor pfa are includedin the concretemix asapartial replacementfor Portland

cement,therevisedGuidanceNotesrequirethe inclusionof the water-solublealkalicontentof whicheverdiluent is used.

In the caseof ggbfs.the control of alkalis can be achievedin one of two ways:

(a) replacementof cementby ggbfs at a minimum level of 50% so that thecombinationhasan acid-solublealkali contentof lessthan 1.1%,

(b) replacementof cementby ggbfsat a level greaterthan30% such that the acid-solublealkali in the ggbfswhencombinedgive atotal alkali contentof not morethan 0.6%.

Suitable pfa can be usedas a replacementof 30% or moreof the Portlandcement.provided that the total alkali level in the concretedoesnot exceed3kg/in3 ~vhenthe acid-solubleandwater-solublealkalis of the Portlandcementand pfa respectivelyare takeninto account.

Thereareothermatterscoveredin the Guidance.Notes~vhichthe Code refersto butdoes not cover in detail. In the absenceof a recognisedtest a list is given of those

aggregateswhich areconsideredto be non-reactive.In addition, the reactiverock typeschert andflint areconsideredto be safeprovidedthat theyarepresentat alevel greaterthan 60% of the combinedcoarseandfine aggregates.

Structureswhichare consideredto be particularlyvulnerableto attackby ASR includethose subjectedto high humidity and those buried in waterloggedground. Highwaystructurescomeinto this categoryandare in additionsubjectedto frequentsaturationwith de-icingsalts. In such cases,more rigorousprecautionsmaybe necessary.

For further information. seealsoreferences6.2. 6.3. 6.4.

6.2.6 Placing, compacting, finishing and curing

6.3 Concrete mix specification

6.3.1 GeneralFollowing the publicationof CPI1Oin 1972. the British StandardMethodsfor specifyingconcrete(BS 5328) was publishedin 1976. and revised in 1981. It was intendedthatBS 5328 shouldprovidea singlestandardfor concreteto be referredto in all codesandspecifications for concrete. Unfortunately’ the publication and revisions of thesedocumentshavenot kept in stepanddifferent terminologyhasbeenusedfor the typesof concretemixesas shown in Table H6.S.

Irrespective of the detailed terminology, the fundamentaldifference between adesigned’mix anda ‘standard’or (special)‘prescribed’mix lies in the responsibilityfor

selectingthe mix proportions.the form of specification.the materialswhich can be usedandtheparametersfor judgingcompliance.Thesedifferencesareshownin TableH64.

It is the Engineersresponsibilityto selectthe concretegradetogetherwith any limitsrequiredon the mix proportions.the requirementsfor fresh concreteandthe typesofmaterialswhich ma’- or may not be usedto meet his strength.durability andany other

135

Hanabook to BS8IIO:1985

Table H6.3 Types of concrete mixes in British Standards

CPIlO BS5328 BS8IIO1972 1976,1981 BS 5328(revision in preparation)

Designed Designed Designedspecialordinary

Prescribedspecial Specialprescribed Prescribedordinary Ordinaryprescribed Standard*Table50 Table1

1n the Codethis is incorrectly termed‘Ordinary Standard’.

Table H6.4 Characteristics of different types of mix

Typeofmix Designedmix Prescribedmix Standardmix

Permittedgrades All grades All grades C7.5— C30Mix specifiedin Performance Mix proportions Mix fromTabletermsof (strengthgrade)Responsibility Producer Engineer Engineerfor mix design selects

Permitted Generally Freetospecify, Complyingwithmaterials complyingwith or restrict,any arestricted

awide rangeof material rangeofBritishBritishStandards Standards

Main parameter Strength Mix proportions Mix proportionsusedfor judgementofcompliance

special requirements. Wherever possible, limitations on materials and mix proportionsshould be kept to the minimum neededin order that the concrete producer can makethe bestuseof his knowledgeandexperienceof local materials.

6.3.2 Selectionof compressivestrength gradeThe grades of concreterequired should be selectedfrom thosegiven in BS 5328 (TableH6.5).

Table H6.5 Compressive strength

Concretegrade Characteristic compressivestrength at 28days

N/min (~ MPa)C2.5 2.5CS 5.0C7.5 7.5CIO 10.0C12.5 12.5ClS 15.0C20 20.0C25 25.0C30 30.0C40 40.0C45 45.0CSO 50.0C55 55.0C60 60.0

The minimum grades and/or other specifiedrequirements for reinforced. prestressedand unreinforced concreteand different condjtions of exposure are given in

- - .. ... K

..K KKK K. K.~ ,K —

K K

[IF

F

F

I,

I

C

I—I

136

I

[

Ii

- ---K-K- - K--K K - -. - . K , K K ~ -. -4 —. — K K -

-- ..- - - K-4 ~KKK K K- K ~K K K

K-K..-... 4K~ ~ K ~ K~K ~~KKKKKKKK 4-,--—’ K..--.

Part!: Section6

Table 3.4 for reinforced concreteTable4.8 for prestressedconcreteTable 6.2 for unreinforcedconcreteTable 6.1 for concreteexposedto sulphateattack.

BS 8110 deals primarily with concrete for structural purposes.However, if concreteis requiredfor non-structuraluses.such as blindingor backfill, thenthe mixes given inTable H6.5 may be appropriate.

6.3.3 Limitations on mix parameters for durabilityThe free water/cementratio is an importantfactor governingthe durability of concreteand should always be the lo~vest value compatiblewith producing fully compactedconcretewithout segregationor bleeding.A minimum cementcontent is a prmaryrequirementfor durability. The cementcontent requiredfor a particularwater/cementratio canvary significantly for different mix constituents.Whereadequateworkabilityis difficult to obtain at the maximum free water/cementratio allowed, an increasedcementcontent,the useof ggbfsor pfa and/or the use of plasticizingor water-reducingadinixturesshouldbe considered.

Mixes are frequently specified in terms of prescribedmixes. In such cases,theimportanceof minimum cementcontent and free water/cementratio in determiningdurabilitysuggeststhatconcretemixesshouldpreferablybe specifiedin termsof (special)prescribedor standardmixes.

With prescribedmixes. the Engineerhas the responsibility for specifying the mixproportionsand ensuringthat thesewill providethe requiredperformance.Moreover,with a prescribedmix, strengthtesting is not a meansof judging compliance.

Therearesome occasionswhen a prescribedmix may be suitablesuch as:

(a) where the Engineer has had successfulexperience in the past of a prescribed mixmadewith particularconstituentmaterialsfrom known sources

(b) where the concreteis to be providedby a contractorand thereis insufficient timefor the collectionof data. or the scaleof work or economydoes not justify theapplicationof mix designprocedures

(c) wherespecialarchitecturalfinishessuchas exposedaggregateare required.

In Table 1 of BS 5328,ordinary prescribed (or standard) mixes are given in nominaltermsby massof dry aggregateto be usedwith 100kgof cement,for the lower gradesof concrete from C7.5 to C30.

As far asBS 8110 is concerned. the ordinary prescribed (or standard) mixes will onlycover the grades C25 and CSO. Since they have to takeaccountof sucha wide rangeofmaterials,limitations areapplied to the typesandgradingsof aggregateswhich canbeusedandthe cementcontentsareconservati~’elyhigh.

As strength is not a criterion with prescribed mixes. compliance with the specifiedmixproportionshas to be assessedby either:

(a) observationof the batching.(b) examinationof the autographicrecordsof the batchweights used,or(c) resultsof analysistestson the fresh concretewith the requirementthat proportions

shall be within ±10% of the valuespecified.

Compliancewith the specifiedmaximum free water/cementratio may be assessedusingworkability testresultsprovidedsatisfactorvevidenceis availableon therelationshipbetweenfree water/cementratio andworkability for the materialsused.

The cementcontentwill affect the appearanceof the hardenedconcrete,the handlingand placing characteristicsof the fresh concrete and performanceduring setting.hardening and curing when, for example. bleeding and ‘settlement’ after initialcompactionmay occur. If a mix is specifiedonly by referenceto the size of aggregate.slump and strength,then some qualities of the fresh or hardenedconcretemay beinadequate.Variability and deficiencies in grading of aggregatemay necessitateaminimum cementcontent to reducethe sensitivity of the mix to bleeding,grout loss.colour variation, poor local compactionetc.

Therefore.when checkingthe cementcontentof a proposedmix for any concrete.assessmentshouldbe madeof:

(a) the likely variability in mix materials 137

Handbookto BSS!I0:1985

(b) the workability requirements(c) the surfacefinish(d) otherspecialplacing requirements.e.g. pumping(e) the permeabilityof the hardenedconcrete.

When concretemixesare specifiedeitherin terms of a minimum cementcontentora maximum water/cementratio. some difficulties may occur in establishingcompliancewith theserequirements.Analysis of freshconcreteis not a generallyacceptedtestatpresentandcontinuousobservationof the materialsbatchedis not alwayspracticable.As an alternative,assuranceof compliancewith mix proportionscan be obtainedbyadoptinga compressivestrengthgradeas suggestedbelow~6~~.

From a comprehensivesurveyof concretemixes manufacturedthroughoutthe UK.basic relationshipswereobtainedbetweenthe averagecompressivestrengthat 28 daysandcementcontent,and freewater/cementratio andcementcontentasshownin FiguresH6. I andH6.2. Thesedataapply to ordinarvPortlandcements.75mmslumpandcoarseaggregateof nominalmaximum size 20mm. From such dataappliedto local materials,it is possible to establishthe averagecompressivestrength level which will satisfy anycombinationof maximum free water/cementratio andminimum cementcontent.

For anygiven requirement.theequivalentstrengthgrademay thereforebe obtainedfrom oneor other of the following methods.

MethodAUse of datafrom recordsor from trial mixesrelating to cementcontent,water/cementratio and meanstrength(M). representativeof the particularmaterialsandworkabilityproposedfor use.The equivalentgradeis takenas (M—I0) N/mm-.

50

45

ST~ENGTI4REFEREN~40 cURVES(N/mm:) K

~35

E2

z0zwI-

wnU

0

wC,

20

10

55

[IF

rrC

C

r[

C

C

EC

70

60

50

40

30

Figure H6.l:

139

30

U-- “•..—--

300 350

cEMENT CONTENT (kg in3)

Relationship bet wee,i strength and cement content of concrete madewith OPC. 75,nm shimp and coarse aggregatewith maximumsize20mm.

- K K - - K -. ~ ~~.:-:: - -: - —-.: . .4 444 . - . — . — 4 4 - , 4 ,~ 444 44~ 4- 4

K-- ..- K K~ K K K..4~4::44 ,. -- -. K K -. ~ ~K:K K— K — — K — , K K

K — . — K K— - -_______ ~:I- ---:-.Z--..-...--..C’-.- K --

Part]: Secnono

0.8

0.7

QI--( 0.6I-zw‘UU

‘UI.-~ 0.5‘U‘UCU-

0.4

0.3

230

WATERDEMANDREFERENCECURVES(kg~in

2)

220210

200

190

180

170

160

150

Figure H6. 2: Relationshipbetweenfree water/cementratio and(0 PC, 7Smmslump, 20mmcoarse aggregate).

cementcontentof concrete

Method BFor any given cement content and ~vater/ceinentratio. Tables H6.6 and H6.7 may beused to determinethe controlling grade.Thesevaluesprovide a probability of about95% that the cementcontent andwater/cementratio requirementswill be met usingUnited Kingdom materials (British ordinary Portland cement, aggregateof 20mmnominal maximum size in a concretehaving a slump of 75mm). Modifications to theequivalentgradesgivenin TablesH6.6andH6.7to allow forotherspecifiedrequirementsare given in Table J-16.8.

Table H6.6 Equivalent grades for cement content

Minimum cementcontent(kg/in3) Equivalent grade

220—230 C20240—260 C25270—280 C30290—310 C35320—330 C40340-360 C45370—39(1 CSO

400andabove Usemethod(A)

139

I

200 250 300 350 400 450

CEMENT CONTENT 1kg/rn3)

Handbook to 8S8!I0:1985

Table H6.7 Equivalent grades for free water/cement ratio

Maximum freewater/cementratio Equivalent grade

0.70 C250.65 C300.60 C350.55 C400.50 C450.45 C50

lessthan0.45 Usemethod(A)

Table H6.8 Modifications to Tables H6.6 and H6.7 to allowfor other specified requirements

Specifiedrequirement Adjustment to theequivalentgradein TablesH6.6 andH6.7

Workability (slump) 25mm100mm 0

Nominal maximum 10mm 0aggregatesize 14mm 0

40mm 0

Aggregatetype Lightweight Usemethod(A)

Cementcomplying BS 4027 0with BS 12(rapid +5

hardening)BS146 0BS6588 0BS 1370 Usemethod(A)

Admixtures Waterreducingagents

Only whencementcontentis the critical parameterdetermininggrade.

6.3.4 SpecIficationof constituentmaterials

6.3.4.1 GeneralA list of the clausesgiving informationon the effectsof materialson thepropertiesofconcreteis given in Table H6.9.

6.3.4.2Cements,ggbfsandpfa

6.3.4.3Aggregates

6.3.4.4Admixtures

6.3.5 Fresh concreteWorkability andcohesionof the freshconcreteshouldbe suitablefor the conditionsofhandling and placing so that after compactionconcretesurroundsall reinforcement.tendonsandductsandcompletelyfills the formwork. Excessbleedingshouldbe avoidedas this can lead to plastic settlementcracking and/or poor quality surfaces.

The characteristicswhich have the major effecton the propertiesof fresh concreteare

workability (6.3.5.2)air content(6.1.5.5and6.2.3.2)temperature(6.2.2.1.6.7 and6.8)minimum and maximumdensity(6.3.5.3)

140 aggregategrading(6.1.3.4).

-K.. K~ -~K K K K - - K KKK..K — K K K K — K KKK — K — K K K K KK. ., KKK K K K K — KKKKKK ~KK~KK K , K.4K — K K — ~KKKK 4

K —, — — — — . K K K 4 K KK KK~ ..KKKK~ K K KKKK•~KKKKK~K~KK~KKKKK~ K K K K — K~ — —K K-,~ K K~ ~ K~ ,K •.. ~

IC

[

Li

I[

[3

r[

I

C

II!

I

nUI

—.-~-~: ..4 K - K, K~ K -- ~4. ~ K K~ K K K K - K

Part1. Section6

Table H6.9 Clauses relating to the effects of materials on thecharacteristics of concrete

Propertyofmaterialorconcrete

Cement,ggbfsorpta

Aggregates Admixtures

StrengthEarlydevelopmentVery high

6.1.2.16.1.3.9

6.1.5

Low heatevolution 6.1.2.26.1.2.5

MaterialsLimits on:

gradingsulphateschlorides

shellcontentflakinessdensity

Avoidance of:highmoisture movementsother effects

6.2.5.36.1.3.46.2.5.3

6.1.3.56.1.3.6

6.1.3.11

6.1.3.76.1.3.3

6.2.5.36.1.546.2.5.2

Improved resistanceto:sulphateattack

ASRfreezingand thawing

wearfirechemicalattack

6.1.2.46.2.3.36.2.5.4

6.1.2.26.2.3.3

6.2.5.4

6.1.3.126.1.3.10

6.1.5.56.2.3.2

Of these characteristics,the Engineer would not normallyworkability, sincethiswould be decidedby the Contractor.

be concernedwith

6.3.5.1 General

6.3.5.2 Worka.bility

6.3.5.3Density

6.3.6 Concrete to meet specialrequirementsConcretemixesmadewith mostBritish cementandaggregatecanbe designedto meetthe requirementsof strength,durability and workability undernormal conditions ofexposure.Where specialrequirementsare needed.guidanceis evenin the Code andthis is summarisedin Table H6.9.

6.4 Methods of specification, production, control and tests

6.4.1 Specificationand acceptanceof mixSpecificationcan bestbe done by the useof forms.as in AppendixesA, B andC:

Appendix A Form for specifying a designedmix or a prescribed mix, in accordancewith BS 5328Section 1: EssentialitemsSection2: Optional items

AppendixB Form for specifyinga standardmix in accordancewith BS 5328AppendixC Materialsfor usein standardmixes 141

Hcjndbook to BSS/IO.198S [IThe exchangeof information should include: F(a) natureandsourceof constituentmaterialsandanyalternativeswhich maybeused:(b) manufacturers’certificatesfor cement.ggbfs and pfa:(c) proposedquantityof eachmaterial for prescribedmixes: C(d) detailsof admixtures:(e) any changesin mix composition:(f) for designedmixes. information on suitability of proposedmix proportionsto meet

a specifiedstrengthbasedeither on previousproductiondataor on trial mixes: L

(g) suitability of proposedmix proportions to meet a specifiedmaximum free water/cementratio or minimum cementcontent:

(h) any other information.

6.4.2 Production, supei-i-isionand tests CCompliancewith characteristiccompressivestrengthCompliancewith the characteristicstrengthis based on individual test results andongroupsof test results.

Wherecompressivestrengthis specified.theirst resultalonecannotbe usedto judgecompliancewith the specified characteristicstrength. Compliancewith the specifiedcharacteristicstrengthshall be assumedif: F(a) the averagestrength determinedfrom the first 2. or the first 3 consecutivetest

results.or from consecutive,but non-overlapping,groupsof 4 testresultscomplieswith the appropriatelimits in column A of TableH6. 10. and U

(b) any individual testresult complieswith the appropriatelimits in columnB of TableH6.10.

C.rable H6.1O Compressive strength compliance requirements

A BSpecifiedgrade

Testresults

Averageo(first2,orflrst3, orof4consecutive,non-overlapping test

results exceedsthe;specified

characteristic

Anyindlvidualtestresultisnot lessthan

thespecifiedcharacteristicstrength

minus:

strengthby atleast:

C20andabove

first 2first 3consecutive4

IN/mm2N/mm3N/mm-

3N/mm3N/mm3N/mm

C7.5 toC15

first 2first 3consecutive4

ON/mm-IN/mm-2N/mm-

2N/mm2N/mm2N/mm

E

CC

[

II;LiL[

U

Compliancewith specifiedmix proportions(prescribedandstandardmixes only)BS 5328 details the compliance requirementsfor mixes specified in terms of mixproportionsand theseare summarisedin TableH6.11.

6.4.3 Additional testson concretefor specialpurposesOther requirementsmay be specifiedthat are not describedin detailin this Code.suchas modulusof elasticity of concrete. Compliancewith those requirementsshould bedeterminedonly in associationwith the detaileddescriptionof the method of testandwith tolerances‘vhich take appropriateaccount of variability due to manufacture.samplingandtesting. A British Standardmethodof testshouldbe usedwheneverit ~S

appropriate.Otherinformation on the strengthof concretein structuresis given in BS 6089.

-KJ.

K K — 4— — K — — — ~ K

Part I: Section6

HG.11 Compliance with specified mix proportions

Specifiedproperties Compliancerequirements

Minimumormaximumcementcontent(i) By observtionof batchingor

fromautographicrecords

(ii) Freshanalysistestsin accordancewithDD 83Assessmentofthecompositionoffreshconcrete

Cementcontentnot lessthan95%of specifiedminimumormorethan 105%of specifiedmaximumLimits agreedwith concreteproducer.basedon DD 83

Maximumfreewatercementratio \Vorkabilitv results.basedonrelationshipbetweenfreeWater/cementratioandworkability

Equivalentgrades Compliancebasedonequivalentgradeagreedassatisfyingminimumcementcontentormaximumfreewater/cementratio

Workability (designedandprescribedmixes)Slump

Slump(sampletakenin accordancewith12.2of BS 5328)

Vebe

Compactingfactor

Flowtable

z25mmor±‘/3ofthespecifiedvalue.whicheveris thegreaterSpecifiedvalue Tolerance25mm +35mm

-25mm50mm ,~mm75mmandover ±~~3of specifiedslump

plus 10mm)~,s or 1/5of thespecifiedvalue,whicheveris thegreater±0.03wherespecifiedvalueis090or greater±004wherespecifiedvalueis lessthan090butmorethan0.80±005wherespecifiedvalueis080or lessSpecifiedvalue±50mm

Air contentofconcrete Individualsamples±1.5% of therequiredvalueAverageof4 consecutivedeterminations±1.0%of therequiredvalue

Temperatureof freshconcrete Not lessthanspecifiedminimumvalueless20CNot morethanspecifiedmaximumvalueplus20C

Densityof fully compactedconcrete Not lessthan95%of specifiedminimumvalueormorethan 105%ofspecifiedmaximumvalue.

6.5 Transporting, placing and compacting concrete

6.5.1 Transportof concrete

6.5.1.1 GeneralConcreteshould be transportedas rapidly as possiblebecauseany undue delay maycausethe workability to decreaseto the extentthat it cannotbe properlycompactedasrequiredby Section6.5.2.Therateof lossof workability with time dependson a numberof factors including cementtype. admixtures.concretetemperatureand the rate ofevaporationof waterfromtheconcrete.Guidanceon acceptableintervalsbetweenmixingandplacingconcreteis given in BS 5328.

6.5.1.2 Transport anddeliveiy of ready-mixedconcreteBefore ready-mixedconcreteis delivered,the siteshouldensurethat the truckcangainaccessto the intendedpoint of dischargeand that.when discharged.the concretecanbe transportedto the point of placing in accordancewith the requirementsof Clause6.5.1.1.Furtherinformationon thehandlingof ready-mixedconcreteis givenin BS 5328.

