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AJ Handbook of

Building Structure

EDITED BYAllan Hodgkinson

The Architectural Press, London



AJ Handbook of Building Structure Introduction

Allan Hodgkinson

Consultant editor and authors

The consultant editor for the Handbook is Allan Hodgkinson

MEng, FICE, FIStructE, MConSE, Principal of Allan Hodgkin-

son & Associates, consulting civil and structural engineers.

Allan Hodgkinson has been the AJ consultant for structural

design since 1951; he is a frequent AJ contributor and is the

author of various sections of this handbook.

The authors of each section will be credited at the start of

the section of the Handbook in which their material appears.

The original Architects' Journal articles were edited by

Esmond Reid, BArch, and John McKean, BArch, MA, ARIBA,

ACIA, ARIAS.

The frontispiece illustration shows one, of the most

magnificent building structures from the era of the Eiffel

Tower, the Forth Bridge and the great railway stations.

The Palais des Machines for the Paris Exhibition of 1889

(Contamin, Pierron & Charton, engineers) was a pioneer

example of three hinged arches.

Preface to the second editionThere have been considerable changes in some British

Standards, Codes of Practice, and Building Regulations

since 1974; and unlike the reprints of 1976 and 1977, this is

a substantially revised and updated re-issue of the now

well-established AJ Handbook of Building Structure.

The principal changes are in the sections on Masonry (re-

written to take account of the 1976 Building Regulations,

and the new BS 5628 'limit state' code of practice); and on

Timber (substantially revised to take account of the new

timber gradings).

Steel handbooks have been replaced for all types of struc-

tural sections; and technical study Steel 3 has therefore

been revised accordingly.

In general, the new 'limit state' approach to design is dis-

cussed (eg in the section on Masonry); but in view of the

rejection of the limit state Codes and draft Codes in their

present form, by the majority of practical designers, it has

been thought prudent to retain the allowable stress methods

of design as the basis of the handbook.

Finally, it should be mentioned that the opportunity has

been taken to bring all references in this Handbook up to

date; and to correct a number of misprints of the first edition.

ISBN 0 85139 273 3 (paperbound)

First published in book form in 1974 by

The Architectural Press Limited: London

Reprinted 1976, 1977

Second edition 1980, 1982, 1983

Printed in Great Britain by

Mackays of Chatham Ltd

This handbook

Scope

There are two underlying themes in this new handbook on

building structure. First, the architect and engineer have

complementary roles which cannot bo separated. A main

object of this handbook is to allow the architect to talk

intelligently to his engineer, to appreciate his skills and to

understand the reasons for his decisions. Second, thebuilding must always be seen as a whole, where the success-

ful conclusion is the result of optimised decisions. A balance

of planning, s tructure or services, decisions may not neces-

sarily provide the cheapest or best solution from any of

these separate standpoints, but the whole building should

provide the right solution within both the client's brief and

his budget.

The handbook provides a review of the whole structural

field. It includes sections on movement in buildings, fire

protection, and structural legislation, where philosophy of

design is discusssed from the firm base of practical experi-

ence. Foundations and specific structural materials are also

covered, while sufficient guidance on analysis and design

is given for the architect to deal with simple structures

himself.

Arrangement

The handbook deals with its subject in two broad parts.

The first deals with building structure generally, the second

with the main structural materials individually.

The history of the structural designer and a general

survey of his field today is followed by a section on basic

structural analysis. The general part of the handbook

concludes with sections on structural safety—including

deformation, fire and legislation—and on the sub-structure:

foundations and retaining structures.

Having discussed the overall structure, the sections in thesecond part of the handbook discuss concrete, steelwork,

timber and masonry in much greater detail. Finally there

are sections on composite structures and on new and

innovatory forms of structure.

Presentation

Information is presented in three kinds of format: technical

studies, information sheets and a design guide. The technical

studies are intended to give background understanding.

They summarise general principles and include information

that is too general for direct application. Information sheets

are intended to give specific data that can be applied

directly by the designer.

Keywords are used for identifying and numbering technicalstudies and information sheets: thus, technical study

STRUCTURE 1, information sheet FOUNDATIONS 3, and so on.

The design guide is intended to remind designers of the

proper sequence in which decisions required in the design

process should be taken. It contains concise advice and

references to detailed information at each stage. This might

seem the normal starting point, but the guide is published at

the end of the handbook as it can be employed only when

the designer fully understands what has been discussed

earlier.

The general pattern of use, then, is first to read the relevant

technical studies, to understand the design aims, the

problems involved and the range of available solutions.The information sheets then may be used as a design aid, a

source of data and design information. The design guide,

acting also as a check list, ensures that decisions are taken

in the Tight sequence and that nothing is loft out.





ScopeThree aspects of structural safety are brought together inthis section of the handbook. Building legislation andattitudes towards the analysis of building safety arediscussed in the first technical study. The second studyconsiders the movement of a structure either under changingenvironment or loading. Without discussing details of jointing techniques and sealants, it examines possible

sources of movement which must be considered in anystructural design. The third topic is fire protection of struc-tures. This problem is discussed generally and by looking ateach structural material separately. The study is supple-mented by four information sheets which provide tables of ‘deemed to satisfy’ situations for structural members inconcrete, steal, timber and masonry. Much of this informa-tion has never been published before and, as far as the AJ cantell, gives the most up to date information available.The tables on concrete protection have been provided bythe Code Servicing Panel of the Institution of StructuralEngineers. These tables have since been published in CP 110(the Unified Code of Practice for concrete). The AJ is gratefulto officers of TRADA for general advice and the table on stud

partitions, and to H. L. Malhotra of the Fire ResearchStation (BRE) for general advice and the tables on masonry.It is understood that these masonry tables will also becomethe basis of a Code of Practice.

Allan Hodgkinson

Author

Allan Hodgkinson, consultant editor to the handbook, haswritten the studies in this section. The help of the Instituteof Structural Engineers, TRADA and the Fire ResearchStation in the preparation of the fire protection tables isacknowledged above.

The illustration on page 71 is can engraving of the 1666 fire of London from Thornton’s History of London and Westminster’ (1743)



1 Legislation

1.01 Legislation with regard to structure resides directly inBuilding Regulations and also indirectly in the Codes of Practice in that the latter are named as a ‘deemed to

satisfy’ condition of the regulations’. Outside London,regulations are administered by local authorities of the UK,and evidence of compliance must be submitted to the localauthority’s engineer in the form of calculations and workingdrawings. The engineer may check these in his own office oremploy the services of a consulting engineer. Calculationsmust be clear, concise, indexed, related to drawings andreferenced to bibliography where special formulae havebeen taken from textbooks; otherwise considerable wasteof time and manpower will occur all round. The localauthority building inspector examines the quality of work during construction.1.02 The London Building Act is administered by thedistrict surveyors, who are each responsible for a particular

area of inner London (old LCC area). They check the calcu-

lations and drawings and inspect the work on site. TheConstructional Bylaws (1972) were published in a newformat early 1973 and rely largely on the Codes of Practice,as do the Building Regulations. While the comments belowmay refer particularly to the Building Regulations, theyare also a reasonable statement, of the present state of legislation for London and Scotland.

2 Structural regulations

2.01 The Building Regulations up to April 1970 werecomparatively simple, as the fundamental requirements

were contained in two clauses D3 and D8.