143

Handbook to BS8/IO-/985

6.5.2 Placing and compacting concretePrior to placing theconcrete,checksshouldbe madeon therigidity andtightnessof theformwork andon the fixing of reinforcementandprestressingducts.Particularattentionshould be given to ensurethat sufficient spacersin number, location and quality havebeenusedand that they are securelyfixed and do not becomedislodgedduring theplacingof the concrete.

Particularcareshould be takenwhen placing concreteunderbox outs, top slopingforms or othercomplex shapeswhere air pocketsmight form. Extra care is requiredwhencastingagainstpermanentformwork if. like woodwool. it canabsorbenergyfromthe concretewhilst it is being compacted.

Thorough compactionis essentialif the hardenedconcreteis to havethe intendedstrengthand durability. For generalguidanceseereference6.9. Furtherinformation onplacing concretein deeplifts is given in reference6.10andon concretingunderwaterinreference6.11.

When concrete is placed in deeplifts with reinforcementnear the upper surface.considerationshouldbe given to re-vibratingthe top surfaceof the concreteto preventplastic settlementcracking.This is a particularproblemwith concretescontainingggbfsor retarders.

Finishingtimes are increasedwith concretescontaining mixturesof OPCwith ggbfsor pfa. This can be an advantagein hot weather,but in mild or cold conditionsit mayrequire overtime working to ensurea properly floated surfaceand adequateabrasionresistance.

66.1 GeneralAppropriatecuring is essentialfor achievingthe strengthanddurability of concreteinstructures.The areasmostaffectedby poorcuring are the surfacezonesandthesearethe critical zoneswith respectto durability. Abrasionresistancedependson the qualityof the concretein the top few millimetresand the protectionof reinforcementdependson thequality of the concretein the cover. If the curing is inadequatethe concretemaynot be durable nor provide adequateprotection to the reinforcement despite fullcompliancewith the specificationin all other aspects.

Severalreferencesprovide information on curing when membersareof considerablebulk or length~6-’2 6,13), the cementcontentof the concreteis hight613),the surfacefinishis critical~6-’4- ~ or specialor acceleratedmethodsareto be applied~6-16~.

6.6.2 Minimum periods of curing and protectionCuring shouldideally be carriedout until the capillary voids arediscontinuous,but atpresentit is not possibleto establishthe precisetimeswhen this occurs.The figures inTable 6.5 provide a useful guide but are minima and may needto be exceeded.Thetimesquotedrelateto both the ambientconditionsandthe averagesurfacetemperatureof the concrete.The ambientconditionscanvaryduring the periodof curing. Conditionswhich startedas ‘good’ can deteriorateto ~average’or ‘poor’ and the curing periodsshouldbe increased.Forexample,if the ‘good’ conditionsdo not last for the ‘averagetime, curing needs to be applied for the remainderof the ‘average~period or theoutstandingproportionof the ‘poor’ period. Proportionsof the curingtimescanbe usedto calculatethe curing period but it may be simpler to cure for the longerperiod. CEBBulletin 166 gives additional information on curing’6 ~

Concreteand ambient temperatureswill vary throughoutthe curing periodand thedaily averagevaluescan usuallybe takenas the mid value betweenthe maximumandthe minimum readings.At 00C the water in concretefreezes,expandsanddisruptstheconcrete.Temperaturesbetween1 and50C are not harmful to concrete,but the rateofstrengthgain is very slow and therefore for practical reasonsClause6.6.2 requiresaminimum concretetemperatureof 50C. BS 5328 states that the temperatureof theconcreteat the time of delivery shall not be less than the specifiedvalue less 20C whichin practicemeansthe acceptanceof any concreteover30C. Whenoneconsidersthat anunheatedform is likely to causethe concretein the surfacezone to cool, a delivCIYtemperatureof 30C is too low. It would be prudentto specify a minimum concrete

[IrCI

rr

I6.6 Curing

C

[

U

L[

C

ITi

U1-14

[

-..~4.~ K - K

K K K K ~ KKKKKK K K K

Part 1: Section6

temperatureof 7”C which in practicewill meanthat any concretenot at 50C or greatercan be rejected.

It should be notedthat the necessarycuringtimesareincreasedwhenthe OPC in amix is partially replacedby ggbfs or pfa. This is to compensatefor the lower rate ofstrengthdevelopment.

Ideally, curing shouldstartas soonas the concretehasbeenplacedand should notbe interruptedduring the whole of the period givenin Table 6.5.

6.6.3 MethodsFurtherinformationon methodsof curingaregivenin references6.10and6.15.Thermalcuring of largepours is describedin references6.12 and6.13.

6.7 Concreting in cold weather

6.7.1 GeneralExperiencefrom NorthernEurope and Canadashowsthat it is possibleto concretethroughoutthe winter,but in somepartsof the UK it maybeworth consideringwhetherit would be acceptableandeconomicto suspendconcretingfor a short period. It hastobe appreciatedthatin verysevereconditionsit maynot bepossibleto transportconcreteto or arounda site.

6.7.2 ConcretetemperatureGuidanceon this maybeobtainedfrom references6.18 and 6.19.

6.8 Concreting in hot weather

Furtherinformationon concretingin hot weatheris given in references6.20 and6.21.

6.9 Formwork

6.9.1 Designand constructionThe designandconstructionof formwork and falseworkhasa significanteffecton theappearanceanddurabilityof concretestructures.Someof thereferencesgiveninBS 8110for loadingor pressureshavebeensuperceded.The currentrecommendationsaregvenin references6.22 and6.23.

6.9.2 Cleaning and treatment of formsThe typeof releaseagentusedcan influencethe appearanceof theconcretesurfaceandthe life of the formface~6-~~.The releaseagentmayalso affectthe bondof paintor othersurfacetreatmentssubsequentlyapplied to the concretesurface.

6.9.3 Striking of formwork

6.9.3.1 GeneralEarly removal of formwork can reducecycle time for both in situ and precastwork.This can be achievedsafely by the use of acceleratedcuring technIques’616’6.Z41

6.9.3.2Striking periodfor castin situ concreteTable 6.6 is basedon a grade20 concreteandas the lowest gradePPFACreinforcedconcreteis C30, this may be usedto determinethe striking timesof PPFACconcretes.Table 6.6 should not be used for slag-basedblendedhydrauliccementconcretesunlessevidenceis producedto show its applicability for the particularmaterialsbeingusedonthe site. 145

ii1~

Handbookto 8581JO:/985

The headingin Table 6.6 “160C andabove”doesnot rule out shorterperiodsbeforestriking if the calculationusingthe equationin the lastcolumn is carriedout.

The effectsof temperatureon the rateof gainof strength(maturity) are differentforblendedhydraulic cementconcretes.The maturity rules used for OPC concretesareunlikely to be applicableand a maturity calculationbasedon activation energiesisrecommended.

Thestrengthof concretein a structuralelementcanbe assessedfor striking purposesby using temperature-matchedcuring’6~, pull-out tests’66~, break-off tests’6~7~ orpenetrationtests~6-8’.Careis neededto ensurethat a safe(conservative)relationshipisusedto convertinstrumentread”’cr to equivalentcubestrength.

An unintentionalchangein the idifl~ntiz~-~ction on striking formworksupportingconcretein flexure from CPI1O’s ‘ION/mm in cubesof equalmaturity to the structur&to BS 8110’s ‘iON/mm strengthin the structure’may leadto an incorrect interpretationof this clause.What is intendedis lONfmm in cubesof equalmaturity to the structureand an amendmentis being issued to changethe text to clarify this point. The idealmethodof curing cubesunderthe sameconditionsas the concretein the elementis byusing temperature-matchedcuring~6~’.

In partially completed structures, local bond stresses on partially embeddedreinforcementmay be the controllingcriterion. Unfortunately,the table of local bondstresseshas not beenincluded in BS 8110 and thereforereferenceshouldbe madetoCPI1O or reference6.24.

Surface finish of concrete

6.10.1 Type of finishGuidanceon the wide rangeof finishes which may be obtainedis given in references6.14and6.29-6.32.

6.10.2 Quality of finishGuidanceon the specificationandproductionof highquality finishes is givenin reference6.33. Where the quality of the surfaceis important, it is essentialthat specifierandcontractor liaise closely before and after the tender stage to develop a mutualunderstandingof the qualityrequired.Demonstrationor trial panelscanbe of immense Chelp in developingthis understanding.6.10.3 Type of surface finish

[Smoothoff-the-form andboard-markedfinishescan form the basisof a wide rangeofinternal finishesproducedby the applicationof paint or othercoatings.

6.10.4 Production LGuidanceon theproductionof surfacefinishesis ~ivenin references6.14and6.29-6.32.

6.10.5 Inspection and making good LObtaininga high-quality durablefinish when making good is a skilled operationwhichshouldbe undertakenby a specialist.

6.10.6 ProtectionVulnerableareasof surfacemay merit permanentprotectionto avoid,discolorationor

Lphysical damage.

6.11 Dimensional deviations

Formwork is madeanderectedwithin sometolerances— whetheror not theyarespecified [and checked— and at somecost which will dependconsiderablyon the level of thetoleranceadopted.Even when no specialattention is given to tolerances,experience fideterminessome upper limit to the deviation from the specified nominal dimensiOns ~becauseexcessiveinaccuracywould causeconstructiondifficulties, resultingin increased

146 cost, or would producework of unacceptablequality. Within the rangeof tolerances L- - — K ~ -- -

K” -.-... ~ - - .-K- I. K -,— .4 — ~ — — ~ K K K . . K — K K — K

Part1: Section6

used in practice.however,an increasein precisionwill generallybe accompaniedby an1increasein formwork costs.

The engineeris usually concernedwith toleranceson the finished concrete,whethercastin situ or precast.Forprecastconcrete,the deviationsof the relevantdimensionofthe mould from the nominal dimensionspecifiedshould normally be smallerthan the1specified tolerancesfor the concrete.to accountfor such factorsas:

(a) the inaccuracyof the measurementsystemusedwhen making and assemblingthe

1 mould(b) the changein size of the mould with repeateduses(c) thechangesin sizeof the mouldwith changesof temperaturewhentheconcreteiscast

~1 (d) the deformationof the mould during casting(e) the changein sizeof the finishedconcretedueto changesin the conditionsto whichit is exposedbetweencastingandchecking.Thesefactorsarealsovalid for in situ concretebut, in addition,the following should

alsobe takeninto account:

(f) the accuracyof setting out

] (g) theuncertaintyof fixing theformworkcorrectlyin relationto thesettingout positions(h) the deflectionof any supportingfalseworkduring casting.

When tolerancesarespecified.therefore,it is essentialthattheybe relatedto aclearly

I defined and statedcheckingprocedurewith a precision of measurementappreciablygreaterthanthe toleranceallowed.The level of toleranceselectedshouldnot be out of

] proportion to the otheruncertaintiesinherent in the construction.

6.11.1 General

] 6.11.2 In situ concrete

6,11.3 Precastconcretemembers

6.11.4 PrestressedunitsFK~.J

6.11.5 Positionof reinforcementandtendons

16.11.6 Positionof connectingbolts andotherdevicesin precastconcretecomponents

[j 6AL7 Control of dimensionalaccuracy

6J1.8Checkingof dimensionalaccuracy

6.12 Construction joints

Furtherinformationon the forming of constructionjoints is availablein reference6.10.Particularcare should be taken ‘~ith constructionjoints in liquid retaining structures(BS 5337). If reinforcementle~els are lo~v. restrainedthermalcontrtion or restraineddrying shrinkageare likelx- to manifestthemselvesat constructionjoints.

The Code is understandablybrief when describing good practice in relation toconstructionjoints becauseof the vervwide rangeof circumstancesin which constructionjoints haveto be made.

Insteadof theweak porousconcretessometimesseen,thequality of concretein kickersshould be at leastequalto thatof the main structure.Since,to obtaina fully compactedconcrete in a kicker. the proportion of fine to total aggregateis likely to have to beincreased,the maximumsizeof theaggregatereducedandtheworkability of the concreteincreasedcomparedwith the concretein the structure,it is evidentthat the proportionof cementin the concretefor kickersshould generallybe greaterthan that in the main

147

Handbook to BS8IIO:1985

part of the structure.This is liable to result in a concreteof darkercolour than the rest Fof the structure:hence,wherethe appearanceof the work is of high importance,kickersare bestavoided.

Ineffectiveclampingof the forms to hardenedconcreteall too often leadsto leakageof mortar from the subsequentpour andthereforeto an unsightly appearance.

The need for full compactionof concrete in the vicinity of constructionjoints ise~.iphasized,particularly as the effectivenessof compactiveeffort is often reducedcloseto a mass of hardenedconcrete.Although the possibility of using a rather higherworkability concreteat the joint is often discussed,this is not practicablewhen ready-mixed concreteis used.The best way of attaining full compactionis thereforeto usevibratorsand to reckonthat theywill have to beused for considerablylonger,perhapstwice as long, nearjointsas comparedwith the bulk of the structure.

No recommendationsare madeaboutthe moistureconditionof thehardenedconcreteagainstwhich a new concreteis to be placed:the concreteshouldnot. however,be sodry as to drawexcessivequantitiesof waterfrom the concretebeingplaced;converselythe concreteshouldnot havepuddlesof freewater laying on it as this might inhibit goodbond betweenthe old and new concretes.The Code also makesno recommendationsabout the useof a layer of freshmortaror grout on the hardenedconcretejust beforethe new concreteis to be placed. Although good joints havebeenproducedby usingmortar or grout. the techniqueappearsto presentmore problemsthan it solves: it istherefore best avoided in most situations and emphasisplaced instead on a highcompactiveeffort.

Note that the surfacetreatmentselectedfor joints may be definedby considerationsof interfaceshear(5.4.7). [

6.13 Movement joints [Poor performanceof movementjoints canaffect the serviceabilityand durability of astructure.If joints cannotmove, this may causethe structureto crackexcessivelyor forotherjoints toopenexcessivelyallowingingressof deleteriousmaterials.Specialattentionneedsto be paid to joints which include a waterbaras sometypes are particularlyvulnerable to the consequencesof poor workmanship.For further information, seePart2. Section8. [

6.14 Handling and erection of pre-cast concrete units [L

REFERENCES6A HARRISON, TA, and sPOONER, D.C. The propertiesand use of concretesmade with composite

cements.WexhamSprings.CementandConcreteAssociation.Interim Technical Note 10.1986. L

6.2 SOMERVILLE. 0. Engineeringaspectsof alkali-silica reaction. Wexham Springs,CementandConcreteAssociation.Interim TechnicalNote No.8, October1985.

6.3 HOBBS. 0 w Expansionof concreteduetoalkali-silicareaction.TheStructuralEngineer.January1984, Vol62A. NoA. pp 26-34. [

6.4 THE CONCRETE SOCIE~I4Y. Minimising the risk of alkali-silica reaction.GuidanceNotesand Model

SpecificationClauses.Draft for Public Comment.London.The Society, 1985.6.5 DEACON. C. and DEWARK 1.0. Concretedurability — specifyingmoresimply and surely by strength. r

Concrete.February 1982. Vol.16. No.2. pp 19-21. L6.6 KLEINLOGEL. A. Influenceson concrete.New York. Frederick Ungar PublishingCo. 1950.

28lpp.6.7 EGLINTON. ‘i s. Review of concretebehaviour in acidic soils and ground waters.London. f

ConstructionIndustryResearchandInformationAssociation.TechnicalNote69. 1975.52pp. L6.8 BARRY. D.L. Materialdurability in aggressiveground. London.ConstructionIndustry Research

and Information Association.Report98. 1983. 60 pp.6.9 BLACKLEDGE. o.i~ Man on the job: Placingandcompactingconcrete.WexhamSprings.Cement

and ConcreteAssociation. 1980. 45. 108. 28 pp.6.IOCEMENT AND CONCRETE AssOCIArION Concretepractice.WexhamSprings.CementandConcrete

148 Association.1984. 48.037. 63 pp. -~

<K -K ~ ,~ -~ -K. --~ - ~ ~ K. K K K- -.K K .44•.. ~ K. .~. .•~ K

K K — K K

_ _____ — - K K ... .. — - - —

K— K - 4KKKK~ ~4 - ~4 - _ - —. ~1..A••44 4 K~4~ K K~ - - -

Parr 1: Section6

6.11TIlE C~~~R~TE socieTy.Underwaterconcreting.London. 1971.52.018.13 pp. TechnicalReportNo. 3.

6.12BAMFORTN. i~a. Massconcrete.London. ConcreteSociety DigestNo. 2. 1985. 53.046.8 pp.6.13HARRISON. TA. Early-agethermal crack control in concrete.London, ConstructionIndustry

ResearchandInformationAssociation.Report91, 1981. 48 pp.6A4MONKS, w. Appearancematters— 1: Visual concrete:Designandproduction.WexhamSprings.

Cementand ConcreteAssociation.1980. 47.101.28 pp.6.15DEACON, R.C. Concretegroundfloors: their design,constructionandfinish. WexhamSprings.

CementandConcreteAssociation. 1986. 48.034. 28 pp.6.16pip Guide to good practice— Accelerationof concretehardeningby thermalcuring. London.

FederationInternationalede Ia Precontrainte.1982. 15.907.6.17cm Guide to durableconcretestructures.Lausanne.Comite Euro-Internationaldu Beton.

Bulletin 166. 1985.6.18 PINK. A. Winter concreting.WexhamSprings.CementandConcreteAssociation.1978. 45.007.

19 pp.6.19HARRISON. TA. Tablesof minimum striking times for soffit and vertical formwork. London.

ConstructionIndustry ResearchandInformation Association.Report67. 1977. 23 pp.6.20SHIRI.EY. D.E. Concretingin hot weather.WexhamSprings.Cementand ConcreteAssociation

ConstructionGuide. 1980. 45013.7 pp.6.21 AcInOSR47. Hot weatherconcreting.Detroit. AmericanConcreteInstituteManualof Concrete

Practice.1985: Part 2. pp 305R-1 to 305R-17.6.22CLEAR. CA. andHARRISON. TA. Concretepressureon formwork. London,ConstructionIndustry

Researchand Information Association. Report 108. 1985. 31 pp.6.23TIlE CONCRETE soCIETY/INSTrrUTIoN OF STRLCTIJRAL ENGINEERS. Formwork. A guide to goodpractice.

London. 1986.6.24HARRISON,TA. The applicationof acceleratedcuringto apartmentformworksystems.Wexham

Springs.CementandConcreteAssociation. 1977. 45.032. 8 pp.6.25BRmsH STANDARDS INSTTTtYnON. Methodfor temperature-matchedcuringof concretespecimens.

London.BSI DD92:1984. 4 pp.6.26BICKLEY. LA. North American experiencewith pull-out testing. Paperpresentedat Nordisk

Betonkongress.22 may 1980. 12 pp.6.27JOHANSEN, R. In situ Strength evaluationof concrete— the Break-off’ method. Concrete

International.Vol. 1. No. 9. September1979. pp 44-51.6.28BUNGEY.J H. Testing by penetrationresistance.Concrete.Vol. 15. No. 1.January1981.pp30-32.6.29MONKS. w. Structuralconcretefinishes:a guide to selectionandproduction.WexhamSprings.

Cement and Concrete AssociationReprint 1/80. l6pp.6.30MONKS. w. Appearancematters—7: Texturedandprofiled concretefinishes.WexhamSprings.

CementandConcreteAssociation.1986. 47.107.pp 12.6.31MONKS. W. Appearancematters— 8: Exposedaggregateconcretefinishes. WexhamSprings.

CementandConcreteAssociation1985. 47.108. pp 16.6.32MONKS. W. Appearancematters— 9: Tooled concretefinishes.WexhamSprings.Cementand

ConcreteAssociation. 1985. 47.109. pp 8.6.33wiLsoN. jo. Specificationclausescoveringthe productionof high quality finishes to in situ

concrete.WexhamSprings.Cementand ConcreteAssociation.47.010.pp 12. 1970.