D3 The foundations of a building shalla safely sustain and transmit to the ground the combined dead load and imposedload in such a manner as not to cause any settlement or other movement whichwould impair the stability of, or cause damage to, the whole or any part of thebuilding or of any adjoining building or works; andb be taken down to such a depth, or to be so constructed, as to safeguard thebuilding against damage by swelling. shrinking orfreezing of the subsoil; andc be capable of adequately resisting any attack by sulphates or any otherdeleterious matter present in the subsoil.D8 The structure of a building above the foundations shall safely sustain andtransmit to the foundations the combined dead load and imposed load withoutsuch deflection or deformation as will impair the stability of, or cause damageto, the whole or any part of the building.

2.02 In April 1970 an amendment (5th Amendment) wasissued as a precaution against the collapse of buildings dueto accidental damage. This arose directly from the collapseof part of an industrialised building and loss of life as theresult of a town gas explosion. The incident received suchadverse national publicity that logic was replaced byemotion in the ensuing considerations. The action taken was

2.03 This Regulation requires that in a building of five or

more storeys, it should be possible to remove any oneparticular member from the structure with only restrictedconsequent collapse. Alternatively the member must becapable of supporting a load of 5 psi (34.5 kN/m2) from anydirection (including load transmitted to that member fromadjacent elements of structure similarly loaded) in additionto its normal loading, but at enhanced allowable stresses.2.04 The intention is to avoid progressive collapse of thebuilding, although the damage so sustained may meanrebuilding at a later date. Although the new Regulationarises from an explosion in a special use of structure inindustrialised building with large concrete panels, theRegulation must be applied to all forms of structureindiscriminately. Continued efforts by the engineering

institutions have secured some relaxation of the require-ments in the case of structural steelwork and reinforcedconcrete frames, and testing work by the British CeramicResearch Association has shown that 178 mm and 229 mmbrick internal walls can satisfy the requirements provided aspecific vertical load is applied to the walls.2.05 However, it should be appreciated that there is not theslightest guarantee that the added requirement of D19produces a structure which would contain a gas explosion,only a structure with a variety of alternative paths of support could hope to have such resistance. It should alsobe appreciated that a block of dwellings of four storeyscould collapse progressively along its length, causing equiva-

lent damage to that of a five-storey structure collapsing

progressively vertically.

3 Codes of practice

3.01 The Regulations avoid any direct rules for applicationto structures of aluminium, steel, reinforced concrete,prestressed concrete, timber and masonry but state thatD8 shall be deemed to be satisfied if there is compliance withthe Code of Practice or British Standard appropriate to thematerial. At first sight this appears to be a logical approach.The codes are technical documents containing guidelines

and parameters for design. They provide norms on whichthe structural design can be assessed, and without whichthe agreement to calculations by an appraising authority

might prove impossible.3.02 However, the procedure does have two unfortunateaspects. First, Codes of Practice are intended to be a generalrecommendation of good practice and to be implementedby a qualified engineer. The slavish following of rules is not

expounded in a new Regulation, D19 (incorporated withinthe Building Regulations 1972) which has been bitterly

attacked by both individuals and representative engineeringopinion.

The aim of ALLAN HODGKINSON'S technical study is to provoke thought on the approach to design safety. He alsoexamines both the traditional and the recent CIRIA 'limit state' approaches, comparing them in a worked example

Building legislation and

structural safety



Technical study Safety 1 para 3.02 to 5.06

in itself an assurance of a satisfactory whole structure.Secondly, in competitive design there is a tendency just tosatisfy the code by using its letter rather than its intent,as there is then the opportunity of sheltering behind itsapparent statutory respectability when liability is demanded

for failure.

4 Safety

4.01 Generally, safety has been referred to already under the

heading of legislation, but how safe is ‘safe’? How is safetymeasured, and is the engineers’ definition of safety the sameas that of the public who use their structures? The RonanPoint aftermath in 1968 gave the impression that thepublic considers ‘safe’ to mean ‘impregnable’ and it wasclearly not understood that total safety from all damageincidence cannot be achieved whatever the cost, thoughwith increasing cost an increasing measure of safety can beprovided.

4.02 Until recently, the method of ensuring a measure of safe construction in a particular material was to define safeworking stresses in the appropriate Code of Practice or

British Standard and then proportion each element of thestructure so that the safe working stresses were not exceeded.The relationship of Yield Stress or its equivalent to thesafe working stress was the- ‘factor of safety’.4.03 In terms of the few failures which occurred in

siderable record of success, despite constant narrowingof the safety factor as detailed knowledge and testing andcontrol of the materials improved. Nevertheless, the methodwas not logical, for it treated all structure types as equal, allconditions of loading and quality of construction as equal,and all consequential failure effects as equal.4.04 A committee of the Institution of Structural En-gineers was set up in the mid-1950s to consider safety instructures. Out of the work of this committee has grown themodern statistical approach to all aspects of structuraldesign introduced into a code of practice for the firsttime in November 1972.

4.05 When considering safety the following aspects requireexamination:1 Loading-how accurate is the determination of load-ing used in analysis, what is risk of overload?2 Materials-what is the risk that materials of constructiondo not comply with specification?3 Design skill-what is the risk of a failure of the design inits basic conception or mistakes in calculation? (This ispartly covered by the checking procedure referred to

earlier)4 Inadequate fabrication or construction-what is the risk of a weld in a steel structure being understrength, or therisk of concrete as mixed being understrength and inade-quately compacted or reinforcing steel being fixed wrongly?(this is partly covered by the inspection process)5 Seriousness of failure-what is the risk to life, to con-tinued use of an industrial process, and further consequencesof a failure?6 What is failure? Failure can be collapse, excessive deflec-tion or excessive cracking, all of which in appropriatecircumstances can mean extensive repair or rebuilding of the structure.

practice, it may be said that this method achieved a con-

5 The CIRIA approach

5.01 Obviously the old concept of the safety factor of working stress related to yield stress cannot give eitherlogical or factual discrimination between various combina-

74

tions of the above-mentioned items. Thus, over the last 20years there has evolved first the concept of load factors, andmore recently the statistical assessment of ‘characteristic

stresses’ and ‘characteristic loads’.5.02 A CIRIA report2 represents seven years of thinkingabout the problem, with the object of suggesting an opti-mum balance between level of safety and overall economyof structural design. Its authors consider that their recom-

mendations can be incorporated in the various Codes of Practice in the next five to 10 years. The new Unified Codeof Practice for Concrete, BS CP 110, is the first to appear

of the new series.5.03 The attitude and method recommended by the CIRIAreport is nowhere better summarised than in the report’s

appendix quoted below:

Limit state of collapseThe limit load factor to be adopted in design is to be evaluated in terms of threepartial factors as follows:y1 = to cover unusual and unforeseen deviations of loading from the character-

istic loads, and unusual combinations of such loadsy2 = to cover unusual and unforeseen deviations of strength from thecharacter-

istic strengths of the structure as builty3 = to cover the seriousness of the effects of collapse whether general or

partial, sudden or gradual, including danger to personnel and associatedeconomiclosses.

Of these partial factors, yl. the load variability factor, should for establishedforms of construction be chosen in the range from 1.2 to 1.8; y2, the strengthvariability factor, in the range 1.1 to 1.6; and y3, the economic factor, in therange 0.9 to 1.4.Having chosen values for these partial factors, the load factor for collapse to beused in design is given by the product Y1 x Y2 x Y 3.

Limit state for local damageThe limit load factor to be adopted in design is to be evaluated in terms of two

y4 = to cover the nature of the loads in service, whether static or dynamic and

y2 = to cover the nature and extent of the damage likely to arise in service.Of these partial factors, which are to be applied to characteristic loads, y4should,for established forms of construction, be chosen in the range 1.0 to 1.2, andy5 in the range of 1.0 to 1.4.Having chosen values for these partial factors. the load factor for local damage

to be used in design is given by the product y4 x y5.