149

Handbookto B58110 198S

Appendix A: Form for specifying a designed mix or aprescribed mix in accordance with BS 5328

Section 1: Essential items

Typeof mix (ringone)

Permittedtype(s)ofcement(ring thosepermitted)

Combinationswithggbfsorpfa(Permittedor notpermitted)

Permittedtype(s) Coarseof aggregate(ringthosepermitted)

Fine

Nominalmaximumsizeofaggregate(ring one)

DesignedorPrescribed

BS 12(OP) BS 12(RH)

BS 4027 B.S4246

BS 4027(low alkali)

ggbfsto BS Permitted6669pfatoBS Permitted3892:Part I

B5882 B5877

B53797

BS882

40mm

Designedmixesonly

Grade(ringonein GroupA. Bor C)

GroupA GroupBCompressive Flexural

C2.5 CiS C40 F3CS C20 C45C7.5 C25 C50 F4 1T2.5ClO C30 C55C12.5 C35 C60 F5 1T3

Minimum cementcontent kg/in3

Rateof samplingfor strengthtesting

Numberof cubicmetrespersampleorNumberof batchespersample

BS 877

20mm

GroupCIndirect-tensile

ff2

BS 146

B54248

NotpermittedNotpermitted

BS 1047

BS 1165

14mm

B51370

B56588

Required(seeSection2)Required(seeSection2)

BS 1165

B53797

10mm

Prescribedmixesonly

Mix proportions

cementfine aggregate

coarseaggregate

kgkg

kg

[IU

U

C

I

C

11

I

154)

- iK K

— K - -. -4 -~ . ~4. - K KK — - - — K K ,K ~ , -

- ~KK--K. K-K-K.-... K, K-K.- K KKKK~K~ K K- - - K KK K K .-:...———~ K ~., K

Parr I: S~crion0

Section 2: Optional items

Workability (entervalueforoneonly)Slump mmVebe sCompactingfactorFlow table mm

Maximumfree-water/cementratio

Maximumcementcontent kg/in’

Specialcement(s)andcombinationsof ggbfsorpfa

Special requirementsfor aggregates

AdmixturesPigmentsggbfspfa

Air content%by

volume

Coarse:Fine:

Specified: Quantityrequired

Prohibited

Temperatureoffreshconcrete

0C maximum0Cminimum

Density of . . kg/in’ maximumfreshconcrete kg/m’minimum

Maximum chloridecontent . . . . % by weight of cement

Specificationoftrial mixes

Details of testprocedures

Methodof assessmentofconcrete

As BS 5328Or (detailsof method)

Other requirements

I

151

Handbook to B58110:1985

Appendix B: Form for specifying a standard mix inaccordance with BS 5328

Standardmix number(ringone)

Permittedtype(s)of cement(ring thosepermitted)

Combinationswithggbfsor pfa(ring thosepermitted)

Permittedtype(s)ofaggregate(ringthosepermitted)

Nominalmaximumsizeof aggregate(mm)(ringone)Workability (ringone)

Any additionalinformation

II

[

LLL

0152

[IC

SMi5M25M3

5M4SMS5M6

BS 12(OP)BS 12(RH)ES 146

B56588

ggbfstoBS6699

pfatoB53892Part1

BS12(OP)BS 12(RH)ES146ES6588ES4027

ggbfstoES6699

pfatoES3892Part1

B5882BS1047ES882N/A

rI

I

r[

aCoarse

FineAll-in

B5882BS 1047B5882ES882

4020

MediumHigh

402010MediumHigh C

——4 KK — — K K

I— ~K ,~ K ~ K K K K K

4—. K. KK --

Part1: Section6

Appendix C: Materials for use in standard mixes

StandardMixes SMi, SM2,5M3 SM4,5M5,SM6

ES 12 (OPor RH)ES146B56588

ES 12 (OPor RH)ES146ES6588ES 4027

Groundgranulatedblastfurnaceslag

Pulverized-fuelash

Coarseaggregate(seeNote 1)

FineaggregateAll-in aggregate(seeNote3)Admixtures

WherecementtoBS 146 ispermitted.ggbfscomplyingwithES6699maybecombinedwith cementtoBS 12 at themixerasspecifiedin 6.1.2.

Wherecementto ES 6588ispermitted.pfacomplyingwith ES3892:Part1 maybecombinedwithcementtoES12 atthe mi.xerasspecifiedin 6.1.2.

ES882ES 1047

B5882BS882

Notallowed

ES 882ES 1047

ES 882 (seeNote2)Not allowed

Not allowed

Note 1. Single-sized coarse aggregates.Where several sizes of single-sizedcoarseaggregatesare used, they shall be combined in such proportionsthat thecombinedgradingfalls within the limits given in BS 882 or ES1047 for gradedaggregateof the appropriatenominalsize exceptthat a toleranceof up to 5%may be applied. This tolerancemay be divided betweenthe sieveswithin thetotal of 55%.

Note2. The purchasershould satisfyhimself that concretemadewith crushedgravelorrock fines, blendedfine aggregateor fine aggregategradingtype F will providethe performancehe requires.

Note3. With all-in aggregates,it is preferablethat the fine contentis atthe higher endof the range.

Cement

153

SECTION SEVEN. SPECIFICATiON ANDWORKMANSHIP: REINFORCEMENT r7.1 General

Hot-rolled steel barsin grades250 and460 are requiredto conform to BS 4-449, Grade250 barsare of circularcross-sectionwith a plain surface,andare availablein sizesfrom6mm to 50mm. Grade460 bars.which ha\-ea highercarboncontent.are usuallyrolledwith a ribbedsurfaceto meet the requirementsfor Type 2 barsandareavailablein the rsamenominal sizesas grade 250 bars.

BS 4461 specifiesrequirementsfor rolled steelbars.other thanplain roundbars, thathavebeencold-workedto increasetheir yield stress.The methodof cold-workingis not rspecified in the Standard.but it normally consistsof twisting the bar while it is heldlongitudinally: extensionin tensionalone is not satisfactorysince this treatmentraisesthe yield stressin tensionbut lowers it correspondinglyin compression.The barsmaybe eitherof plain square(or chamferedsquare)cross-section(Type I) or of circular [cross-sectionwith ribs (Type 2). The latter form is normal nowadays.

Cold reducedsteel~vireto BS 4-482 issuppliedmainlyfor themanufactureof reinforcingfabric andfor the massproductionof reinforcementfor precastconcreteproducts.The

Fwire may be plain, indented(Type I) or deformedin sizesfrom 5mm to 12mm. Fortheindentedwire, the size. spacinganddepthof indentationsare given. It shouldbe notedthat this wire is not of high enoughstrengthto be used for prestressingconcrete.

ES 4483 dealswith reinforcing fabric manufacturedfrom plain or deformedwires orbarscomplyingwith BS 4-149. BS 446tor BS 4482. Its main usesin bui1din~are in floor U

slabsandwalls. assecondaryreinforcementfor developingfire resistance.andin precastproducts. C,Eachof the specificationssets a numberof requirementsfor mechanicalproperties.which are summarizedin Table H7.1 with someof the otherdetailedrequirements.

The Standardalso setlimits for the deviationsfrom the nominalmassof the material Kassupplied.As the test resultsarecalculatedon the cross-sectionalareaof the barsandthe designcalculationsarebasedon the nominal cross-sectionalareaor mass.anyunder-sized barswill be over-stressedin design and this over-stressmust be accommodatedwithin the partial safety factor. -y~, for the steel.For hot-rolledbarsof 12mm size and

above,the cross-sectionalareais allowedto be as much as2.5%under-sizefor the batch Land4% under-sizefor individual bars: the correspondingfigures for cold.~vorkedbarsare 3% and4.S’/o respectively. r

In exceptionalcircumstances,whenenhanceddurability is required(see2.2.4ingeneral [terms), reinforcementthat offers some integral protectionagainstcorrosionmay needto be considered.Three main types are available commercially,namely: galvanized.epoxy-coated and stainless steel reinforcement. Galvanized and epoxy~coatedreinforcementis obtainedby coatingnormal reinforcementto BS -1-449 or ES 4461 andstainlesssteelreinforcementis availablewhich complieswith the requirementsof ES 4449other thanweldabilitv. LEvidenceabout the performanceof galvanizedsteel in concreteis confusing,eventhoughit has beenused for some years.While in some marineconditionsits corrosionprotection has considerably improved the durability of reinforced concrete. manyexposurestudieshaveshown that corrosion protectionin chloride-bearingconcreteiS Jincreasedonly marginally over the performanceof uncoatedsteel.However. it offersan undoubtedincreasein corrosionresistancein carbonated.but chloride-free,concrete.The durability of galvanizedreinforcementis enhancedby chromatetreatmentwhich [alsodepressesinitial alkali-inducedcorrosion(SeeBRE Digest 109). Epoxy-coatedbarshavenot beenusedextensivelyin the UK but havein the US.

The protectionprovided by an epoxy coatingdependson the continuity of the filmand its freedom from gaps.evenof pinhole size. occurring during manufacture.The Lprotectionis also affected by the coatingsability to withstanddamagefrom abrasionand impactduring bendingandhandlingon site. There is a wide rangeof epoxycoatinL’sbut thoseapplied as powdercoatings and thermally cured provide the lowest risk of

154 [

-— j -

I——4 - - ~ - - - K -— KKK — .4 —K K . , . K~K K

~ - — .. ~ --- ... -

Part1: Section 7

pinholing of the epoxylayer.Cut endsof barsshouldbe protected.e.g.by painting witha compatibleepoxy. The quality achievableis improving but it should be appreciatedthat a defectivecoating which permits oxygen and moistureto penetratebeneaththecoatingmay allow sub-filmcorrosionof the reinforcementto occur.Coatedanduncoatedbars should not be mixed. The greatadvantageof epoxy-coatedbars is that they limitcorrosion-induceddamageand so facilitate repair.

Stainlesssteel,sofar, hashadlimited useasreinforcementfor economicreasons;theonly technicalreasonis the needto providesufficient oxygento maintain the oxide filmon the steel.Thus,stainlesssteelis of leastvaluein anaerobicconditionsin the presenceof chlorides. Under low coverswhere the environmentis aerobicthe advantagesoverplain carbonsteelare significant. In the absenceof a product standardit is importantto selectan appropriategradeof stainlesssteel.An austeniticnon-freemachininggrade.such as 304 or 316, shouldbe used.Type 316 is not essentialfor low coverapplications:the cheapertype 304 gives adequateperformancein carbonatedconcrete.Testsmay benecessaryto establishsuitableconditionsunderwhich barsof this materialmay be bentsatisfactorily,becauseresidual stressesmay be above the thresholdto initiate stresscorrosioncrackingin certainenvironments.

Further information on galvanized,epoxy-coatedand stainlesssteel reinforcementmaybe obtainedfrom reference7.1.

J Table H7.1 British Standard requirements for reinforcing bars for concrete

BSTypeof Preferredreinforcement sizes

(mm)

Specifiedcharacteristicstrength(N/mm2)

Compositionofsteel Otherrequirements

4.4.49 Hot-rolledplainroundbarsGrade250

I (6)8

10121620253240

(50)i~

250

(Yieldstressorstressfor0~33%totalstrain:the tensilestrengthshouldbeatleast15%greaterthanactualyield stress,theyieldstressshouldnotexceed425)

C ~0~25%5 ~‘0~06%P >0~06%

K(Carbonequivalent>042%)

Minimumeloncationon5•65varea=22%.Bendtestthroughl800aroundaformerwithadiameterdoublethenominalsizeofbar.

Rebendtest(if specified)through450 aroundaformerwith adiameterdoublethenominalsizeofbar.thenheatedto 1000Cfor 30mm. cooledand bentbackthroughat least230.

Hot-rolledhigh-yielddeformedsteelbarsGrade460

4-449

—.

4461 Cold-workedsteelbarsGrade46O

Asforgrade250bars

460(Yieldstressorstressfor 0~43%totalstrain:the tensilestrengthshouldbeatleast15%greaterthanactualyieldstress)

C *0~30%S =0.05%P 4~ 0~05%

(Carbonequivalent>0~51%)

Minimumelongationon5.65vTh=12%.

Bendtestandrebendtestsasforgrade250barsbut~vitha formerof diameterthreetimesthenominalsizeofbarfor thebendtestandfive timesthenominalsizeof thebarfor therebendtest,

Asforhot-rolledbars

460(Yieldstressorstressfor0~43%totalstrain:thetensilestrengthshouldbeatleastbob

greaserthanactualyield

C =0-25%S ~-0~06%P ~0~06%

(Carbon I

Minimum elonEationon565varea= 12%.

Bendandrebendtestsasforhot-rolledhi2h-vielddeformedbarsto BS4-449,

-1482 plainandCold-reduceddeformedsteelwire

111

1012

460(Yieldstressorstressfor0~43%totalstrain:thetensilestrengthshouldbeatleast5%greaterthantheactualyield stressandnotlessthan510)

C> 0~25% Rebendtestasfor coldworkedsteelbars.S =0~06%P >0~06%

(Carbonequivalent=t).42%)

*If a bar smaller than 8mm is required.6mm is recommended.

If abar largerthan 40mm is required.50mm is recommended.155

Handbookto BS8JIQ:198S

7.2 cutting and bending

Attention needs to be given to the correct dimensionsin bending and cutting ofreinforcementif the requiredtoleranceson thepositionof reinforcementandthicknessof concretecover in 7.3 are to be achievedin construction.Bendingdimensionsaregiven in ES 4466 which specifiesthe cuttingand bendingtolerancesin Table H7.2.

Table H7.2 Cutting and bending tolerances

Dimensionsofbenthars(mm)uptoand

over including

Tolerances(mm)

plus minus

1000 5 51000 2000 5 102000 — 5 25

Dimensionsofstraightbars—all lengths 25 25

The minimum permissiblediametersfor bendsin barsareset out in Table H7.3.It is not always practicableto bend binders, links and stirrups to diametersthat

correspondto the diametersof the main bars to which they arefixed. Allowance for theeffectof anylack offit on thepositionof themainbarsshouldthereforebemadein design.

Table H7.3 Minimum diameter of former

TypeofmaterialMinimum diameter

offormer

Grade250steeltoBS4449Grade4.60steelto ct~20 6q~B54449andB54461 4~25

t is the size of the bar. i.e. the diameterof a circle of equivalentarea.

7.3 Fixing

Seecommentaryon 3.12.1.3,3.12.1.4and7.2.

7.4 Surface condition

The surfaceof the reinforcementshould be free from any material which is likely tO

reducethe bond with the concreteor lead to corrosionof the steel.When loose rustand scalehavebeenremoved, the remainingrust is known to benefit bond with theconcrete.particularly for plain bars. If steel must remain in position in the forsriwOrkfor morethana few daysbeforecasting,it may be coatedwith cementgrout to preventrusting.

In particularly aggressive environments the desirability of grit blasting thereinforcementshouldbe considered.

7.5 Laps and joints

See3.12.8.9.156

C- ~‘:~-

[IrF

E

I

~ ~ - --K. ~ K- K , -- -I- K ~ K~KK4 ~ . - K. K - K K~.K . — — — K K-4 K ....,...--. , - -. K, -K K KKKK K K K K

Part 1: Section 7

7.6 Welding

The ease with which reinforcing bars may be welded depends upon the carbon equivalent

value of the steel. Bars to BS 44-61 and grade 250 bars to ES 4449 are considered to be

readily weldable, provided that the requirements of ES 5135 and the manufacturer’s

recommendations are observed. Grade 460 bars to ES 4449 are considered to be weldable.

Welding should be avoided where reinforced concrete members are to be subjected to

large numbers of repetitions of substantial loads. The fatigue strength of beams in which

the links have been welded to the main bars can be reduced by as much as 50%.

7.6.1 General

7.6.2 Use of welding

7.6.3 Types of welding

7.6.3.1 Metal-arc welding

7.6.3.2 Flash butt welding

7.6.3.3 Electric resistance welding

7.6.3.4 Other methods

7.6.4 LocatIon of welded joints

7.6.5 Strength of structural welded joints

7.6.6 Welded lapped joints

REFERENCE

7.1 MARSDE.N. A.F. Special reinforcing steels. Concrete Society Current Practice Sheet No. 103.

Concrete. Vol.19, No.9, September 1985. pp 19-20.

157

8.1 General

SECTION EIGHT. SPECIFICATION ANDWORKMANSHIP: PRESTRESSING TENDONS

Although specific referenceis made to BS 4486 and BS 5896. it is reasonableto useother typesof steeltendonthat havebeenshown to havepropertiesnot inferior to thematerials described in the British Standards. There are no explicit requirements regardingsteel makingor chemicalcomposition.exceptthat the air andair/oxv~enbottom-blownprocesses should not be used and the cast analysis should not show more than 0.04%sulphur or morethan0.04% phosphorus.

The main typesof steel used in the UK for prestressingare all coveredby the BritishStandardsnoted.i.e. tendonsprocessedfrom hotsteelbaror fabricatedfrom cold-drawnsteel wire, often in the form of seven-wirestrand.A summaryof the preferredsizesandstrengthsof tendonsis given in TableH8. I togetherwith the main requirementsof theBritish Standards.

Table H8.1 British Standard requirements for prestressing tendons for concrete

Specifiedcharacteristicload

breaking’load

(kN)

0.1%proofload

(kN)

Minimumelongationatmax.loadorfracture

(%)

3-5at

max.load

Modulusofelasticity

kNImm)

205±10for allwires

Relaxation(% initial load)

Initialload asOf f,oO

breakingload

607080

Niax. relaxationafter1000hours

4.58-012-0

for allwires

1-02-54-5

for allwireS

4486 Hot-rolled 20 1030 314 325 260 6~0 165±12 60 [5bar 25 491 505 410 at forbars 70 3.5(smoothorribbed)

3240

8041257

8301300

6701050

fracture asrolledand

stretched206±10in othercases

Sf) 6-0forallbars—

Relax. Relax.classI clasS2

Hot-rolledandprocessed

202532

1230 314491804

385600990

340530871)

4~0at

fracture

7-’~ire 8-0 1860 38 70 59super I 9-6 1860 55 102 87strand 11-3 1860 75 139 118

12-9 1860 100 186 158K 15-7 1770 150 265 225

21-022-325-832-734.747.350-160-464-3

92125164232

209300380

17-518-5

21-427-’28-839.341-650-I53-4

78106I3q197

178

323

3.5at

max.load

195±10for all

strands

607080

4,58-0

12-()for all

strands

1-i)2-54-5

for allstrands

Note. SeeBS -1486 and BS“ire in mill coil.

~896for further information,includingtolerancesand ductility tests,and requirementsfor cold drawn

158

[Ir

Nominaldiameter-

orsize

(mm)

Typeoftendon

Cold-drawnwire(stress-relieved)

Nominalsteelarea

(mm!)

12-612-615-919.619-628-328-338•538~5

5896

Nominaltensilestrength

(N/mm)

167017701620167017701670177015701670

1770177017701670

44

4.5S56677

7-wirestandardstrand

9.3

11-0

15-2

527193

139

7-wire- drawnstrand

12-715-218-()

186018201700

112165223

U

L

I;

___C- - . — —: . — . —

K K4~ ~ — — — — 4~ K K K KKKKKKKKK —

K - -1~--~ - - •-. ->~-- ,-...; K .~ K K K - K K K K K KK ~ - .. — K K. . K ,,K, . - K ~4KK K K - — - —— . — .. 4 ....... ,.. - ~. K ~. . KKKKKK.K~4~KKK K..., ~K K K K. K — K— K . K K

Part]: Se-tiun ~

ES -4486 gives the requirementsfor barswhich areproducedby hot rolling lo~v alloysteels under controlled conditions, so that. as the bar leaves the last rolls, the temperatureis at the right stage in the cooling cycle to give a fine pearlitic structure. The bars arenext cold worked by stretching under about 90% of the characteristic strength. permanent1 stretch being carefully controlled. The bars have a maximum length of 18m. the endsbeing machined and threads formed by cold rolling in the machined length. Longertendon lengths are obtained by coupling bars together. Ears are normally smooth but

1 max’ also be rolled with a ribbed surface.ES 5896 gives the requirements for round cold-drawn wire, which may either have aplain surface or be indented or crimped to improve bond with the concrete. The Standarddifferentiates between three categories of material: pre-straightenedwire with normal1 relaxation properties; pre-straightened wire with low relaxation properties: and ‘as drawn’wire in mill coils. The last-mentioned type of ~virewas used ~vhenprestressing wasintroduced into this country some fifty years ago. Since then. substantial improvementsI -in propertieshavebeenobtainedby thedevelopmentof newtechniquesin production.The wire is drawn from hot-rolled rod. ~vhichhas been patentedby heating in acontinuousfurnaceto about 10000Cfollowed by cooling in a lead bath at about5000Cto impart to the steela suitablemicrostructurefor drawing. After removal of scalein apickling treatment,the rod is passedcontinuouslythrougha seriesof water-cooleddieswhich reduceits cross-sectionalareaby between60% and80% and increasethe tensilestrengthby betweentwo and three times. After passingthrough the final die, the wireI is wound on to the drawing capstanand then has a coil diameterof 0.6 to 0.7m. Tomaintain continuity of the processes.one length of rod is ‘velded to the next. Theseweldsare cut out beforethe steelis suppliedas wire, unlessspecial lengthsof wire are

I required;in that case,weldsmadebeforethe patentingprocessmay be accepted.In this form. the wire is usedfor pre-tensioning-inthe manufactureof someprecastproductssuch as pipes and railway sleepers.Under stress. it exhibits substantialpermanentdeformationeven at low, levels of stresswith a considerablerelaxationofI stressat levels correspondingto thoseat transfer.Sincewire in mill coils doesnot pa~-out straight,it is unsuitablefor post-tensionedtendons.For post-tensioning,wire shouldbe purchasedfrom the manufacturerin a pre-straightenedform: straightening~virefromI mill coils by the purchaseris not recommended.Pre-straightenedwire is obtainedby straighteningthe mill coils; it is then eithersubjected to heat treatment at a fairly lo’v temperatureto relieve the effects ofI straighteningor to special treatment(sometimestermed stabilizing) to further reducerelaxation losses.The former material is describedin ES5896 as wire with class 1relaxationand the latter aswire with class2 relaxation.Eachis ‘vound into coils of largediameterto pay out straight,2.Om for 6 and 7mm wire. 1.Sm for 5mm wire and 1.25mI for 4 and4.5mm wire.Most cold-drawn wire usedin prestressinghas a plain smoothsurface. It has beenshown,however, that greaterreliability of bonding with the concretecan be obtained

by indenting the surface.andbond may be substantiallyimproved by crimping. Wiresof 3mm size or larger may be supplied‘vith an indentedsurface,the form of indentbeing agreedbetweenthe manufacturerand the purchaser.Relevant information fordesignis given in 4.10, but for thepurposeof specificationtheonly effecton the requiredmechanicalpropertiesis to reducethe specified bend test from four reversebendstothree reversebends.ES 5896 also gives the requirementsfor seven-wire strand. which is producedbyspinningsix cold-drawnwires in helical form arounda straight core wire of slightly largersize. Welds are permittedin the indi~’idual wires provided that they were madebeforepatenting.After the strandingprocess.standardand superstrandsare subjectedto heattreatmentto produceclass 1 normal relaxationstrandand to an additionaltreatmenttoproduceclass2 low relaxationstrand.Drawn strandis producedby drawinga seven-wirestrandthrougha die undercontrolledtensionand temperature.As a result, the producthaslow relaxationand thewiresexhibit a characteristicnon-circularshape.Strand,whichis producedin sizesrangingfrom 8 to 18mm. is either coiled or put on to reels with aminimum diameterof 800mm.The Standardsfor tendonsrequire the manufacturerto keeprecordsof test resultsfor inspection by the purchaseror his representative.This is necessaryto determinecompliancewith the requirementfor the specifiedcharacteristicstrengthof the tendons.which requiresthat not more than5% of the test resultsshould fall below the specified159

[IHandbookto BS8IIO:1985

characteristicstrength~ and that none should be less than 0.95 fm,. Both Standards ralso requirethe manufacturerto provide load-extensioncurvesfor the estimationof theextensionof tendonsin stressingoperations.