Limit state of excessive deflectionThe limit load factor to be adopted in design is to be evaluated in terms of twopartial factors, as follows:y6 = to cover the nature of the loads in service, their duration and fluctuation

y7 = to cover the nature and extent of the deflections likely to arise in service.Of these partialfactors, which are to be applied to characteristic loads, y4should,for established forms of construction, be chosen in the range 0.2 to 1.0. andy7 from 1.0 to 1.2.Having chosen values for these partial factors, the load factor for excessivedeflection to be used in design is given by the product y6 x y 7.

5.04 In limit states, the actual loads in service are to berepresented by systems of ‘characteristic loads' relating tothe particular limit states. The loads will eventually be

specified in the codes with the aim of approximating to themost severe conditions to be expected in service during thelife of the structure concerned. Eventually they will bedetermined statistically from experience, but initially loadswill be based on those currently quoted in BS CP3 chapter v3

or on greater loads known to the designer.5.06 Strengths of materials used in the structure are to berepresented by ‘characteristic strengths’ specified in thecodes. These will be based on tests and specified with theaim of approximating to the smallest strength* that is likelyto be incorporated in the structure.5.06 The proposed CIRIA partial factors for steelwork andreinforced concrete building structures are shown in table I.

of rare or frequent occurrence

during the life of the structure

*strength with not more than 5 per cent probability, ie the strength below

partialfactors, asfollows:

which not more than 5 per cent of the test results would fail.



75 Technical study Safety 1 para 6.01 to 6.06

Table I Proposed CIRIA partial factors for reinforced concrete and steelwork compared

Limit state Collapse Local damage Excessive deflection

Reinforced concrete δ 1 1.25 permanent

1.5 imposed

1.25 wind

δ2 1.4 to 1.7 concrete

1.15 steel

δ3 0.9 -1-1*

δ4 1.0 all loads

δ5 1.3 concrete1.0 steel

δ6 1.0 all permanent loads

0.8 short-term imposed and wind together

1.0 short-term imposed and wind separately0.25 to1.0 long-term loads†

δ7 1.0 concrete1.0 steel

Steelwork δ1- 1.2 permanent1.5 imposed

1.25 wind

d 2 1.1 continuous frames1.2 structural elements and non-continuous frames

δ3 0.9* collapse by plastic bending

1.1 to 1.2* collapse by buckling or fracture

δ4 1.0 all loads

δ5 1.0

δ6 1.0 all permanent loads

0.8 short-term imposed and wind together

1.0 short-term imposed and wind separately0.25 to 1.0 long-term loads†

δ7 1. 0

*Factor 8, depends on probable nature of failure, and frequency or duration of occupation by people and valuable commodities

†Depending on the use of the structure and the likelihood of the characterist ic loads acting for long periods

6 The two methods

6.01 Summarising the two approaches to design safety, in

the traditional method the unfactored loads are applied to

the chosen structure and an analysis determines certain

stresses which can be compared with the 'allowable stresses'

as set out in the Codes of Practice. The measure of safety is

the relationship of the 'allowable stress' to the 'yield stress'

or equivalent yield stress of the material in question.

6.02 In the new method, factored characteristic loads are

applied in an analysis (which may use either plastic or

elastic theories) and the strengths of the chosen structural

members are compared with the 'characteristic strengths'

attributed to the material in question. The applied factors

are then the measure of safety.

6.03 Plastic analysis is perhaps best known to architects at

present for calculation of steel frames, and the following

examples will illustrate the way in which a simple portal

frame can be designed by both methods for safety against

collapse.

6.04 A 15 m span warehouse portal is to be designed with a

spread load 8 • 8 kN/m run of portal beam of which 4 • 4 kN/m

is live load 1. Ignore wind and deflection for this example and

assume that a similar steel section will be employed for

both beam and stanchion. The calculation of this portal by

elastic analysis (BS 449) and plastic analysis (CIRIA re-

commendation) are compared on the next page.