Since the characteristic strength of a tendon is specified in terms of breaking load. Fdimensionalaccuracydoesnotdirectlyaffecttheultimatestrengthof prestressedconcretemembers.

Furtherinformationon the manufactureandpropertiesof steelfor prestressingtendonsmay be obtainedfrom reference8.1. [

C8.2 Handling and storage

Since nearly all the types of tendon in generalusehavea high tensilestrengthimparted I-by cold-working, it is importantthat theyshouldnot be subjectedto temperatureswhichwould impair their properties. In handlingand storage.therefore,the tendonsshould rnot be nearcuttingor welding operationswithout propersafeguards.Experiencedoesnot suggestthat corrosioncausesseriousproblemswhenreasonable

care is taken to provide good conditionsof storagefor tendons.It mustbe recognized.however,that the steelusedin tendonsissusceptibleto severecorrosionin circumstances rwhere ordinarv reinforcement would suffer little damage. Protection from ground damp Lis essential,becauseseverecorrosionhasresultedwhensulphatesor othersalts in thesoil havecomeinto contactwith thesteel.Corrosionmay also becausedby straywelding pcurrentsoreventhepresenceof bacterianearthesteel.In coastalconstruction,protection

-jfrom airbornesprayandsalt is needed.

Where storage is prolonged, provision should be made for regular inspection of tendonsfor pitting. Visual examination for surface pitting is required andmetallurgicalinspection Kshould be made in cases of doubt. Reductions in tensile strength resulting from severeand unacceptable levels of corrosionmay be quite small and changesin mechanicalpropertiesmay be assessedbetter from the changesin ductility revealedby bendtestsor the extensionat fracture.

8.3 Surface condition

To avoid superficial rusting, themanufacturerusuallygivesthe steela protectivecoatingwhichneedsto beremovedby themethodsuggestedto obtain good bond: if light surfacerustinghas developed,however,removalof thecoating is not necessary.

It has beenestablishedexperimentallythat light surfacerustingof hard-drawnwirehas little or no effect on the static or fatigue strength of membersin which it isincorporated.

8.4 Straightness

Except for cold-drawn wire suppliedin mill coils, wire andstrandcomplyingwith BritishStandards should pay out reasonably straight. As straightening modifies the properties Lof steel substantially and in some respects adversely, it should be done only under themanufacture(s control.

8.4.1 Wire L8.4.2 Strand

8.4.3 Bars160

_ K K... -I-- - - - - .- -..-.---....-~---.---K---,KK-K -‘-K-.-.

Rart 1: Section 8

~J8.5 cutting

Pre-tensionedtendonsof wire or strand may be cut flush with the endsof units. Nospecial measuresfor protecting the ends of the tendonsagainst corrosion are then1required.

The methodof cutting shouldnot impart shock to the tendon.as this might impair1bondor causeslip in the anchorageif the tendonshavenot beengrouted.

8.6 Positioning of tendons andsheaths

The recommendationson accuracy of placing apply to both pre-tensionedand post-tensionedtendons.Pro”ided that the toleranceof ±5mmis maintained,thereshould

71 be no difficulty in satisfying the requirementthat the actual covershould be not lessthan the nominal co’-er less5mm.These tolerancesare so small that they are unlikely to affect compliancewith

requirementsfor serviceabilityandultimatelimit statesexceptforveryshallowmembers.

~1 In practice,it will often be necessaryto agreelargertoleranceson positioning.In shortmemberswith pre-tensionedsteel,it is usuallysufficientto positionthetendonsat their endsonly. but for long memberssome intermediatesupports,which may bewithdrawn before the completionof casting,may be requiredto prevent the tendonsbeingdisplacedby vibration or othercauseduring the filling of the moulds. For post-tensioning,the positioningof the tendonsis usually governedby the positioningof thesheathsor duct-formerswhich should thereforebe fixed firmly during concreting. Ifsheathsareused.it-may bedesirableto placethetendonin the sheathbeforeconcreting.thereby stiffening the sheath, and to support the sheathseither temporarily orpermanentlyat centresof at least0.75m. Maintenanceof the true cross-sectionalformof sheathsand ductsand avoidanceof leakageis neededto minimize frictional effectsduring the stressingoperations.

8.7 Tensioningthe tendons

:ij 8.7.1 GeneralIf prestressedconcreteconstructionis to resistcrackingandcomplywith therequirementsfor construction,the prestressingforces imposed musi be as requiredin the design;successthereforedependson theskill andaccuracywithwhich theprestressingoperations

]arecarriedout in the field and in the factory.All tensioningshouldbe done underthedirectcontrolof asupervisorwith thoroughexperienceof the variousstressingoperationsinvolved.

] The choiceof systemof prestressingto be usedin particularcircumstancesdoesnotusually presentdifficulty. Pre-tensioningis normally usedfor the massproductionofsimilarunits. such as floor beams.If theycan be readily transported.theyare precastin the concreteproductsfactory,but if theyare too largeto be handledeasily thentheymaybemadeonaprestressingbedatthesite.Post-tensioningismostfrequentlyemployedin largestructuresandcarriedout in situasconstructionproceeds.However,wheresitesarevery constricred.it may be moreconvenientto precastthe membersin shortsections

11 in the factory andto assemblethem on site: oneadvantageof this is that it gives bettercontrol of the concreteproduction.

Eachof themethodsof post-tensioningavailablehasparticularadvantageswhich maymakeit moresuitablein certaincircumstances.For short members.bars with threadedendsaremostsuitablebecauselossesof prestressdueto draw-in.whichcouldbeexcessivewith wire orstrand.arecompletelyavoided.Earscancarrythe largestprestressingforces

K in individual tendonsbut strandhas the advantageif they haveto be curved. Largetendonscan be built up from groupsof strandswhich may be anchoredtogetheror inindividual anchorages.or from individualwiresanchoredin groupsby wedge-anchoragesor by button-headingin a specialanchorageassembly.For particularly long tendons,

I both wire and strandhave the merit of being availablein long lengthsand so do notneedconnectors.The longerthe tendon,the less the significanceof the lossof prestress

dueto draw—in of the grips. lb I

Handbookto B58110:198S

8.7.2 Safety precautionsDuring the life of a prestressedconcretestructure.the concreteandthesteelare usuallymostseverelystressedduring the operationsassociatedwith tensioningandtransfer,ata time whenthe strengthof the concreteis not fully developedandthe anchoringof the Fsteelmay be only temporary.It is then,therefore.that the risk of failureandof accidentis greatest.Although it is not possibleto safeguardpersonnelcompletelyfrom the risksof suchan accident,reasonableprecautionsshouldalways be takenwhenworking withor neartendonswhich havebeentensionedor arein the processof beingtensioned.Personnelshouldnot standin line with the tendons,anchorageor jacking equipment.Simple protective measuressuch as stout timber shieldsshould be placed in line wi th~thetendonsandbehindthejacksto protectthosepassingin thecourseof theirduties. I

Eachfactory or sitewill call for separateconsiderationof the most reasonableform’-’of protection. It mustbe emphasized,however,that the mosteffectivesafetyprecautionis the proper supervision and training of personnel in prestressingtechniques.Manufacturersinstructionsfor theuseof stressingequipmentshouldalwaysbefollowed~closely.

Notes for guidancewith regard to safety precautionsfor prestressingoperationsareprovided in references8.2 and8.3. [8.7.3 Tensioning apparatusItem (c) requiresthat the elongationof the tendonbe measured.This measurement I

should be checked against the elongation calculated from the load—elongation% Lrelationshipsuppliedby the manufacturersof thetendonsfor thebatchof material beingused.

8.7.4 Pre-tensioning

8.7.4.1 General

8.7.4.2Straight tendons

8.7.4.3Deflectedtendons

8.7.5 Post-tensioning r8.7.5.1 Arrangementof tendons

8.7.5.2Anchorages [8.7.5.3 DeflectedtendonsThe requirementrefersto deflectorsfor externaltendons,as the curvatureof internal Ltendonswill be determinedby that of the ductswhich will be usuallylessonerousthanthe limit given here.

The useof deflectorsof smallerradiusof curvatureor with a largerangleof deflectiois permittedas longas test dataon the lossof strengthareobtained.SomeexperimentaILresults(seereference8.4) for strandof 12mm diametershow that the loss of strengthis lessthan 10% fora ratio of deflectorradiusto tendondiameterof 2. Thereis therefore Iconsiderablescopefor testing, but it should be noted that the secondarystressesthatdevelopat sharpchangesin curvaturewould havean adverseeffecton fatiguestrengthundercyclic loading and could aggravatean otherwisepassivesituation shouldmildlycorrosive conditions develop in the region of the deflector. [8.7.5.4 Tensioningprocedure

[8.8 Protection and bond of prestressing tendons U

The recommendationsapply only to post-tensionedsteel; pre-tensionedsteel is

162 adequatelyprotectedby theconcretecover.providedthat the requirementsfor thickness

I.- - — — -- — - :~<-. ~.K. .• . K —— . — —.— ..~ . .~K. — —4 -K... — K~ -

K.KK KK. K~KK K - — - K K

Parr 1~- Section8

of nominal cover and for quality of concrete in 4.12.3.1.2 aresatisfied. The primaryreasonfor poutinginternal tendonsandfor encasingexternal tendonsin concreteis toprotectthe steelagainstcorrosionandfire.

A secondarybut important consideration is the effect of grouting and encasementonstiffnessand strength.If othermaterialsbasedon bitumen.epoxy resinsor rubberareused,theremaybe a reductionin stiffnessandstrengthwhich will needto be allowedfor and the fire resistancemaybe affected.Greatcare is neededin selectingmaterialswhich are not boundby cement, to ensurenot only that theyare not harmful to thesteelbut that they cannotbecomeso underconditionsthat could developin the ductsover a prolongedperiod.

8.8.1 General

8.8.2 Protection and bond of internal tendons

8.8.3 Protection and bond of external tendons

of prestressing tendons8.9.1 General

Furtherguidanceon preparingandgroutingductsis given in reference8.5.

8.9.2 Ducts

8.9.2.1 Ductdesign

8.9.2.2 Construction

8.9.3 Propertiesof grout

8.9.3.1 General

8.9.3.2 Fluidity

8.9.3.3 Cohesion

8.9.3.4 Compressivestrength

8.9.4 Composition of grout

8.9.4.1 General

8.9.4.2 Cement

8.9.4.3 Water

8.9.4.4Sandandfillers

8.9.4.5Adinixtures

8.9.4.6 Chloride content

8.9.5 Batching and mixing of grout163

Handbookto BS8JIO:1985

8.9.6 Grouting procedure

8.9.6.1 Trials

8S.6..2 Injection

8.9.6.3 Injection procedure

8.9.7 Blockagesandbreakdown

8.9.8 Maintenanceand safety

8.9.9 Groutingduringcold weather

8.9.10 Precautionsafter grouting

8.9.11 Checking the effectivenessof grouting

REFERENCES8.1 LO~~GsO-1-rON4. ~.w, Steel for prestressedconcrete.ConcreteSociety DigestNo. 4. 1984. 8 pp.

Publication53.048.8.2 mc co~’cac-rcsocic-ry. Safety precautionsfor prestressingoperations(post-tensioning).Notes

for guidance.ConcreteSociety DataSheet.1980. 2 pp. Publication53.031.8.3 mc coNc~xrcsocic-ry. Safetyprecautionsfor prestressingoperations(pre-tensioning).Notes

for guidance.ConcreteSociety DataSheet.1982. 4 pp. Publication53.036.8.4 VANDEPrI-rE. D. RAThE. .r, andKERcI4AER-r. p• Lossof strengthof prestressingstranddueto severe

curvature.RevueC Tijdschrift. Vol 4. No 9. 1966. pp 275-283.8.5 BCDGE. ci. Preparingandgrouting ductsin prestressedconcretemembers.WexhamSprings.

CementandConcreteAssociationConstructionGuide. 1981. 8 pp. Publication47.012.

IB

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~ PART 2. CODE OF PRACTICE~R SPECIAL CIRCUMSTANCES

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PART 2

SECTION- ONE. GENERAL

1.1 Scope

1.2 Definitions

1.3 Symbols

167

PART 2SECTION TWO. NON-LINEAR METHODSOF ANALYSISFOR THE ULTIMATE LIMIT STATE

2.1 General

2.2 Design loads and strengths I2.2.1 General

2.2.1.1 Choice of values

2.2.1.2 Analysis phase U2.2.1.3 Elementdesignphase

22.2 Selectionof alternativepartial factors

2.2.2.1 General

22.22 Statistical methodsThe situationswhere adequatestatisticaldataare likely to be availablefor this approachto be contemplatedare currentlyrare. Nevertheless,thereare possibilitiessuchas whereacceptancetests are carried out on mass producedunits. In this case,a statistical Ievaluationof •y~ might be feasible.

2.2.2.3 .4ssessmentof worstcredible valuesSomeexplanatoryremarkson Table2.1 may be helpful as it shouldbe interpretedwithsomecare.

(a) Adverseloads.The greaterthe numberof different, independent,types of loading consideredtobe acting togetheron a member. the lower is the probabilityof them all being attheir worst credible valuetogether.hencethe lowerpartial factorsuggestedfor (c). UCase(c) assumesdeadload, andat leasttwo other typesof loading acting together(e.g. dead+live+wind).

(b) Beneficial loads. IIt must be noted that the ~vorstcredible beneficial load is its minimumcrediblevalue.The factorof 1.0 is thereforenot directly comparablewith the partial factorsapplied to characteristicloadsin Part 1 of the Code: it is being applied not to a Icharacteristicvaluebutto anestimateof thelowestvalueof the loadthat isconsideredcredible.

2.2.2.4 Worst crediblevaluesfor earth and water pressures 3The -worst credibl& approachto assessingdesignloadswasfirst developedin the groundengineeringfield~’~ and this is an area where it is particularly applicableand where

Iothermethodsof assessingpartial factors are particularly suspect.

2.2.3 Implications for serviceabilityIf si~nificantlv lowerpartial factorsareusedthan thosegiven in Part 1 of the Code.thenthe stressunderserviceloads will be significantly higher. This, in turn, will meanthatdeformationsunderservice loads (crackingand deflections)will be greater: hencethenecessityfor this warning. I

I 6S B

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F] Part 2: Section 2

2.3 Restrictions on use

This is the wrong title — it should read ~Non-linear methods’.

2.3.1 General

2.3.2 Basic assumptions

2.3.2.1 Design strengths

2.3.2.2 Material properties

Figure 2.1 has been taken from the CEE Model Code: 1978(22) with minor modifications

to convert from cylinder to cube strength.

2.3.2.3 Loading

2.3.3 A.nalvsis methods

2.4 Torsional resistance of beams

2.4.1 General

2.4.2 Symbols

2.4.3 CalculatIon of torsional rigidity (G X C)

2.4.4 Torsional shear stress

The nominal torsional shear stress is calculated by assuming a plastic shear stress

distribution. Torque levels giving a nominal shear stress less than Vt mm in Table 2.3 do

not cause a si~niflcant reduction in flexural and shear strengths’2-3~ and may be ignored.

The upper limit v~ for the sum of the flexural and torsional shear stresses is the same

as for shear alone and is to prevent a premature compressive failure of the concrete.

The background to this requirement and the special limitation to prevent corner spalling

in small sections is described in reference 2.4.

2.4.4.1 Rectangular sections

2.4.4.2 T-, L- or I-sections

In the treatment of T-, L- or I-sections, it has been assumed that a whole section can

be divided into rectangles each of which attracts a torque approximately proportional

to its theoretical torsional stiffness.

2.4.4.3 Hollow sections

The treatment of thin-walled box sections has been omitted because the provision of

diaphragms or transverse reinforcement to resist cross-sectional distortion is outside the

scope of this clause. Where distortion has been properly provided for, the St Venant

torsion may be designed for by calculating the maximum nominal stress from the Eredt

formula.

2.4.5 Limit to shear stress

2.4.6 Reinforcement for torsion

169

Handbookto BS8IIO:198S ii2.4.7 Area of torsionalreinforcementThe reinforcementformulaearebasedon the equationsfor an orthogonaltrusswith 450compressivestruts’2-5~. The reasonsfor the efficiency factor of 0.8 are describedinreference2.4. All of the torsionshouldbe resistedby reinforcementalthoughthe capacityof the concretein flexural shearis unaffected.

2.4.8 Spacing and type of linksThe detailing requirementshave beenchosento give reasonablecontrol of cracking.They are more stringent than those for shearbecauseof the different overall stressconfiguration.It is particularlyimportantto resistthetendencyfor thecornerstospall~2-’1owing to theresultantof thediagonalcompressiveforcesin adjacentfacesof the member.Forthis reason,thepitch andcoverto theclosedlinksshouldbeassmallaspracticable.

2.4.9 Arrangementof longitudinal torsionreinforcement

2.4J0 Arrangement of links in T-, L- or I-sections

Effective column height

2.5.1 GeneralReference2.6 gives further informationon the rigorousdefinition of effective heightsof columns.

2.5.2 Symbols

2.5.3 Stiffnessof memliers

2.5.4 RelativestiffnessThe assumptionof a~ = 1.0 for basesdesignedto resistmomentis appropriateto padfootings. Where a massconcretebaseis providedwith depthandwidth greaterthan.say, four timesthe depthof the column cross-section.it will be reasonableto assumearigid fixing, i.e. a~ = 0.

2.5.5 Braced columns: effectiveheight for framed structuresEquations3 to 6 aresimplificationsof the bucklingequationsderivedfor regularframes.Figure H(2)2. 1 showsthe buckling modesconsidered,from which- it will be seenthatthe effective length mustalways be less than the actuallength for a braced frameandalways greaterthan the actual length for an unbracedframe.

2.5.6 Unbraced columns: effective height for framed structuresSeeabove.

? t t 4 t I

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Part2: Section2

26 Robustness

2.6.1 General

2.6.1 Key elementsWhere key elementshave to be designed,the Ym factors should be thosegiven forexceptionalloadsor localiseddamage.The valuesof y~ on the deadload andon anylive load that it may be consideredappropriateto include, should be 1.0 and 1.05respectively.No factorneedbe applied to the 34kN/m.

Specialconsiderationsapplywhen it is desiredto checkwhethera lengthof plain wallwill withstand34kNlm2; the following methodis suggested.

Step 1: Assessaxial preloadon the wall, dueto deadloadonly.

Step 2: Considermodeof collapseto be by formationof yield lines attop, bottomand

centreof wall (Figure1-1(2)2.2).

yield lines

Figure H(2)2.2. Formationofyield lines in a wall subjectedto lateral loading.

Figure H(2)2.3 Configuration ofwall at ultimateconditions.

Step3: Sketchdeflectedshapeunderultimateconditions,takingultimatedeflectiona~at centralyield line equalto ea = le/2500h asgiven in 3.9.4.16of Part1 (FigureH(2)2.3).

Step4: Determineneutralaxis depthx at yield lines from

x = (n~/0.3f~)(I3/1.5)mm

wheren~ is the preloadin kNlm run.The equationfor x is derivedfrom the simple rectangularstress-blockas

explainedin the commentaryto 3.9.4.15of Part 1. The 1.3/L5 ratio adjustsfor the different Ym factor.

Step 5: Determineultimate lateral load q~ l~t by taking momentsabout centre ofstress-blockat centralyield line. This gives

q~ ~ = 8n~(h—a~——x)ll2~

Step6: Check for shear.Shearforce at upper and lower yield lines should be lessthanor equalto one-quarterof the axial load n

Example.Wall. Smlong. 150mmthick.f~~ = 30N/mm,clearheightbetweenfloor slabs2.3m. preloadl8OkN/m run. Check resistanceto 34kN/m.