1 Portal frame example

6.05 The difference between elastic design (with the

allowable working stress approach) and plastic design (in

the limit state of collapse) will be carried a stage further in

section 6 of the handbook: ST E E L . In theproblem described,

it is shown that the latter method results in a reduced

section member representing an economy but giving a

greater deflection. Had the portal beam been of reinforced

concrete, it would also have been necessary to check for thesize of cracks in tension areas if there was the possibility of

deterioration of the reinforcing steel. Hence the three

checks for collapse, deflection and local damage.

6.06 This leads to a critical assessment of the CIRIA recom-

mendations in practice. Assuming that sufficient evidence is

eventually obtained to make the 'characteristic' values

meaningful, can the whole process be handled in the design

office? The Code CP 110 has certainly brought considerable

doubt on this point from practising engineers and time

alone will tell whether it will be workable. In any case,

it is presumed that Codes CP 111, 112, 114 and 115 will

be preserved in their existing form for several years.



Technical study Safety 1 para 7.01

Portal frame calculation: comparison of traditional and CIRIA recommended methods

7 Design responsibility

7.01 In the event of failure attributable to design error, a

client may sue the designer responsible to him for negligence.If the architect is tho only person under contract to theclient, clearly he will be sued; this liability can only betransferred to the engineer when he too is appointed directlyby the client. The architect can, of course, in turn sue for

76

negligence an engineer working to his direction who is notemployed by the client. The surest way of minimising

failure is for one person to be in charge of the structural

design and at all times to appraise the superstructure andfoundation as a whole. The practice of having elements of the building designed by various people (who may well bespecialists in their own field) leaves the process open bothto failures in communication between the parties and failure

Elastic analysis: BS 449Dead load 4. 4 kN/mLive load 4.4 kN/m

Total 8.8 kN/mPortal span 1b = 15 m, moment of inertia = Ib

Stanchion height 1s = 6 m, moment of inertia = I s

It can be proved that the bending moment at the knee

Bending moment diagram is therefore:

The allowable working stress = 165 N/mm2

from M = fz (see technical study ANALYSIS 1, AJ 25.4.71 forexplanation of symbols)

Elastic z =

Section from Handbook is 406 x 178 by 54 kg/m (z=922.8)

130000 x 1000

165= 789 000 mm3 or 789 cm3

The stanchion would have to be checked for the combina-tion of the direct load and bending moment

Notional safety factor =yield stress

working stress250

165-- = 1.52

Plastic analysis: CIRIA recommendationActual dead load 4.4 kN/m

Factored dead load = 1.2 x 1.1 x 0.9 x 4.4= 5.21 kN/m

Factored live load = 1.5 x 1.1 x 0.9 x 4.4= 6.51 kN/m*Frame braced against buckling

= 328.0 kNmHinges will form at collapse thus:

Therefore the bending moment at centre and knee will be328- = 164 kN/m

2Bending moment diagram is therefore :

Characteristic stress (yield stress) = 250 N/mm2. FromMp = fzp

164 000 x 1000

250Plastic Zp = = 656 000 mm3 or 656 cm3

Section from Handbook is 356 x 171 by 45 kg/m (Zp=771.7)

Reduced plastic modulus for stanchion section (direct loadwith bending moment) = 7 13.0Both OK

NB the reduced section of the steel members will lead in thiscase to a larger deflection than the 'elastic design' portalNotional safety factor (from load factors)on dead load 1.2 x 1.1 x 0.9 = 1.19on live load 1.5 x 1.1 x 0.9 = 1.48



77

in the overall achievement of the project conception.7.02 If an architect considers a project beyond his ownstructural design capabilities he should appoint a structuralengineer or preferably ask the client to make such anappointment. A chartered engineer in the modern contrac-

tor’s organisation can be as efficient as, and of equalintegrity to, his counterpart in the office of the consultingengineer, but his employer, the contractor, may have a

commercial axe to grind in the conception of a project.Equally it should be realised that the unlimited liability of a professional consultant is of no greater value than thesubstance of the consulting engineer: his personal wealthand professional indemnity insurance.