I

I-

171

Handbook to BS8IIO:1985 iile = 0.75x2300= 1725mm I

= 1725/(2500x150) = 8mm =x = (180/0.3x30)(1.3/1.5)= 17mm

Thereforeq~ lac = 8X 180(0.1500.025)/2.3Z= 34kN/m. which checks. EShearat base= 34x1.15 = 39k.N/mrun

Permitted= 180/4 = 4SkN/mrun. which checks.It should be noted that l8OkN/m run is a substantialpreloadwhich would

Ibe unlikely to be realizedin the topmoststoreysof a structure.In suchcases.it will be necessaryeither to provide reinforcementtying (see3.12.3.7ofPart 1) over the wholewall areaor to providelocal strengthening. i

2.62.1 Designof key elements(where required in buildings offive or more storeys)

2.6.2.2Loads on key elements I2.62.3Key elementssupportingattachedbuilding components g2.6.3 Design of bridging elements(where required in buildings of five or more storeys)Any building componentthat is normally not load-bearingmaybe takeninto account. pThereare limitlesspossibilitieshereover the rangeof typesof structure(andtheir usage)coveredby this Codeandthe valueof the loadingis left to thediscretionof the engineer;in general. all permanentloads would be consideredand somefraction of imposedloading — this will dependon usageandspecialconsiderationmay have to be given to 0warehouses,plant rooms. etc. Only rarelywill it be necessaryto considerdebrisloading.becauseof the relativemagnitudesof the safetyfactorsfor normalandexceptionalloadsandalsobecauseof the force requirements. IThe bridgingmethodoutlined abovewill be themostappropriatefor precastconcretestructuresmadeof load-bearingelements.This involves the furthernecessityto definea lateralsupport’: this maybe eithera substantialpartition at right-anglesto the wall

Ibeing consideredand tied in to it or. alternatively,a narrow width of the wall itselfwhichhasbeenlocallystiffenedandiscapableof resistingaspecifiedhorizontalforce.

2.6.3.1 General 52.6.3.2 Walls

2.6.3.2.1 Length consideredlost C2.6.31.2Lateral support [REFERENCES [2.1 sI~~tPSoN. B, PAPPIN. iw andCROFT. DO, An approachto limit statecalculationsin geotechnics.

GroundEngineering.Volume 4. No.6.September1981.2.2 cEn-Fip. Model Code for ConcreteStructures.Paris. Comite Euro-Internationaldu Beton.

Bulletin d’Information N t24/125-E.April 1978.2.3 HSU. T.T.c and KEMP. EL. BackEroundand practicalapplication of tentativedesigncriteria for II

torsion.Journalof the AmericanConcreteInstitute. Vol.66. No.1. January1969. pptZ-J- L2.4 SWANN. RA. The effect of size on the torsional strengthof rectangularreinforced concrete

beams.London. Cementand ConcreteAssociation.March 1971. Spp. Publication42.453-2.5 LAMPERT. ~.Torsionandbendin2in reinforcedandprestressedconcretemembers.Proceedings Iof the Institution or Civil Encineers.Vol. 50. December1971. pp 487-505.2.6 CRANSTON. WE, Analysis and design of reinforced concretecolumns. London. Cementand

ConcreteAssociation.1972. 2Spp. Publication41 .020.

ca

172

CK KKK•K

2.K~. — — K...... K — KK ,~ K — K K — — — . — — — — —

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HPART2SECTION THREE. SERVICEABILITY CALCULATIONS

] 3.1 General

This Section containsa substantialamountof explanatorymaterial— far.more than iscommonly provided in Part 1 of the Code. Additional information is thereforeonly1 requiredfor relatively few clauses.

There is. however,one generalpoint about serviceabilitycalculationswhich, eventhough it is touchedon in the Code,needsreiterating.By their nature,serviceability

.4~] calculationscannotbe accurate.This arisesfrom our inability to predict the propertiesof concretewhich influence the deformationsof the structure.Some of the problems.7 with predictionof thesepropertieswill be consideredbelow.

(1) TensilestrengthThe deformationunderload, particularlyof lightly reinforcedmembers,is criticallyaffectedby the tensilestrengthof the concrete.This is illustratedschematicallyinFigureH(2)3.1.Normally, theinformationabouttheconcreteavailableto the persondoing the calculationis just its compressivestrength.There is no reliable,uniquerelationshipbetweenthe compressiveandtensilestrengthof concrete.Unlessa greatdealmore is known aboutthe concretethanjust its cubestrength.it is doubtful ifthe tensilestrengthcan be estimatedto a betteraccuracythan±30%-

0 LOAO

Serviceload

1-~Range of

uncertainty in

cracking load

Li

in

(2) Behaviour of concrete in the tension zone after crackingLI After cracking, the concrete between the cracks continues to carry some tension

and therefore this concrete adds significantly to the overall stiffness. The amount of

this contribution is very variable and cannot be predicted with precision.

] (3) Creep and shrinkageThese characteristics are not known with any precision in normal circumstances andyet contribute 50% or more to the total deformation. They will depend on the exact

j details of the mix used, the loading history and the environmental history. Therecan be significant differences in behaviour between members cast in early summer

compared with those cast in late autumn. Differences in formwork striking timesand propping procedures could also have a substantial effect. The person attemptingcalculations is unlikely to be able to define any of these factors, but needs to put

~4J

bounds on their effects.

The calculation methods used in this Section are based on simplified assumptions

about section behaviour; however, in view of the major uncertainties discussed above,

C

~eJ

DEFORMATIONFigure H(2)3. 1.- Influence of uncertainty about tensile strength of concrete

on deformation.

173

Handbook to BS8IIO:198S

it is doubtful if a morerigorousapproachcould be justified.Anotherfeatureof serviceabilitycalculationsis that they canbe checkedagainstthe

actualbehaviourof thestructure.Sincecalculationandreality are mostunlikely to a~rcc.this tendsto underminetheconfidenceof the designer.Calculationscannotbe expectedto predict what the deflectionor crack width will actuallybe: they can be used to setboundson the likely values,and it is important that they areused in this way. so thatpositive practical action is takenby the designerto ensureserviceableand durablestructures(see Section7, regardingthe requiredaccuracyof calculations).

3.1.1 Introduction

3.1.2 Assumptions

3.2 Serviceability limit states

32.1 Excessivedeflections due to vertical loads

7’Is

Is

[

I!

I:3.2.1.1 AppearanceThe limit on saggingof span/250follows the recommendationsof a committeeof theInstitution of Structural Engineers on the testing of structures’3-’1. A survey of beamsand slabs in Germany conducted by Mayer, where sag had given rise to complaints’3’produced about 50 examples.The measuredsagwas less thanspan/250in only two oftheseexamples.and span/300was the smallest sag which gaveoffence-. The selectedlimit thushas somepractical justification. When the designercanshow that greater sagis unlikely to give rise to trouble,this limit might be increasedto span/200.A limit toprecamberis not given; ho~vever.a reasonablelimit would seemto be aboutL/250. Ifgreaterprecambersare needed,thenthe structuremustbe averv flexible oneandcouldgive rise to problemsdueto generalliveliness’.

3.2.1.2 Damageto non-structural elementsThe basic problem is that partitions, since they tend to be vertical, arevery stiff andgenerallycannotfollow the deflectionof the floor or beamwhich supportsthem.If thepartition possessesa reasonabledegreeof tensile strength.the floor can deflect awayfrom the partition, leavinga gap betweenthe partitionand the floor of muchthe samemagnitudeasthedeflection.This, of course,can be hiddenby skirtingsorsimilar details.Unfortunately,with permanentpartitions(e.g.blockwork),the bottomsof thepartitions

— 4.—.

/ N/ crack ~q

I7[71~

—-4..

N

/,~crack

Figure H(2)3. 2.- Partition wall damage — cracksbetweento a self-supporting wall.

wall andfloor due

4.....,.

.---,KK....K-K.KK.KK K -i- 4.—

- . K - K - -- r—

K — K K K K

Parr 2, Section3

crack

I

crack crack

crackcrack

crackCrack

17i W iThFigure H(2)3.3: Partition it-all damage— cracksat joints betweenwall

andceiling and towardsexteriorwall due to rotationor movementofindividual wall panels.

I’

LI

Figure H(2)3.4: Partition it-all damage— inclinedcracksdueto shear.

Figure H(2)3.S: Partition ‘tall damage— t-ertical crackingdueto f7exure.175

[IHandbook to B58110:198S

m

Figure H(2)3.6: Partition wall damage— n’pesofdamagerelated to differentstructural configurations.

arefrequentlyhelddown by the floor screeds.In this case.the bottomof the partitionis pulled down and cracks appearin the body of the wall. Thesecan be large andunsightly.The presenceof openings(e.g.doors, windows)in partitionstendto produceweaknessesand will often act as crack initiators. A paper by Clarke, Neville andHoughton-Evans~33~gives illustrationsof the typeof damagethat can occurin particularcasesand the relevantFiguresare reproducedhere(H(2)3.2 — H(2)3.6).

Masonryis generallyfairly brittle andit seemsclear that, if a floor or beambelowamasonrypartition deflects.then that deflectionwill be accommodatedvery largely bycracks (i.e. 10mmof deflectionoverthelengthof awall is going to produceatotal crackwidth of the orderof 10mmsomewhere).While theaccommodationof verticaldeflectionby horizontalcrackingcanproducelargecracks,the cantileverexampleshownin FigureH(2)3.5 is capableof producingquite startlingly large cracksevenwherethedeflectionis relativelysmall.

FigureH(2)3.7, takenfrom reference3.4, showsthe damageproducedin modelwallswherevariousformsof deformationareimposedon the lower edgesof the walls.It can

[

LLI

[

UFigure H(2)3. 7: Crackingof model walls dueto saggingor hogging(from Ref. 2.3.4)-

j

,K,,KK K KK K KK~~ - K K —K.- ;.:.v--.. .---. -- - -

rI!

C

I

I

[

C

CI

IL

K--K -

— K- :- -.7. - -. -K--K.-.-- K K ~ K~ K~4•4K•K~, K~ - ~.. K—K~..4~K,K KK~~ K— K ,. - .~-KKKKK K K K

K — K ~~~K..44K~KK KKK~KK K — — , K

44K4.~ .4 K.4. KKK~ K , .•1Part2: Section3

Ibe seen that hogging deformations cause much greater disruption than saggingdeformations.

This concludesthe discussionof the typesof damagethat can commonlyoccur. It is

1 far from completebut 2ives a general idea of the general formsof damagewhich canbe met.The nextquestionto consideris the limitations to deflectionsrequired to keepdamagewithin acceptablelimits.

Much of the work done on damageto partitions has beenconcernedwith allowable

1 settlementof foundations rather than the deflection of memberssupporting walls:ne~’ertheless.the datashouldhavesomerelevance.

Oneof theearliest investigationsof thisproblemwasthat by Skemptont3-5~.Herelated

1

damageto angular distortion (Figure H(2)3.S).

angular distortion=L

Figure H(2,l3.8 Skempton‘s definition ofangulardistortion

He concludedthat cSIL should be limited to £/300. This would appear to be moreequivalentto a limit to mid-spandeflectionof a beamof L/600.

Mayer. who collected information from buildings in which deflectionshad causedcomplaint,reportedthe resultsshownin Figure H(2)3.9.The deflectionsoccurring afterconstruction of the partitions could not. of course, be measuredand are thereforeestimatedvalues. It will be seenthat theseresults, obtainedfrom studiesof deflectionproblemsin buildings.are not inconsistentwith the resultsobtainedfrom considerationsof damagedue to foundationsettlement.Both sourcessuggestthat deflectionswouldhaveto be limited to around£11000if damageis to be avoidedwith any certainty.

It must be concludedthat it is impossibleto give universally applicable limits forallowable deflectionsand the designershouldreally establishlimits appropriate to theparticular structure and typeof partition. The valuesgiven in this clauseare thosegivenin the ISO standardISO 4356-1977whichhasbeenapprovedby the UK, but the pointis madethat the valuesare only indicative.

3.2.1.3 Constructionlackoffit

3.2.1.4Lossofperformance

A cracksbetweenwall and supporto crackswithin wallC bendingand shearcracks• nature of crack not reported

zi0l‘-I ZW 0.

-~ ~WI0I 0 ‘.i~O 0

0 • •

L350 - •~AT~~@~A. &0 A -

L500 - e.——P—AA -~ —;~—-—r—OK.L1000 -

0,5 10 15

SPAN(ml

Figure H(213.9: Damageto partitions as a function ofcalculateddeflectionofsupportingsrructure (Ref 3.2,.

177

[1Handbookto B58110.-198S

3.2.2 Excessiveresponseto wind loads

3.2.2.1Discomfortor alarm to occupants

3.22.2Damageto non-structural elements

3.23 ExcessivevibrationResearchhasbeencarried out on the responseof humansto vibration. For furtherinformationon this, for example,seereference3.6.

3.2.4 Excessivecracking

3.2.4.1 AppearanceClearly the width of crackwhich will be acceptableis very dependenon the particularcircumstancesand a codecould not possibly give more than a guide. Factorslikely toinfluenceacceptablecrackwidths are:

(a) surfacetextureof concrete(b) distanceof observersfrom surface(c) exposureconditions(in surfacesexposedto weather,crackscan becomeaccentuated

by dirt and by exudationsof calcium carbonate) [(d) aestheticimportanceof element(a crack in. say. the entrancelobby of a prestige

office blockis likely toleadtomorecomplaintsthanthesamecrackin, say,apigsty)

3.2.4.2 Corrosion FIn recentyearsa considerableeffort hasbeenput into establishingwhat relationship,ifany, exists betweencrack width and corrosion~3-71.The generalconclusionfrom cuch ITstudiesis thatsmallcracks(say lessthan0.5mm)veryrarelyposeanyparticularcorrosionU~risk. whatever the nature of the eiivironment. However, very few studies have beencarriedout in circumstanceswherethe cracksfollow the line of areinforcingbarratherthan crossingit. In the absenceof reliable information on this question,it thereforeseemsprudentto limit widths to about0.3mm.

3.2.4.3 Lossof performanceLeakageis probably the commonestmanifestationof loss of performancecausedbycracking. Investigationsof leakagethroughcrackshave beencarriedoutt38~ but havenot beenentirelyconclusive.It seemsprobablethatcrackspassingright throughasection rwith widths lessthan0.2mmwill fairly rapidly seal themselvesandwould thereforenot Eucausesignificant loss of water in a water-retainingstructure.

[3.3 Loads

3.3.1 General L3.3.2 Dead loads L3.3.3 Live loads

[3.4 Analysis of structure for serviceability limit states35 Material properties for the calculation of curvature and stresses [I3.6 Calculation of curvatures U

The approachusedfor calculatingshrinkagecurvaturesis given in reference3.9.

178 [—

K.... K-K...--- -- .- - - - K - -~— .~ - 4~ ~ ~ - -

K--K---.- ,-~:.-. --~ -K-K..K K — K K.4 — — K KK K — —

Parr2: Section3

3.7 Calculation of deflection

3.7.1 General

3.7.2 Calculation of deflection from curvatures

3.8 Calculation of crack width

3.8.1 General

3.8.2 Symbols

3.8.3 Assessmentof crack widthsThe basis of equation 12 is given in reference3.10. Equation 13 can be derivedapproximatelyfrom the assumptionsgiven in 3.6 as follows:

If the tensilestressat the tensionface is f, then the force carriedby the concreteintension(Figure3.1) will be given by:

= [b~(h—x)f]/2

This can beconvenedto an effectivestrainreductionin the steelof:

- f/AIES = [b~(h—x)fJ/2A,E.

Clause3.6gives valuesof 1 and0.55N/mm2for the stressin the concreteat the steellevel underinstantaneousandlong-term loadsrespectively.The correspondingvaluesof f will be slightly largerthan the values but x, calculatedon the basis of a crackedsection.will be underestimated.Whathasbeendoneis to assumea valueof ~sN/mmforf andassumethatthiswill copeadequatelywith the variousuncertainties.This livesan approximatevalue for the steelstrainas:

Earn = 8s [b~(h~X)I/3AsEa

Equation13 assumesa linear distributionof strainover the tensionzone.Figures H(2)3.1OandH(2)3.11 may be usedfor calculatingthe propertiesof cracked

sections.In the Figures,a~ is the modularratio andh is the secondmomentof areaofthe crackedsection.

For crackingin puretension,equation12 is somewhatapproximate.Recentwork byWilliamst3lZ) providesmoreinformation.

aA.bd

0.4

0.3

0.2

0.1

0

0.3 0.2 0.1 0

K—K .7.

K

0.1 0.2 0.3 0.4 0.5 0.6

xd

Figure H(2)3.10: Neutral axis depthsfor rectangularsection.

U

179

HandbooK to BSSII’kl%~

0.4 0 0.1 aA, C0.2

aA, 0.3 0.3 I,bd

0.2

01

0 ____________________________________

002 0.040060.08 0.1 0120.140.16

IFigure H(2 3.11. Secondmomentsofarea ofrectangularsections. r3.8.4 Early thermalcracking

3.8.4.1 General [The informationused in the draftingof this sectionhasbeentakenfrom references3.12— 3.14 which give considerablymoredetailedinformation.

3.8.4.2 Estimatingearly thermal crack widths

REFERENCES

3.1 INSTITLT?ON OF STRICrURAL ENGINEERS The testingof structures.1964.24pp.3.2 MAYER. H. Bauschadenals folge derdurchbiegungvon stahlbeton— bauteilen.ReportNo.68. IMaterialprufungsamtfur dasbauwesender technischenhochschuleMunchen. 1966.3.3 CLARKE. C.V. NE\1LLE. AG. andHOUGH-roN-evANs. w. Deflection— problemsandtreatmentin various

countries. American Concrete Institute. Deflections of concrete structures. SpecialPublication5P43. I3.4 i~s-nTunoNop STRCCTCRAL ENGINEERS. Structure— soil interaction— A stateof the art report.The Institution. 1978.

3.5 s~.~tr’ro~. AW. and McDONALD. OH. The allowablesettlementof buildings. Proceedingsof theInstitution of Civil Engineers.Vol.5. December1956. C3.6 AMERICAN CONCRETE INrn-rUrE. Vibrationsof concretestructures.AmericanConcreteInstitute.PublicationSP6O.Detroit. 1979.

3.7 BEEBY, .~w. Cracking and corrosion. ConcreteIn the OceansReport.NoA. CIRIAIUEG.London. CementandConcreteAssociation.Dept of Energy. 1978. C3.8 CLEAR. c..~. The effectsof autogenoushealing upon the leakageof water throughcracksinconcrete.WexhamSprings.CementandConcreteAssociation.TechnicalReport.May 1985.42.559.

3.9 HOBBS, D.W. Shrinkage-inducedcun-atureof reinforcedconcretemembers.WexhamSpringS.UCementandConcreteAssociation.DevelopmentReport4. November1979.3.10 BEEBY. ‘..w The predictionof crackwidths in hardenedconcrete.The Structural Engineer.

VoL57A. No.1. January1979.3.11 ~~ILLIAMS. A Testson largereinforcedconcreteelementssubjectedto direct tension.Wexham

ISprints.Cementand ConcreteAssociation.TechnicalReport.April 1986. 42.562.3.12 HARRISON TA. Early agethermalcrack control in concrete.CIRIA ReportNo.R91. 198143A3 roE L-O\c-RETE ~OCIETh Non-structuralcracks in concrete.Technical Report No.22. 1982-

Publication53,038. C3.14 BA\IFOR1~H. p Mass concrete.ConcreteSociety Digest No.2. London 1985. 8pp. PublicatiOn

C

I ~(iJ U— . . K — — K K.K~ KKKK K K~K•~ KK--K.. K~,, , K~,:. K K K K K K K K- ~>--~ K •~>7•~~ K-4. - -. K,., K..

K K K - -- K K K K K •~;.~~7.•:;.-: ~. - - K K-K--K:-.-. K

I-,.4 -4w. K-

.44 - K ...... K ~. - - - K - K ,KK K K

PART 2SECTION FOUR. FIRE RESISTANCE

A SUMMARY OF THE APPROACH TO FIRE RESiSTANCE

In the thirteen yearsbetweenthe introduction of CP110 andthis Codetherehavebeena numberof important reportsissuedon the fire resistanceof concreteelements.

From the viewpoint of the designof structuresin the United Kingdom theseare(inchronologicalorder):

1975(TheOrangeBook)

1978(The RedBook)

1980(TheBRE Guidelines)

Fire resistanceof concretestructures— Report of a JointCommitteeof.the Institution of StructuralEngineersandthe ConcreteSociety.Designanddetailingofconcretestructuresforfire resistance—Interim Guidanceby a Joint Committee of the Institutionof StructuralEngineersand the ConcreteSociety.Guidelinesfor tile construction of fire resisting structuralelementsby R E H Read.F C Adamsand G M E Cooke— A Building ResearchEstablishmentReportpublishedbythe Departmentof the Environment.

The contentof thesethreereports(whererelevant) hasbeenincorporatedinto this Code.The four principle changesfrom CP1lO:1972are:

I. The concept of continuity

CP110:1972distinguishedbetweensimply-supportedconcreteelementsandthosewhereconditionsof restraintcouldbeincorporatedinto thestructure.suchthatthefire resistanceof aconcreteelementcould beincreased.In the useof the old Codefrom 1972 onwardsverv few constructionshave beenable to demonstratethe advantagesof higher fireresistancefrom such restraint. The 1978 report put forward the conceptof achievingbetterfire resistancethroughcontinuity in structuralelements,whichwasadoptedastheprinciple on whichthe concreteelementssectionof the 1980 BRE Reportwas prepared(Tables4.3. 4.4 and4.5).

2. Variation of width ofsectionand cover

The presenttabulardatain the 1980BREGuidelinesstatefor any concreteelementtherequiredminimum width andappropriateconcretecover. In the majority of casesatthelower endsof fire resistance(Le. up to two hours)therequirementsfor minimumwidthsare belo~’- thosewhichmostdesignerswouldwish to usein practice.Consequently,someincreasein the minimum width requirementcould be justified on practicalgrounds.

Table H(2)4.1. Variation of minimum width of member andcover to reinforcement

Minimumincreasein width(mm)

Decreasein coverDense Liehtweicht

concrete concrete(mm) (mm)

2550

10015020(1250350450

110151515202()2t)

10[52025

SO

(Refer to clause4.3.5 in Part2)

181

Handbook to BS8IIO:198S riIt was alsoevidentthatthe resultsof fire testingbothin thiscountryandabroadhave rshownthat flexural elements.(Le. beamsandribs) havehigherperiodsof fire resistance

whenthewidth of the beamor rib is increased.Consequently.thereemergedaviewpointthat somesmall reductionsin concretecover for beamsandribs could be establishedif

Cthe minimum values for widths of these flexural elementswere increased.A table ofadjustmentsto concretecover for increasein minimum width was thereforeproduced,Table H(2)4.1.By adoptingthe morerealisticpracticalminimum widths for beamsandribs, it was found possible to reducethe concretecovers required,particularly in the [continuoussupportsituation.

3. Provision of supplementaryreinforcement

FThe requirementsof BS 8110 centre around the concept of nominal cover toreinforcementor prestressingsteel for durability, as well as fire resistance.An areaof conflict that had to be resolvedwithin CP11Owas the requirementto position a Ilayerof D49 meshat 20mm from the face of beams.ribsandcolumnswherevercoverto the mainsteelexceeded40mm fordenseconcreteor50mm for lightweightconcrete.This requirementwas negatedby the new requirementsfor durability, whereby acover of 20mm to a steel meshwas unacceptable.A working party examinedtherequirementfor supplementaryreinforcementanddecidedto relegateits importancein favour of threeothermethods:

— an applied finish to enhancefire resistance— provisionof sacrificial steel in the main tensilezone— provisionof a fire resistantfalseceiling to the undersideof floors

Supplementaryreinforcementin the form of a D49 mesh implanted in concrete rcoverwas not, however,rejectedoutright for the BRE haveproved in fire testsonbeamsthatsuchaconstructionimprovesfire resistanceof flexural elements.However,this meshcannotnow be used wheredurability requirementsset a nominal coverabove20mm. (Refer to Table 3.4 in Part1).

4. Designfor fire resistancebycalculationIn thirteen yearsof Code developmentandthe issue of major reportsit has beenpossibleto promulgatedesignfor fire resistancebasedon first principles.Consequentlythis Codeis the first to includesuchdesignprinciplesas opposedto compliancewithtabulardata on minimum width of sectionand requiredconcretecover to steel.

Apart from the changesintroducedby the four principal itemsstatedabovethisCodealsomakesfurtherminor changesfrom thoseto be found in CPI1O:1972viz:

— rationalisationof tabulardatafor columns— additional tabulardatafor reinforcedconcretewalls— simplification of constructiontypesfor ribbed floors— removal of dataon additionalprotectionfrom tables.

PARTS I AND 2 OF THE CODEFire resistanceis treatedat two levels in the code.

Part 1 gives simplified recommendationsfor use in the majority of denseconcretestructureseither reinforced(3.3.6) or prestressed(4.12.3.1.3).

Part 2. Section 4 gives detailedrecommendationsfor fire resistancein any concretestructure.

Part2. Section5givessimplified recommendationsfor usein themajority of reinforcedlightweight concrete structures. No simplified recommendationsare given forprestressedlightweight concretestructures.

This treatmentof fire resistancebetweentwo partsof the Code is unfortunateas thedesignerhas no single point of referenceas existed in Section 10 of CP11O:1972.Thereasonfor the split in treatmentwas the CodeCommitte&s insistancethat referenceto f ‘~

cover to steel whetherfor fire. durability or othershould be availablein onesectiOn.This was achievedin Section3.3of Part 1.

I 82

UK K — K — K K — K — ~.

K. KK~ KK~K~KKKK K .— K K — K KK K-- . .---.-r--.~Z---- -. :-.->.-.----. - —, -- .. -. - K.,.-.. ..- -K ——44K~ K — K.,-- - K~ - K -K -. - K K -K K K — - K •K-K-~KK — K K K - K K K K K

Parr 2:Section4

fl KT

L fire

(-11> —~ resistancecover

.1~

nominal cover

Figure H(2)4.1: Nominalcoverandfire resistancecover in beamsor columns.

• ~— fire r~istance cover

OK

• • nominal cover

a I

Figure H(2)4.2: Nominalcoverandfire resistancecoverin walls or slabs.

Beforeembarkingon aclauseby clauseexplanationof Section4of Part2 it is necessaryfor the designerto appreciatethe significanceof the word ‘cover usedin this Code.

CONCRETECOVER TO STEELSection 10 of CP11O:1972madereferenceto concretecovers to reinforcementorprestressingsteelswhich were the distancesfrom the exposedfaceof the concretetothe edgeof the main tensilesteel— bar, strand,wire.

It is very importantfor the designerusingthis Codeto appreciatethat the useof theterm“nominalcover”will givedifferingvaluesto theterm~cover”usedin fire resistancedependingon theuseof secondarysteelsuchasstirrups,lacers,linksetc.Forexample:

Beams— links mustbe used(3.4.5 of Part 1)Columns— links must be used (3.12.7of Part 1)Ribs — links not usually required,thereforethe nominal cover is usuallyequalto the

fire resistancecoverW~iI1s — vertically reinforcedwith lacershorizontally

Thereforegreatcareis neededin referenceto the wordcovertodistinguishbetweenthe two meanings.For easeof referenceto fire resistancethe designershouldnote:

Part 1 generallyrefers throughoutto nominal coversPart2, Section4 refersthroughoutto fire resistancecoversPart2, Section5 refers to nominalcovers.

4.1 General

The handbookon the Unified Code(i.e. Section 10 of CP1lO)gaveadescriptionof thebehaviourof concreteelementsin fire with figures showing the effect of temperatureon material properties.This descriptionis still valid thirteen years later but is notreproducedherein the interestsof brevity.

In the interveningthirteenyearsa much better understandingof the roles of thestructurein fire resistancehasemerged.Apart from the threereportsmentionedin theopeningsummaryto this section therehave beenother reports,technical papersand

183

Handbookto BS8JIO:1985

Date

1975

conferenceswhich haveadvancedknowledgeconsiderablyreferredto thesewhich, amongothers,can be listed as:

Title and author(s)

FIP/CEBRecommendationsfor thedesignofreinforcedandprestressedconcretestructuralmembersforfireresistance

1978 FIP/CEBReporton Methodsofassessmentofthefireresistanceofconcretestructuralmembers

1978 Assessmentoffire-damagedconcretestructuresandrepairbvgunite.(TechnicalReportNo 15)

1979 Concreteforfireresistantconstruction,Cembureau

1979 Spallingofnormalweightandlightweightconcreteonexposuretofire. IrWJ Copier

1980 An internationalreviewofthefire resistanceoflighweightconcrete, JCM Forrest

1982 Designofconcretestructuresforfire resistance.PreliminaryDr-aft of an Appendixto theCEB-FIPModelCode

1982 Theeffectsofelevatedtemperatureson tilestrengthpropertiesofreinforcingandprestressingsteels.RHolmes.RD Anchor.Di CookandRN Crook

1983 A basisfor tile designoffireprotectionofbuildingstructures,MargaretLaw

1983 lnternationalSeminar.Threedecadesofstructuralfiresafety.22-23February1983

1984 Guidancefor theapplicationoftabulardataforfireresistanceofconcreteelements,JC M ForrestandMargaretLaw

1984 Spallingofconcreteinfires,HL Malhotra(TechnicalNote 118)

1985 Fire resistanceofribbedconcretefloors.RM Lawson(ReportNo 107)

The designeris therefore

Publisher

CementandConcreteAssociation

CementandConcreteAssociationTheConcreteSociety

CementandConcreteAssociationHeron(Netherlands)Vol 24.No?. 1979TheConcreteSociety& TheInternationalJournalofLightweight ConcreteVol 2. No?.June1980CEB (Lausanne)

TheStructuralEngineer. Vol 60B.No 1. March 1982The StructuralEngineer.Vol 61A.No 1.Januarv1983Buildinit ResearchEstablishmentInstitutionofStructuralEngineers

CIRIA (London)

CIRIA (London)

C

C

I

rC

C

I;(

C

4.1.1 MethodsThreemethodsare availablefor use by the designerto determinethe fire resistanceofa concreteelement.

4.1.2 Elements

41.3 Whole structures

4.1.4 Surfacesexposedto fireThe presentstateof knowledgedeniesdeterminationof the fire resistanceof anassemblyof concreteelements.Some researchcentresin Europe and the USA are currentlyworking on the fire resistanceof completeframes by mathematicalmodelling but nopractical applicationscapableof being usedin codes.are yet available.

41•5 Factors affecting fire resistanceConsequentlythe designeris required to ensurethat the detailsof concretemembersizes.cover, disposition of steel reinforcementor prestressingstrand.and choice ofmaterialsachievesthe desiredresponseof the structureunder fire attack.

UI X4

LII;[

3

K ~. — — — K~ — — ~..~ ~ -- ~ ..... - -~ - K~~K - K K--K...- -

Part 2: Section 4

4.1.6 Spalling or concrete at elevatedtemperaturesDuring preparationof the Codefurtherresearchin theUK wascommissionedby CIRIA.The report was written by H L Malhotraand publishedin 1984 as TechnicalNote 118underthe title Spallingofconcretein fires. A summaryof this report from CIRIA News.January/February1985 is given below.

‘Spalling is the breakingoff of layersor piecesof concretefrom the surfaceof astructuralelement.andcan occurwhen reinforcedconcreteis exposedto the highandrapidly rising tmperaturesexperiencedin fires. Spallingmay be insignificant inamountand consequence,such as surfacepitting or the fall of a small piece froman arris. or it can seriouslyaffect the stability of the constructionbecauseof theextensiveremoval of concretefrom reinforcementor becauseit causesholes toappearin slabsor panels.It can occursoonafterexposureto heat. accompaniedbyviolent explosionsor it may happenwhen the concretehas becomeso weakafterheatingthat, whencracksdevelop,piecesfall off the surface.All thesephenomenaare coveredby the expressionspalling.

Over the last twentyyearsor so.various national and internationalcodeshavesuggestedthat measuresare neededto preventspallingwhen dealing with certaintypesof concreteorwhenthe coverto the reinforcementis large.Theserequirementscreateddifficulties on site. leadingto increasedcost andproblemsof control. Theresulting criticisms necessitateda re-examinationof the basis of the earlierrequirementsandfurtherkno~vledgewhichhasbecomeavailableoverthelastdecadeby experimentsas well as by studyof actual fires.

Technical Note 118 Spalling of concretein fires presentsthe resultsof the firstpart of a CIRIA researchproject intendedto provide a basis for future coderecommendations.It describes,collates and assessesexisting information in theliterature,from laboratorytestsandreportsof the effectsof actual fireson buildings.This leadsto a reviewof the causesof spalling as recently understood.the ways inwhich it may be controlled and.particularly. the areasin which more researchis

needed.including recommendationsfor furtherexperimentalstudv.~

4.1.7 ProtectIon against spallingIn concretestructurestheconcretecoverprotectsthesteelreinforcementor tendonfrombecomingoverheatedandandlosingstrength.This concretecovercanspall away underfire attackexposingthe steel.

Subject,to the method of detailing employed by the designersuch spalling can beignoredif alternativepathsfor loadtransferenceor capacityareavailable.Wheretheyare not then the designerhas to ensurethat the concretecover remainssufficientlyunimpairedfor the period of fire resistancerequired.

CP11O:1972recommendedthe useof a secondaryreinforcementsystemas follows:

“Supplementaryreinforcementwill be requiredin thosecasesindicated in the tablewhen the cover to all the barsandtendonsunderconsiderationis more than40mm.Whenused.supplementaryreinforcementshouldconsistof expandedmetal lath ora wire fabric not lighter thanOSkg/m2(2mmdiameterwires at not morethan 100mmcentres)or a continuousarrangementof links at not more than 200mm centresincorporatedin theconcretecoveratadistancenot exceeding20mm from theface.”

This requirementarosefrom the test programme.by the Fire ResearchStation in1968.on concretebeamswhenthe useof such a meshas secondaryreinforcementwasfound to be advantageousin increasedfire resistancefor simply-supportedbeams.Thetestspecimensweremanufacturedunderlaboratoryconditionswherebythe meshcouldbe accuratelylocatedat 20mm from the facesof the beamand without regardto costor time implicationsin manufacture.

Regrettablyvia CPI1Othis requirementwas imposedon the constructionindustryatlarge with the result that the use of such a meshled to many cases.well documented.of poorly compactedconcrete.displacedmeshpositioningandincreasedcostsof concretesite production. The useof such mesh becameabhorrentto thoseconcernedwith theproductionof good qt~ality homogeneousconcrete.During preparationof this Codetheimportant aspectsof durable concrete were incorporated as the concrete coverrequirementsfor durability (refer to 3.3 in Part I). It wasrealisedthat the requirementsfor supplementaryreinforcementmeshwould be in conflict with thosefor durability and

185

Handbookto BS8IIO:198S [1alternativemeasuresforprotectionagainstspallingwouldbe necessary.A workingpartyof the Code Committee (including representativs from the Building ResearchEstablishment)advisedalternativemeasuresas given in this clauseso as to resolvethematter.

However the use of a mesh as supplementaryreinforcementwas not removedfromthe Code as otherwisethe test experienceby the BRE would be negated.Insteadtheuseof a meshwasdown-gradedin favourof the four alternativesandsuitablecautionarynotesincludedin the clause. [4.1.8 DetailingAn understandingof the requirementsfor good detailing is given in the 1978 Report— rDesignand Detailingof ConcreteStructuresfor Fire Resistance— Interim Guidanceby La Joint Committeeof the Institution of StructuralEngineersandthe ConcreteSociety.Two principal aspectsfrom reportsof existing fires are: rBottom reinforcementin slabs— The designershouldcheckthat at least50% of themain steel is anchoredat both ends (i.e. avoid the staggeredstraight bar arrangementwherebyonly oneendis anchoredin a beamzone). r

Top reinforcementin beamsand slabs— Doesthe length of steel from the supportenablethe beamor slab to adopta ‘near cantilever’ structuralaction at the limit statein fire? [

4.2 Factors to be considered in determining fire resistance [4.2.1 General

C.4.2.2 AggregatesFor the purposesof the Code,concretematerialsaredivided into two classes(i.e. denseand lightweight). The aggregatesusedin theseclassesaregiven. CUnlike CPI1O:1972no separatesubdivisionof denseconcretesmadefrom calcareousas opposedto siliceousaggregatesis made.This is a retrogradestepafterthirteenyearsbut reflectsthe evidenceobtainedby the BRE that thereis no superiorfire resistance Ifrom limestones(calcareous)overgravels(siliceous) in a flexural modein fire. Concretesectionsizesfor beamsandslabsarethereforeidenticalforeithertypeofdenseaggregate.For the compressionmode howeverthereis no just causeto penalisecolumnsand -

wouldbereasonableto useCP110:1972valuesforconcretesectionsmadefromcalcareousaggregates.The valuesfor fully exposedcolumnsare as follows: I’

Periodofflreresistance(hours) 0.5 1 1.5 2 3 4

Minimum dimensionof column(mm) 150 190 200 225 275 300 [(ComparewithTable4.2 150 200 250 300 400 450)

4.2.3 Coverto main reinforcementThe designer is referred to the summary introduction to this Section for a clearunderstandingof the useof the word ‘cover’. As statedthroughoutthisSection,reference Iis made to the fire resistancecoverunlessspecificallycalled nominal cover.

(a) Floor slabsDuring the preparationof the Code significant stridesin new knowledgeof the Ibehaviourof one-wayspanningandtwo-way spanninitribbed floorswas made.Theinformationgainedsuggeststhat the conceptof averagecovershouldnow only applyto solid slabswith multi-layer reinforcementor to one-wayspanningribbedfloorsTwo-way spanningribbedfloors havebeenexaminedby CIRIA in a programmeOf Ltestingduring 1984/85leading to CIRIA Report No 107 in 1985 Fire resistanceofribbed concretefloors. The designeris referredto a full descriptionof the tests.summaryandconclusions.In relationto coverthe reportstates:

~‘Toclarify the use of the tabulardata in BS 8110, the definition of minImumI 86

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Parr 2: Section4

cover to the barsin the ribs of two-way spanningfloors (suchaswaffle slabs)is takento be to thelower bars.The increasedcoverto the barsin thetransverseribs is not important.becauseof the ability of the slab to redistributemomentfrom anyheat-affectedarea.~’

The last sentenceis invariably valid becausewaffle floors are usually cast as insitu constructionwhere adequatecontinuity of structural action is availablefromeachof the two spansconcerned.

(b) RectangularbeamsNote2 rivenin thisclauserefersto therelativeheatingeffecton cornerbarscomparedwith the others.Cornerbars areheatedfrom two directionswith equal intensity.All otherbarshavethemain heatappliedfromonedirectiononly. It is gooddetailingpracticeto arrangefor the majority of the fully stressedtensile bars/tendonsto begroupedaway from the corner.

(c) I-sectionbeamsOn apracticalnotethesebeamtypesarerarely found in in-situ constructionastheyareusually the productof precastworks. Wherevolume productionis requiredforsuch applicationsas floor beamsin systemconstructionit is usualto undertakeafire test (method2). The requirementsof this clausearethereforeusuallycheckedagainstthe behaviourof the test modelafter initial sizing of the concretesection.

Of greatimportancein thesebeamtypesis the retentionotTheweb of the beamin fire. andthe useof web reinforcementas shown in Figure4.2 is essential.Therehavebeensomenotablefire testfailureswhereearlycollapseof thewebhasresultedin the prematurefailure of the beam.

4.2.4 Additional protectionThe values given in this clauserepresenta more conservativeapproachto the use ofappliedmaterialsto enhancefire resistancethanthe valuesobtainablefrom CP11O:1972.In putting forward thesevaluesthe BRE consideredthat insufficient test evidencewasavailablefrom the use of modernapplied finishes to be specific for eachperiod of fireresistancefor eachmaterial.The intention of this clauseis thereforeto aid the designerwith initial sizing of concretesection andapplied finish and then to seek moredirectinformationon thefire resistantqualitiesof theappliedfinish from selectedmanufacturersatthe detaileddesignstage.

4.2.5 Floor thickness

4.2.6 Width of beamsThe designeris referredto Section4.3.5 for the influenceof width of beamson coverrequirementsfor given periodsof fire resistance.

4.2.7 Distinctionbetweenribsand beamsThe requirementto distinguishbetweenribs and beamsin this clausewas insertedtoensurethat rib spacingdid not movetoo far apartandconsequentlynegatethe role ofa ribbed floor as a floor ratherthana topping over a seriesof beams.The originalmaximum spacingproposedwas 1.2m but representationsfrom the precast industryenabledthe Code Committeeto. raise the spacing to 1.5m so that double‘T’ units ofwidth 3.Om could be madeif required, to matchconstructionwidths in the USA forexample-

4.2.8 Beamsand floors

4.2.9 ColumnsFigures4.2 and4.3havebeenpreparedto illustrate the generalintention of constructionon which the tabulardata in Tables4.2. 4.3, 4.4 and 4.5 are based.The sketchesareidealised and the designershould ensure the relevance of the important factors ofminimum dimensions,thicknessesand coversin the design of the concreteelementconcerned. 187

Handbookto BS8IIO:198S Iii4.3 Tabulated data (method 1)

I.

4.3.1 Method by designfrom BRE GuidelinesThe summarvintroductionto this Section of the Handbookexplainedthat the BRE FGuidelines,publishedin 1980,were usedas the basisfor the tabulardata in this Code.In turn the BRE Guidelineswerebasedon the tabulardatacontainedin the 1978Report— Design and detailing of concrete structuresfor fire resistance (the Red Book).

[Consequently the tabular data given represent the combined input of the threeorganisationsover sevenyears deliberations.It is anticipatedthat any alterationstoBRE Guidelinesin the future will be in conjunction with BSI revisionsto the text of rthis Code.4.3.2 Support conditions: simply-supported and continuous r4.3.3 Use of tabular dataExamplesof continuousconstructionsaregiven in the 1978 Report.Designanddetailingof concretestructuresfor fire resistance(the RedBook). The 1978 report alsoindicateshow the fire resistanceof concreteelementsvaries with applied load, viz increasesinfire resistancefor lightly loadedelementsandvice versa. [

4.3.4 Spallingof nominal coverThe designershouldnotethat the onsetof the requiredprotectionagainstspallingnowstartswhenthenominalcover,i.e. thecoverto theoutermoststeel.exceeds4Omm/SOmmrespectivelyfor denseandlightweight concretesand not the fire resistancecoveras inCPI1O:1972.(Refer to this Handbookcoveringclause3.3.6 of Part 1.) C4.3.5 Variation of cover to main reinforcement with memberwidthTable 4.1 was producedfollowing modernevidencefrom fire teststhat as beamwidths~vereincreasedthere were small permissiblereductionsin concretecover to maintainthe sameperiod of fire resistance.No such facility was available in Section 10 ofCP11O:1972andin manycasesdesignerswereusingconcretebeamandrib sectionswellabovethe minimumrequiredbut havingno benefitin decreaseof requiredconcretecover.