8 Supervision

8.01 As design processes tend to produce structures withminimum mass members and errors of fabrication become of greater potential hazard, supervision at all stages of con-struction should be an essential contribution to buildingwith an adequate reserve of strength. Neither architect norengineer can relieve the contractor of his responsibility to

supervise the work under construction, although by asystem of routine testing and inspection, they can cause ageneral level of quality to be established. The achievementof this level requires great determination on the part of thesupervisor and the client’s full support.8.02 It is of no profit to the client to demand compliancewith the programme and a high quality of workmanship if he has already accepted a price for the job which will notyield an adequate return to the contractor. Equally theclient should be prepared to pay for the level of super -

vision required and not expect the architect to supplementthe contractors’ staff free of charge. It is unfortunate thatthe various RIBA and ACE (Association of Consulting En-

gineers) agreements are not as clear as they might be on this

matter.

The future8.03 Relationships between architect, engineer and con-

tractor are changing as both design and fabrication of buildings become more involved. Unless the training of architects can keep pace with the refinements of analysisand the understanding of structural legislation there willbe very little structure with which the architect can work without the services of a chartered engineer.This is a regrettable state of affairs as there are many simplestructures where no Building Control Authority should evenask for calculations and many more where the simplest of codes of practice should give an adequate guide to a properlytrained architect.In the search for more design refinement and in the applica-tion of more theory and laboratory testing, Code Committeeshave lost sight of the bread and butter structures of thebuilding industry and have instead created a method of computation in the limit state codes which produces almostthe same end result with at least twenty percent more effort.CP 110 has had several years of practical testing in designoffices and has been found sadly wanting. It is to be hopedthat a lesson has been learned and much simpler limit stateguidance will be provided in future codes of practice.

Bibliography

1 The Building Regulations 1976 SI 1676. HMSO £3.30; TheBuilding (First Amendment) Regulations 1978 SI 723.HMSO£0.60p2 The London Building (Constructional) Bylaws 1972 GLC

[(Ajn)]


3 The Building Standards (Scotland) (Consolidation) Regu-lations 1971 SI 2052 (S218) HMSO £1.30; The BuildingStandards (Scotland) Amendment Regulations 1975 SI 404(S51) £0.29p; The Building Standards (Scotland) Amend-ment Regulations 1973 SI 794 (S65) £0.21p [(A3j)]4 Construction Industry Research and Information Associa-tion Report R30 CIRIA study committee on structuralsafety, London 1971 [(21) (K) (E2g)] o/p

5 BS CP 110: Parts 1-

3 1972 The structural use of concrete6 BS CP 111: Part 2 1970 Structural recommendations forload bearing walls7 BS CP 112: Part 2 1971 The structural use of timber8 BS CP 114: Part 2 1969 Structural use of reinforced concretein buildings9 BS CP 115: Part 2 1969 Structural use of prestressedconcrete in buildings



Technical studySafety 2 para 1.01 to 3.02

Technical study

Safety 2Section 3 Structural safety

Movement in building

1 Introduction

1.01 The subject of this study, movement of structure under

changing environment and under applied loading, is one

which does usually receive appropriate consideration in large

engineering structures. Buildings, however, have both the

added complications of applied finishes, attached elements

and services work, and the contradictions of architectural

planning requirements and structural logic. When the

problems associated with movement are considered, these

conflicting requirements often lead to the replacement of

considered common sense by optimism.

1.02 Buildings seldom fail by total collapse, but the adverse

effects of movement and excessive deflection can lead to

cracking and deformations, which involve repair and

maintenance costs and perhaps consequential loss of use.