Table 4.1was thereforeusedto preparerevisedconcretecoversto thosegiven in BREGuidelinesto suit practicaleverydaywidths of beamsat 200mmwide andribs at 125mmwide. These concretecovers are given in Tables 3.5 (reinforcedconcrete)and 4.9 F(prestressedconcrete)of Part I of the Code.

Table 4.1 can alsobe usedto vary the coversof beamsandribs from thosegiven inthe BRE Guidelinesviz Tables4.3, 4.4 and4.5 of the Code.

The proviso that the coverto beamsteelshouldnot be reducedto a valuelower than[

thatforasolidconcretefloor isnecessarytoensureretentionoffire engineeringprinciples.

4.3.6 Reinforcement LBRE Guidelinesandhencethe tabulateddata in the Code are basedon fire testsheldin the UK where the variation of material propertiesof UK reinforcing steels and

K prestressingtendons (to British Standards)under heating are well known and Idocumented.This clausealerts the designerto considerthe heatingeffectson othersteelsthe strengthpropertiesof which do not conformto the patterngiven in Figure 4.5or to British Standards.

Tablesapplicableto methodI i~.Table4.2 Reinforcedconcretecolumns

The exposuregradingsare illustrated in Figure 43. The designershould note that for Lpracticalpurposesthe coversgiven in this tableshould be reducedb~ the dimensionofthe link to arrive at the nominal covergiven in Part 1.

Table4.3 ConcretebeamsThe designershouldnote the requirementfor the onsetof protectionagainstspalling. Lii.e. whenthe nominalcoverexceeds40mm fordenseconcreteand~0mmfor lightweightconcrete.

188

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Table4.4 Plain soffit concretefloors

Table4.5 Ribbedopensoffit concretefloorsThe footnoteon cover is incorrect as floor reinforcementdoesnot normallx- haveanylinks. Consequentlythe cover to the main reinforcementis also the nominal cover.

Table4.6 Concretewalls with vertical reinforcementWalls are groupedinto three categoriesdepending on the percentageof vertical

2 reinforcementviz:

— less than0.4% (note0.9% is a printing error)— 0.4% to 1%— over 1%

The valuesin the table extendthe rangeavailablefrom CP11O:1972.

Fire test (method 2)

This briefclausecoversthe requirementsfor constructionsnot conformingto the generalarraneementsoutlined in this section.Also whereprecastunits in volume productionwould benefiteconomicallyby a suitablydesignedfire test programmeto verify the fireresistanceof a manufacturedunit.

4.5 Fire engineering calculations (method 3)

4.5.1 General

4.5.2 Principles of design

4.5.3 Application to structural elementsThismethodof determiningfire resistanceby calculationis confinedto the flexural modeof behaviourof beamsandslabsin fire. For an understandingof the principlesto adoptthedesigneris referredto Chapter8 of the 1978Report.Designanddetailingofconcretestructuresfor fire resistance(the RedBook).

4.5.4 Material properties for design

4.5.5 Designcurve for concrete

4.5.6 Designcurve for steelThe designershould notethat the curvesgiven in Figures4.4 and4.5 are designcurvesderivedfrom experimentaldataovermany testregimes.

4.5.7 DesignThe basis of the calculationapproachto the designfor fire resistanceof an elementinflexural mode.i.e. beam.rib or slab,follows fire engineeringprinciples.Theseprinciplesdictate that theelementshould. over the requiredperiod of fire resistance.supportthemoving patternof loadingprovidedby the detailingof the reinforcementltendonswithinthe section-At the end of the fire resistanceperiod the reducedmomentof resistanceof the elementunderattackshould still be greaterthan or at least equalto the appliedmoment.This principle is illustrated in Figure H(2)4.3. For simplicity of approachtheultimate momentof resistance(M~ for the simply-supportedcondition andAl,, for thecontinuouscase)is reducedto 50% by the heatingeffect at the endof the fire resistanceperiod.This approachis basedon the strengthof the steel beingreducedto 50% of itsultimate strengthas indicatedin Figure4.5.

189

[IHandbookto BS8IIO:1985

full anchorageby bond orequivalent

design moment4~~-r morn:nt

capacity

Cal reinforcement at normaltemperature

design moment2

ultimatemomentcapacity

(bI reinforcement at criticaltemperature

restraintr against rotation -~4

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normal temperature

fixed-endmoment

mid-spanmoment

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ultimatesupport total designcapacity 4 mOment

ultimata M T’ijnused moment capacitymid-span ~capacity

(bi Top reinforcement at normaltemperature. Bottom reinforcementat critical temperature

Figure H(2)4.3: Structuraleffectsoftemperature.

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PART2SECTION FIVE. ADDITIONAL CONSIDERATIONSINTHE USE OF LIGHTWEIGHTAGGREGATECONCRETE

5.1 General

5.1.1 IntroductionThe clausesin this Sectionareadditional to thoseelsewherein the Code.anddealwithsituationswherechangeisnecessaryin the generaldesignprovisionsto covercaseswherelightweight aggregateconcreteis used.

The Code stressesthat the propertiesof any particulartypeof lightweight aggregatecan beestablishedfar moreaccuratelythanfor mostnaturally occurringmaterials,andrecommendsobtainingspecific datafrom the aggregateproducer.in preferenceto usinggeneralisedtabulatedinformation. Indeed.with any one sourceof aggregate.a widerange of propertiescan be obtained,by varying the manufacturingand productionprocessesin acontrolledmanner.This permits considerableflexibility to the designer.but makesthe derivationof generalCode clausesdifficult; for that reason,most Codeclauseson lightweight aggregateconcretetendto be on the conservativeside.

For reinforcedor prestressedconcrete,the Code suggestsgrade20 as a minimum.This meansthat the densityof lightweight aggregateconcretewill normally be in therange1,500-1,900kg/in3(6r 60—80% of that for normal-weightconcrete).However,thematerial can have advantagesother than that of reducedweight, notably in terms ofstrain capacity, stability at high temperatures(fire resistance)and good insulationcharacteristics;equally, thereare somedisadvantages,comparedwith normal-weightconcrete.andthesemustbe accountedfor in design.In verygeneralternis,somedesignpropertiesare listedin Table H(2)5.1 below:but, for the designof aparticularstructure.using a particularaggregate.recourseshould be madeto the datafrom the aggregateproducer— as the Codesuggests.

Table H(2)5.1 Some design properties of lightweight aggregateconcrete, compared with the same grade ofnormal-weight concrete

Property

Relation to samegradeofnormal-weightconcrete Notes

StrengthDensityStiffnessExpansionCreepDrying shrinkageShearstrengthAnchoragebondBearingcapacityPermeability

thesame60-80%50-70%65-80%higherhigher75.85%80-85%60-75%hicher

‘I, theupperendof therangeJ obtainsforhighergrades

usuallybecausethepastecontentis higher

The Code makesno specificreferenceto prestressedlightweight aggregateconcrete.In general.the provisionsof Section4, Part1 may be takenas applicable— modified bythe requirementsof this Section.Additionally, attentionmay have to be given to:

Lossof prestress

Transmissionlength

Loss of prestressmay be higher with lightweight aggregateconcrete.This will be due mainly to greaterdeformationof theconcrete,either by elasticshorteningor causedby greatercreepand shrinkage.This requirescareful checking,since overall lossof prestresscan increaseby as muchas 50%-

The valuescalculatedby equation60 in 4.10.3 of Part 1 of theCodeshould be increasedby 500/o, in the absenceof appropriatetest data.

I

191

Handbook to BS8IIO:1985

Shearin prestressed As for reinforcedconcrete(see5.4 below), the design concrete

lightweightconcrete shearstressshouldbetakenas0.8timesthatfor denseconcrete.

5.1.2 Symbols

5.2 Cover for durability and fire resistance -

Table 5.2 sets out the nominal covers to all steel to meet specified periods of fireresistance.This table is preparedin similar format to Tables3.5 and4.9 in Part 1 forreinforcedconcreteandprestressedconcreterespectively.

The table should containthe samefootnotesas Tables3.5 and4.9 referrmn~to:

(a) the 10mmstirrup allowanceto beamsandcolumns(b) coversrelated to minimumdimensionsgiven in Figure 3.2(c) anti-spallingmeasures.

It is tobe hopedthatthesefootnoteswill beaddedin lateramendmentstotheCode.Thedesignershouldnotethat to coverthe full rangeof fire resistanceup to four hours

there is no needto incorporateany additional measuresto reducethe risk of spallingforanycontinuousconstruction.This reflectsthegenerallymuchbetterspallingbehaviour

[in fire of flexural elementsmadefrom lightweight concrete.For exposureconditionsotherthan Mild. Table 5.1 requires10mm additional coverwhencomparedwithTable 3.4 in Part1 of the Code.This is becauselightweightconcreteis usuallymorepermeablesinceit hasagreaterpastewatercontentthananormal-weightconcreteof the samestrength.Lightweight aggregatehas-a porousstructure~andwit hlowergradeconcretesin particular.carefulattentionshouldbe given to curing.to ensurethattheporousparticlesdo not provideaneasypathforcarbondioxide. thusaccelerating

[carbonation-Table5.2. on the otherhand,requireslesscoverfor fire resistancethanthatrequired

by Table 3.5 in Part 1; the differencesrangefrom 5 to 15mm.This reflectsthe generallybetter performanceof lightweight aggregateconcretein fires. Exactly why this is so isnot completelyproven,but it is generallyattributedto agreaterstraincapacity.in firesof limited duration,dueto some combinationof reducedstiffness, a lower coefficientof thermalexpansion(8 x 10~ per0C, comparedwith 11 x l0~’ per 0C) and thermal

[Sdiffusivity.In practice,this meansthatfor lightweightaggregateconcrete,coverwill almostalways

be dictatedby durability requirements,except possibly for deep beams in buildings.With corrosionprotection being the main issue. thereare perhapsgreater incentives Lwith lightweight aggregateconcrete to consider special measuresfor loweringpermeability,such as additivesor evenprotectivecoatings. L

5.3 Characteristic strength of concrete

5.4 Shear resistance LThis clause in effect permits the use of conventionalsheardesign methods.but sets I-limiting design concreteshearstressesat 80% of those for equivalentnormal-weightconcrete.Normal designshearstressesarecalculatedat critical sectionsandsubsequentdesignis basedon how thesestressesrelate to limiting designconcreteshearstresses gThe basicmethod has not changedfor decades.but some of the limiting valueshave. Las more datahavebecomeavailable.

Although the methodis convenientfor design use, it doesnot directly reflect how abeamcarriesshearforces in practice— this is generallyassumedto be acombinationot Ldowel action of the main reinforcement,aggregateinterlock acrossshearcracksand acontribution from the flexural compressionzone.The argumentfor reducingthe shearcapacityof lightweight aggregateconcreteis basedon a reducedcontribution to shedrpfrom aggregate-interlock— this is becausethe aggregateitself can crack, leading tOLsmootherfaceson eachside of the crack, andhenceless interlock. With some typesot

lightweight aggregate.this might not happen,andhigher stressescould be justified. In

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the light of presentknowledge.however,this could only be on thebasisof comparativedatabetweensimilar beamsof denseand lightweight concrete.

5.5 Torsional resistance of beams--.5

1 See5.4.

5.6 Deftections

] In general,directcalculationwill bethe mostrealisticandeconomicmethodof checkingon deflectionssincethe methodgivenin 3.4.6.3of Part I of theCodewill beconservative.The approachusedwill dependon the accuracyrequiredfrom the calculationbut, ingeneral.the input to the calculationshouldbe basedon propertiesdeterminedfrom the

H aggregateto be used:in particular.thereis aneedfor precisedataon elasticmoduli. andon creepandshrinkagecharacteristics.

Columns

The significant change here, compared with 3.8 in Part 1 of the Code. is that theII slendernesslimit for a short column is set at 10, irrespectiveof whetherthe column isbracedorunbraced.Irt part,thisis tofurtherlimit theriskof deflection-inducedmoments.

In addition, it is suggestedthat the clear distancebetweenend restraintsshould notUexceedfifty timesthe minimum thicknessof the column.1] 5.7.1 General

5.7.2 Short and slender columns

[1 5.7.3 Slender columns

~ 5.8 Walls

Similar restrictionsare introducedhereasfor columns— for similar reasons.Forslenderwalls (5.8.3), thechangesin thevaluesfor thedivisorsrepresentconservativeassessmentsof the relativemoduli of denseandlightweight concrete,in the contextof equations349,and44 of Part1.

J 5.8.1 General

5.8.2 Stocky and slenderwalls

5.8.3 SlenderwallsK’

5.9 Anchorage bond and laps

Test data indicate reducedvalues for anchoragebond, when lightweight aggregate.1concreteis used(andevengreaterreductionsfor local bond,which is vervrarely criticalin practice).The Codesetsthisreductionat20%,leadingto increasedlap andanchoragelengths,which can be somethingof a problemfor shallow short-spanmembers.Gooddetailing is essentialin anycase,not just for thesestructuralreasonsbut alsoto ensurepropercover for durability.

5.10 Bearing stress inside bends

The limiting bearingstressgiven in this clause is one-third less than that obtainedby193

Handbookto BS8IJO:1985

equation50 in 3.12.8.25.2in PartI of the Code,for denseconcrete.In reality, bearingstressesdependvery much on the lateral restraintprovided,and this reductionis anattemptto allow for the fact that lighrweight aggregatemay crushmor.~ easilydueto itsporousnature.

REFERENCE

5.1 FEDERATION INTERNATIONAL DE LA PREcONTRAINTE ~FIPt Manual of lightweight aggregateconcrete.Secondedition. SurreyUniversity Press/Blackie& Son.Glasgow.1983.

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PART2-SECTION SIX. AUTOCLAVEDAERATEDCONCRETE

6.1 GeneralIAutoclaved aerated concrete is a li2htweight cellular material and does not normallycontain coarse aggregate. It is made by introducing air or other gas into a slum’ ofcement and sand, pulverized-fuel ash or other suitable material. For structural purposes.Ithe material is autoclaved, i.e. cured in high pressure steam chambers. The material isthus made in a factory to produce various precast structural units, which, in addition totheir low density (400—1,00Okg/m~. o~en dry). have good thermal properties and fire1resistance.

This Section stresses that the manufacturer is responsible for the design of the units.which should meet the general requirements of Section 2, Part 1 of this Code: certainly.the manufacturer’s recommendations regarding the use of these units should always becarefully followed.

The properties of the material. the design considerations. and the production ofstructural units are fully described by Short and Kinnibur~h~61~. Additional informationmay be obtained from the CEB 62)~ particularly on design and detailing. Morerecently, a volume edited by Wittman16~ contains numerous papers on moisture

I movement, and on creep and shrinkage characteristics.6.2 Materials

7’6.2.1 Cement

] 6.2.2 Water

6.2.3 Fine materials] 6.3 Reinforcement

Owing to its porosity and low alkalinity, aerated concrete does not afford the sameI protection against corrosion of the steel as does normal concrete, and so the reinforcement

must be specially treated by protective coatings. These should resist moist heat. bej chemically inert towards the steel and adhere to it. have sufficient mechanical strength

to resist impact and abrasion in handling, and should not be brittle nor deteriorate withage. Coating methods now in use have been well proven, even for long exposures undercorrosive conditions; in general, the~’ are based either on a mixture of rubber latex and

j cement, or on special bituminous compounds.Reliance cannot be placed on the bond between the aerated concrete and thereinforcement, and hence all bars must be provided with suitable anchorages. One of themost commonways of achieving this is by cross-bars, welded to the main reinforcement:

j hence the reference in this clause to the use of mats or cages. This means that it isimportant that units should not be cut on site, except in accordance with themanufacturers’ instructions.

Due to the nature of the manufacturing process. it is difficult to incorporate shearreinforcement. In slabs, shear reinforcement need not be provided, as long as themanufacturer has allowed for the shear in his design by. for example, increasing thedepth of the units. However, in lintels and single beams. shear reinforcement should beprovided, -broadly in accordance with th~ principles in Section 3, Part 1 of this Code.

6.4 Production of units

The compressive strength of autoclaved aerated concrete is directly related to its density,ranging from 2—3N/mmat 400kg/mt to 5—8N/mmat 800k~/m3. Strength is also dependent

I 95

Handbookto BS8I]O:1985

on moisturecontent. and can increaseby 20% or more if the moisturecontentfallssignificantly below 10%. The Code requiresthat the averagestrengthof 12 specimensminus 1.64 times their standarddeviation is not less than 2N/mm: normally thesemeasurementswill be takenat relatively high moisture contents,and hence a furthergain in strengthcanbe expectedasthe unitsdry out in serviceto 3—4% moisturecontentby volume.

6.4.1 General

6.4.2 Quality control

6.4.3 Marking of units

6.4.4 Dimensionsand tolerances

6.4.5 Rebating and grooving

Methods of assessing compliance with limit state requirements

Here the responsibilityfor the designis placedfirmly with the manufacturer.and anindication given that prototypetestingwill be required.In practice,manufacturershaveundertakendevelopmenttestingover manyyears.andthereforehavean extensivedatabank to drawon. in formulatingdesignprocedures.Deformationgenerallyis influencednot just by stresslevel in service,but also by moisturecontent. ambienttemperatureandrelative humidity.

6.6 Erection of units

6.7 Inspection and testing

REFERENCES

6.1 SHORT A and KINNISLRGH w. Lightweight concrete.London. Applied SciencePublishersLtd.Third Edition. 1978. 464pp.

6.2 coNlim EL’ROPEEN DU BETON. CEB Manualof aucoclavedaeratedconcrete— designandtechnology-The ConstructionPress.London. 1978. p90.

6.3 WI-1-rMAN PH. (Editor). Autoclavedaeratedconcrete,moistureand properties.Developmentsin Civil Engineering.6. ElsevierScientific PublishingCo.. Oxford. 1983. p380.

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PART 2SECTION SEVEN. ELASTiC DEFORMATION, CREEP,DRYING SHRINKAGE AND THERMAL STRAINS OFCONCRETE

7.1 General

This Section is not concernedwith providing minimum requirementsto satisfy anyparticularlimit state.It is concernedwith differentaspectsof deformation.andattemptsto provide helpful informationon deformationand movements,for useat the detaileddesignstageperhapsat a time when the concretespecification hasnot beenfinalized.andhencethe precisepropertiesof the mix are not known.

The point is strongly madein this Section that the designershould first decidehowaccuratehis assessmentneedbe, sincethis will affect the way he approachesthe design.In effect,therearethreelevelsof accuracyimplicit in thisSectionasawhole:theseare:

(a) information requiredto assessthe generaloverall responseof the structureto thedesignloads,andhenceto calculatetheresultingforcesandmoments.This reducesto the selectionof a suitablevalue for the modulusof elasticity,which can be usedto define the stiffnessof the structureas a whole. Here, the meanvalues given inTable7.2 areusuallysufficient, butcaremay be necessaryif limestoneor lightweightaggregatesare to be used.

(b) informationrequired,aspartof routinedesign.to assessdeformationandmovements— of concretethe material, of individual elements.andof the structureas a whole— with a view to determininghow to copewith thesein the design.This will involveidenti~ing the source of the movement,quantifying the likely effect (perhapsbracketingthe potential rangeof movementin doing so).prior to taking decisionson whetherto deal directly with the stressesso induced, or to makeprovisionforthe strainsinvolved (by providingmovementjoints for example).It is this level ofaccuracythatSection 7 is intendedto cover.

(c) the assessmentmadein (b) abovemayreveala level of deformationsuchthat moreprecisedata are required,before final design anddetailing decisionsare possible.The Code then suggeststhat the only way to obtain thesedata is by testscarriedout on concretemadewith the materialsto be usedin the actualstructure.

The informationcontainedin Section7 is intendedasguidancein predictingin-servicemovement.This is clearlyshownby the varioussub-headings.whichareconcernedwith:

— Elastic deformation— Creep— Drying shrinkage— Thermalstrains.

However, to obtain a properoverall perspective,it is important to rememberthatother forms of movementcan occur, particularly at an early agewhen the concreteisstill plastic(plasticshrinkage,plasticsettlement)or when it has just hardened(early agethermal contraction, crazing). These deformations are essentially intrinsic, beingdependenton theconstituentmaterials,concretetechnology,workmanship.etc. Dealingwith this type of movementis primarily a concretetechnologyissue.and guidanceisavailable in the literature~7’- 7,2) However, in coping subsequentlywith in-servicemovement,it is important to rememberthat this early age movement(and how it isdealtwith) will all toooftenprovidethebasisordatumline forall subsequentmovement.

Movement and deformation are mainly a serviceability issue: however,excessivedeformationor a failure to deal properlywith movementin designcan causecracking,or the openingof joints or otherdefects,all of which can acceleratedeteriorationandaffect durability.

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7.2 Elastic deformation F1.

The provisionsin this Sectionare basedsubstantiallyon reference7.3. Elasticmodulusdependspredominantlyon the typeof aggregateused,but is also influencedby thegrade flof concrete.The relativeimportanceof thesefactorsis clearly shownin equation17 andTable 7.2. Table7.2 also showsthe wide rangeof valuesthat can occur in practiceforany particular grade,andthe Codesuggeststhat it would be prudentto considera range rof values in a particular case. in order to bracket the movementthat could occur. jAlthough no allowancefor an increasein strengthwith agebeyond28 daysts permittedin Part I of the Code when dealing with limit state requirements,Table 7>1 hasneverthelessbeenincludedhere.since,in dealingwith movements,theassessmentshould rbe as accurateandrealisticas possible. L

7.3 Creep [The recommendationsin this Sectionare basedmainly on reference74. A greatdealof researchhasbeendoneon creepoverthe years.but mainlyundercontrolledlaboratory 7conditionsandsomecareis necessaryin applyinglaboratorydatato actualstructuresin Lservice.Figure7. 1 is an attemptto presentthebest availableinformationin a simplifiedway for designpurposes.Creepdependson the stressin the elementandon its stiffness.As Figure 7. 1 indicates, the creep coefficient also dependson the environmental [conditions,on the maturxt~-ot the concrete,on the aspectratio of the cross-section,andalsoon thecompositionof the concreteitself. Additionaly, creepandshrinkageeffectsare inter-related,althoughit is normal practice. as indkited here, to deal with them rseparately. U

7.4 Drying shrinkage

This clauseis also basedon reference7.4. Figure 7.2 shows the strong influence ofrelative humidity andof the aspectratio of the cross-section.Mix proportionsare also rimportant,andattentionis alsodrawnto the influenceof highly shrinkableaggregatest7.S). IFigure 7.2 relatesto plain concrete,andthe influenceof any reinforcementshouldalsobe takeninto account.The Codegives a simplemethodfor symmetricalreinforcementbut the influenceof non-symmetricalreinforcementon curvatureis more complex.In [this context.across-referenceis madeto Section3. Part2 of the Code— andto equation9 in clause3.6in particular.A moredetailedtreatmentof the subjectisgiven in reference7,6.

7.5 Thermal strains

Figure 7.3 is takendirect from reference7.4. which also containsa much moredetailed [version of Table 7.3.

REFERENCES L7.1 HARRISON TA Early age thermalcrack control in concrete.Report 91, London.Construction

Industry Rcsearchand InformationAssociation. 1981. 48p. I7.2 THECO\CRETESOC!E1~Y Non-structural cracksin concrete.TechnicalReportNo.22.TheConcrete I.

Society. London. 1982. 38p.7.3 rEYCHENNE. 0 C. P.\RROTT. Li. andPO.\IEROY. c D The estimation of the elastic modulusof concrete

for the desienof structures.Building ResearchEstablishment.Garston.Current PaperCP [23/78. 1978. l2p. L

7.4 ~.~utorr. Li. Simplified methodsof predictingthedeformationof structuralconcrete.WexhamSprings.CementandConcreteAssociation.DevelopmentReportNo.3. October1979. lIp.

7.5 ItLILDING RESEARCH ESTAHLISH\IENT Shrinkage of natural aggregatesin concrete. BRS Digest35(secondseries). 1968,

7.6 HOBBS. 0 w Shrinkace-inducedcurvatureof reinforcedconcretemembers.WetthamSprings.Cementand ConcreteAssociation. DevelopmentReportNo. 4. November1979. l9p.

I9X [

PART 2SECTION EIGHT. MOVEMENTJOINTS

8.1 General

Since the first nationalCodeof Practicefor reinforcedconcretewas publishedin 1934,structureshave tendedto becomelighter andmore flexible and hencemorevulnerableto theeffectsof dimensionalchange.At thesametime, materialshavebecomestronger,with theresultthatstructuresareproducedwhicharelesstolerantin theirintrinsic abilityto accommodatemovement,without special provisionsbeing made. This meansthatmoreattentionhas to be given consciouslyto the treatmentof movementin design;themereexistenceof this Sectionof the Code highlights that fact.

Section7, Part2 of the Codegives guidanceon calculatingdeformationsdue to factorssuch ascreep,shrinkageandthermalmovement;thecommentaryon that Sectionbrieflymentionsother factors, andgives referenceswhich permit theseto be assessed.It willbe obviousfrom Section7 that the predictionof deformationis not an exactscience,andengineeringjudgementis requiredin identifyingandquantifying thosefactorswhichareof importance,in individual cases.

Evenmorejudgementis requiredin deciding how to allow for thesedeformationsindesign — eachwith its associatedvariability. In broad terms,sourcesof movementcanbe consideredin one of threeclasses:

(a) Intrinsic i.e. thosedue to change in the inherentpropertiesof the materialsandcomponents.For concrete,this categorywould includeearlyagethermalmovement,plasticshrinkageandsettlement.and,to someextent,dryingshrinkageandcreep.

(b) External ie. thosedue to deadandimposedloading, to temperatureandhumiditychange.etc.

(c) Time-ucp~ndenti.e. seasonal.diurnal.

This classificationis significant in design.sincethe solution to eachcanbe different;for example.someintrinsicsourcesof movementcanbe dealtwith by reducingthem toacceptablelevels via concretetechnology,whereastime-dependentsourcescannotbeavoidedin this way andrequire accommodationas part of the design.

In any particularcase,having identified,classifiedandquantifiedall relevantsourcesof potential movementand deformation, the designeris facedwith making a choicebetweenalternativestrategies.Again, he has threebasicapproachesto choosefrom:

(a) Reductionof the deformation.This might be appropriatefor many of ihe intrinsicsourcesof movement.By the useof protectiveor insulation systems,it might alsobe relevantfor seasonalvariations.

(b) Suppressionof the deformation.In effect, this implies acceptingbuilt-in restraints,andcopingwith theresultingstressesby appropriatedesignof the individualelementsand the structureas a whole.

(c) Accommodationof the deformation.This meansallowing movementto physicallytake place.

In practice.somecombinationof theseapproacheswill generallybe mostappropriate;however,if this is done,therequireddetailingassociatedwith eachshouldbecompatible;the analysesof feedbackon in-serviceperformanceindicatesthat this has not alwaysbeenso, andthat the generalapproachin designingfor movementis often confused.

This Section of the Code effectivelydealsonly with approach(c) asoutlinedabove.The treatmentcan be no more thangeneralin nature, andreferenceshould be madeto the literature for moredetailedinformation18-’- ~

8.2 Need for movement joints

8.3 Types of movement joint

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8.4 Provision of jointsC

8.5 Design of jointsC

REFERENCES

8.1 ALEXANDER. 5,5. andLAWSON. RM. Design for movementin buildings. Technical Note 107.ConstructionIndustry ResearchandInformation Association.London. 1981. S4pp.

8.2 RAINGER. p, Movementcontrol in the fabricof buildings.BatsfordAcademicandEducationalLtd. 1983. 216pp.

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PART 2SECTION NINE. APPRAISAL ANDTESTING OFSTRUCTURESANDCOMPONENTSDURINGCONSTRUCTION

9.1 General

This Sectionis intendedto covercaseswheretestingmay be deemednecessaryduringconstruction. That point is stressedin 9.1; model or prototype testing is specificallyexcluded,nordo theclausesrelateto theappraisalof structuresthathavebeenin servicefor sometime (wherereferenceshouldbemadeelsewhereto the literaturee.g. reference9.1).

Testing,particularlyloadtesting, is expensive,and the implication behindthe wholeof Section 9 is that it shouldonly be used as alast resort: andshould not be regardedas an easyoption in a disputesituation~ The approachis essentiallya structuredone.Firstly, thereis the need to clearly establishthat testing is necessary(9.2). 9.3 thendefinesthe basicobjective, namely to assessthe structureas built and to determineifit meetsthe requirementsof the original design.There then follows —in 9.4 and9.5 —

aprogressiveseriesof stepsto befollowed.Aboveall,anytestingmustbemeaningful.In general,the designand constructionof structureswhich satisfactorilyfulfil their

intendedfunction is madeup of an overall ‘package which can be brokendown into aseriesof discreteelementsas follows:

(a) a properassessmentof loadsandload effects(b) the choiceof performancecriteria (e.g. deflectionor crackwidth values)(c) a choiceof appropriatefactorsof safety,or designmargins(d) the useof representativemodels’ for structuralbehaviour(e) complying with materialspecifications(f) achievingrelevantstandardsof workmanship.

Doubtsgenerallyarisebecauseof deficienciesin (e) and(f). Here, the importanceofinspectionandsupervisioncannotbe over-emphasisedin gettingthe constructionrightin the first place, andparticularattention is drawn to reference9.2. However,wheresomethinghasgonewrong (9.2), thenthe implication of any test resultsobtainedon thewhole package’(a—f above)must be considered.

9.2 Purpose of testing

9.3 Basis of approach

9.4 Check tests on structural concrete

9.4.1 GeneralThis clauseemphasisesthat testing neednot relate solely to strength. In general, ameasureof in situ strengthis a good guide to concretequality, but B58110 as a whole.and Sections3 and 6 of Part 1 in particular,placesgreatstresson the provision ofadequatedurability. Considerationmay then be given to the useof covermeters.NDTtechniques.gammaradiography,chemicalanalysis.surfaceabsorptionmeasurements(or othertechniquesto quantify permeability),etc.

9.4.2 Concretestrengthin structures

9.5 Load tests on structures or parts of structures

9.5.1 GeneralDetailedrecommendationson test proceduresaregiven in reference9.1.

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IHandbook o BS8IIO:1985

9.5.2 Test loadsSincethe basicobjectiveis to ~calibrate’the structureas built againstthe original design.the magnitudeof the testloadmustbesufficientto givereliablemeasurementsof strains,deflections.etc.Variouscaveatsaregiven in 9.5.2. which are importantin interpreting Fthe resultsobtainedand in matchingthe assessedperformanceagainstthat expectedinthe original design. The levels of loading specifiedshould be regardedas minimumvalues,sinceit is alsoimportantto rememberthatmostdesignsarebasedon an‘envelope Fapproach using patterned loading. If the primary concern is about strength or stabilityof the structure. then there is a good case for increasing the test load up to 1.5 timesthe design live load, provided that this doesnot causepermanentdamage.should thetestprovesatisfactory.Engineeringjudgementisabsolutelyessentialin individualcases. [9.5.3 Assessmentof resultsComparisonsbetweenmeasuredand predictedresultsare essential.Where thereare rsignificant differences(say greaterthan 15—20%), then the first step shouldbe to check 5-that the structureis not carrying the load in a way different from that assumedin thedesign (due. say, to archingaction. or the influenceof ‘non-load-bearing’elements).Material propertiesshouldthenbe re-checked.If therearestill seriousdiscrepanciesinthe results— usingthe criteria in 9.5.4as guidelines— thenadditional specialtestsmayhaveto bedevised(9.5.5) to eliminateall extraneousfactorsor. alternatsvetv.remedialaction taken.

9.5.4 Test criteriaThe valuesgiven are for generalguidanceonly. The most Important factor is that thetestloadsshouldbe appliedat leasttwice; repeatability,andthe recoveryof the structureafterthe load is removed.are perhapsthe most importantissues.

9.5.5 Special testsThis clauseis intendedprimarily for precastunits which, in the final structure.will actcompositelywith in situ concrete.However,the approachshouldalso be consideredforload testing of the type describedin 9.5.2, if discrepanciesappearin the results (seecommentaryon 9.5.3 above).

9.6 Load tests on individual precast units CThe two paragraphsin this clausecover quite differentsituations.The first simply says [7

that if thereare doubts about an element—forthe reasonsgiven in 9.2—then that &elementshouldbe treatedlike any otherconcreteelementi.e. clauses9.3 — 9.5 obtain.Thesecondparagraphrelatesto Quality Assurance.A numberof accreditedQA schemes

Lnow exist for variousprecastcomponents;hereit is important to rememberthat QAgenerally relatesto the entire production process.and gives a measureof assessedcapability — testing is only one part of that assessment,and samplingwould not beexpectedto exceedthat laid down in the relevanttechnical schedule. LREFERENCES

9.1 N5T1Tt.TtO’~ OF STRUCTURAL ENGINEERS. Appraisal of existingstructures.July 1980, 60p.9.2 iNs-n-I-u-rto~OFSTRUC~TURAL ENGINEERS. Inspectionof building structuresduringconstruction-Aprtl

1983. 20p.

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Handbookto 8S8110:198S

LIST OF TABLES

PART 1

Section Table Page

3.2.1.24 H3.1 Momentsin columns 2733.4 H3.2 Exposureconditions 323,4.4.4 H3.3 Valuesof K’ correspondingtovariousamountsof 38

redistribution3,4,4.4 H3.4 Designparametersfor rectangularsections 383.8.1.6.2 H3.5 Assumedbeam/columnstiffnesses 603.12.8.13 H3.6 Multiplying factorsfor lap lengths 77

6.1.2 H6. 1 Concretecharacteristicsrequiringtheuseof special 129cementsorggbfs,orpfa

6.1.3 H6.2 Choiceor limitationof aggregatecharacteristics 1306.3.1 H6.3 Typesof concretemixesinBritishStandards 1366.3.1 H6.4 Characteristicsof differenttypesof mix 1366.3.2 H6.5 Compressivestrength(from BS 5328) 1366.3.3 H6. 6 Equivalentgradesfor cementcontent 1396.3.3 H6.7 Equivalentgradesforwater/cementratio 1406.3.3 1-16.8 ModificationstoTablesH6.6andH6.7toallowforother 140

specifiedrequirements6.3.4 H6.9 Clausesrelating to the effectsof materialson the 141

K characteristicsofconcrete6.4.2 H6. 10 Compressivestrengthcompliancerequirements 1426.4.2 H6.11 Compliancewith specifiedmix proportions 143

7.1 H7.1 BritishStandardrequirementsfor reinforcingbarsin concrete 1557.2 H7.2 Cuttingandbendingtolerances 1567.2 H7.3 Minimum diameterofformer 156

8.1 H8.1 BritishStandardrequirementsfor prestressingtendonsfor 158concrete

PART 2

(2)4 H(2)4.1 Variationofminimumwidth of memberandcoverto 181reinforcement

(2)5.1~1 H(2)5.1 Somedesignpropertiesof lightweightaggregateconcrete, 191comparedwith thesamegradeof normal-weightconcrete

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LIST OF FIGURES

PART 1

FigureH31:FigureH3.2:

Figure1-13.3:FigureH3,4:FigureH3.5:FigureH3.6:FigureH3.7:FigureH3,8:FigureH3.9:FigureH3. 10:FigureH3A1:FigureH3. 12:FigureH3.13:FigureH3. 14:FigureH3.15:FigureH3.16:

FigureH3. 17:FigureH3.18:FigureH3. 19:FigureH3.20:FigureH3.21(a):figureH3.21(b):FigureH3.22:FigureH3.23:

FigureH3.24:

FigureH3.25:FigureH3.26:FigureH3.27:FigureH3.28(a):FigureH3.28(b):FigureH3.29:FigureH3.30:FigureH3.31:

FigureH3.32:

FigureH3,33:FigureH3,34:FigureH3.35:FigureH3.36:FigureH3,37:FigureH3.38:FigureH3,39:FigureH3.40:FigureH3.41:FigureH3.42:FigureH4, 1:FigureH4,2:FigureH4,3:

FigureH4.4:

Schematicillustrationof tyingsystemPoorstructurallayouts(a) lack of torsionalstiffness(b) linesof actionofloadandresistancenotcoincidentPermissiblesimplificationof aframeforanalysisComparisonof analysesincludingandignoringthecolumnsAlternativetreatmentof laterally-loadedunbracedframeDevelopmentof bendingmomentsinanencastrdbeamComparisonof CP 110andBS 8110for twohoursfire resistanceComparisonof CP 110andBS81lOfire provisionsIsit awall,beam,columnorslab?EffectiveflangewidthconceptsflangedbeamShearstrengthof beamswithoutshearreinforcementProvisionofshearreinforcementin beamsNormal modeof shearfailureTrusssystemsforshearUltimateshearstressesfor beamsloadedclosetosupports.:u~takenfromCodeLoadson thebottomof beamsShearandaxialcompressionExamplesof torsionduetoimposedrotationLogicbehindtensionsteelmultipliersModification factorsasafunctionof steelpercentageModification factorsfromTable3.11Developmentof bendingmomentenvelopeforslabAreasto beconsideredforsectionpropertiesin equivalentframeanalysisDistributionoflong-spannegativemomentin internalpanelofflatslabReductionofshearperimeternearholesAssumeddeformedshapeof bracedcolumnAssumeddeformedshapeof unbracedcolumnEffectivelengthconceptforabracedcolumnEffectivelengthconceptforanunbracedcolumnProblemsituationfor treatmentof minimummomentsVariationof ultimatecurvatureatdifferentaxial loadsInterpretationofClauses3,8.1.7,3.8.1.3,3.8.2.2,3.8,3.4and3.8,3.9Validity of designchartsfor columnswith reinforcementnotconcentratedin cornersEffective depth of a column sectionEccentricityin bracedwallsEccentricityin asingle-storeyunbracedwallLoaddistributionalongawallStressblock underultimateconditionsinplain wallShearin pile capsModesof lap failureEffectofjoggledlapMinimum barspacings(hagisthe maximumsizeof aggregate)Maximumbarspacingsin shallowmembersNotationusedincalculationsfor slenderbeamsConditionsatendofaClass3 beamSignificanceoftypeof loadingon therelationbetweendeflectionandtimeDesignof prestressedconcretesectionsin flexure

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FigureH4.5:

FigureH4.6:FigureH4,7:FigureH4.8:FigureH4.9:FigureH4.10:FigureH4.11:

FigureH4.12:

FigureH4.13:FigureH4.14:

FigureH4.15:

FigureH4.16:FigureH4.17:FigureH4.18:FigureH5A:FigureH5,2:

FigureH5.3:

FigureH5.4:

FigureH5,5:

FigureH5.6:

FigureH5.7:

FigureH5.8:FigureH5.9:FigureH5.10:FigureH5.11:Figure1-15.12:

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FigureH6, 1:

FigureH6.2:

Designchartforprestressedrectangularbeams(bondedtendon)Designof Tbeamsfor flexureTreatmentof flangedsectionwhereHf=0.9xPrestressedbeamshowingzonesfor shearFlowchartfor shearin prestressedconcretePrestressedbeamwith pdssibleshearfailureEffectsof(a)deflectedand(b)straighttendonsin sectionsuncrackedin flexureTheoreticaleffectof inclinedtendonsinsectionsuncrackedin flexureCritical sectionforshearatendof apre-tensionedbeamTheoreticaleffectof deflectedtendo-nsin sectionscracked in flexureEffectof (a) timeand(b) temperatureon therelaxationof 5mmdiameterwireatvariouslevelsof stt’essInfluenceof tpeofaggregateon creepSplittingatendsof pre-tensionedbeamsBurstingstressesfrom tendonswithhighcurvatureDesignbasisforcorbels(5.2.7.2)Possiblemethodsof anchoringmain tensionreinforcementin corbelsMethodsofprovidingcontinuityof reinforcementforprecastfloorsandbeamsEmpiricaldetailingrulesforachievingcontinuityofreinforcementwithverticalloopbarsExampleofwhereagroutrecesswouldneedits sidesroughened(ajoint betweentwolargecolumnssubjecttomainlyaxialload)Problemsatendsof compressionbars(a) stressdistributioninconcrete,(b)effectof bendingorhookingcompressionbarsTypesofconnectionreferredtoin5.3.4.5— (a) compressionsleeve,(b)compressionandtensionsleeveExamplesof typesof threadingreferredtoin 5.3.4.6Basictypesofconnectionusingstructural steelinsertsForcesystemfordesignof singlesteelinsertsfor columnsEffectivejoint areafor compressionjoints(5.3.6)Suggestedenhancedcompressivestressvaluesin compositebeamsfordifferentformsof constructionContinuity in compositeconstructionCompositesectionsconsideredin designingfor shear(5.4.7)—

(a) original member,(b)with compositeinfill, (c) withcompositetoppingRelationshipbetweenstrengthandcementcontentofconcretemadewith OPC.75mmslumpandcoarseaggregatewithmaximumsize20mmRelationshipbetweenfreewater/cementratioandcementcontentof concrete(OPC,75mmslump,20mmcoarseaggregate)

PART 2

FigureH(2)2.1:FigureH(2)2.2:FigureH(2)2.3:FigureH(2)3.1:

FigureH(2)3.2:

BucklingmodesforrectangularframesFormationof yield linesin awallsubjectedtolateralloadingConfigurationof wall underultimateconditionsInfluenceof uncertaibtyabouttensilestrengthof concreteondeformationPartitionwall damage—cracksbetweenwall andfloor dueto aself-supportingwall

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FigureH(2)3.3:

FigureH(2)3.4:FigureH(2)3.5:FigureH(2)3.6:

FigureH(2)3.7:

FigureH(2)38:FigureH(2)39:

FigureH(2)3.10:FigureH(2)3.11.FigureH(2)4.1:FigureH(2)4,2:FigureH(2)4.3:

Partitionwall damage—cracksatjointsbetweenwall andceiling and towards exterior wall due to rotation or movementof individualwallpanelsPartitionwalldamage— inclinedcracksduetoshearPartitionwall damage— verticalcrackingdueto flexurePartitionwall damage— typesof damagerelatedtodifferentstructural configurationsCrackingof modelwallsduetosaggingorhogging(fromRef. 2.3.4)Skempton’sdefinitionof angulardistortionDamagetopartitionsasafunctionof calculateddeflectionofsupportingstructure(Ref.3.2)NeutralaxisdepthsforrectangularsectionSecondmomentsof areaof rectangularsectionsNominalcoverandfire resistancecoverin beamsorcolumnsNominalcoverandfire resistancecoverin wallsorslabsStructuraleffectsof temperature

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