The actual amount of movement is of importance, but

differential movement between major portions of the build-

ing and building elements is the real problem and a decision

has to be made either to prevent such movement or to

allow it to take place by incorporating joints.

2 Sources of movement

2.01 Sources of movement can be summarised as shown in

table I. All these sources can have either an overall or a

differential resulting movement.

Table I Sources of structural movement

Source Type Short-term Long-term

effect effect

Active loading Dead load

Steady superimposed load

Impact superimposed load

Wind load

Seismic load

Prestress loadChemical expansion of soil or

foundationFreezing expansion of soil

Mining effects

Vibration

Passive loading Soil deflectionSoil settlement

Mining effectsWater movement

Environment Temperature

Moisture

Materials Creep(almost all Shrink age

building Moisture movement

materials)

Insulation*

Conductivity*Elasticity*

Coefficient of expansion*

*Controlling properties

(to lesser

degree)

Possible sources of movement must be considered in anystructural design. Movement of structure under changing

environment and applied loading is discussed by

A L L A N H O D G K I N S O N in terms which do not obscure

principles with details such as specific jointing techniques

3 Major division of the structure

3.01 Major divisions of the structure may have to be made

with regard to foundation considerations, overall length of

the building and horizontal or vertical variations in building

shape. The reasons for soil movement are considered in

greater detail in section 4 of this handbook: F O U N D A T I O N S ;

here the effect on the building is discussed.

3.02 An isolated infinitely stiff building, loaded reasonably

evenly, will settle bodily and if total settlement does not

interfere with access or service connections this may not be

of consequence. Buildings in Mexico City have settled in

amounts ranging from several inches to a whole storey.

There is a practical limitation to stiffness however and in

the average building, unless founded on rock, there will be

differential movements within the building and, in a large

development, between different units of the development.

Movement joints which were required in a large London

development are shown in 1 and 2.

b

1 a Basement plan, Shell Centre, London; upstream building.

Chain lines are joints in both basement and superstructure.

Figures are net increases in pressure on soil (ie load of

building minus weight of excavated material per unit area)

in kN /m2; b superstructure plan; joints isolate stiff corner

areas and stiff lift/stair complexes

78



2 Section through Shell Centre upstream building showing

computed settlements

3.03 Differential movement may arise from uneven soiltypes; perhaps due to sloping strata, a localised peat layeror a large clay filling of a pocket in chalk. It may also arisefrom the use of different foundations in adjacent buildingunits, for instance where one building is piled and the nexton a pad foundation, or from simple pad foundations sizedso as to produce differing ground pressures 3. A notableexample is the Queens Tower of the former ImperialInstitute in London, which had a thick raft foundation

bearing on London clay at 644 kN/m2

. The surroundingbuildings were equally massive but were carried on thick strip footings with a bearing pressure of 215 kN/m2. Thebuildings achieved a differential settlement of 178 mm pro-ducing some very odd effects on the floor levels and in thesurrounding buildings when they tried to resist this move-ment. Tilting can become a problem in high buildings whenadjacent ground is loaded unevenly, as 4, or where theloading within the building is uneven.3.04 In most cases a vertical separation joint of about25 mm width between the buildings is desirable. Belowground this may produce problems in keeping out groundwater, and above ground in keeping out rain, wind andsnow. It may give the architect problems in modelling the

elevation, and in the general appearance of the building.Alternatively the structure may be arranged so thatduring construction there is complete freedom between thehigh and low areas. On soils where settlement takes placequickly, the connections between the high and low areascan be made after the whole permanent load is in positionor, and certainly in the case of long-term settlement, theconnections can be designed with provision for articulation.3.05 The positioning of major movement joints is so funda-

mental to both the architectural planning and the basic struc-tural concept that decisions must be taken in the earlieststages of the project. The order of settlements can beassessed from the results of ground investigation tests;but both this type of movement and overall longitudinal

movement produce different effects in different structuraltypes and the final decision will derive largely from intuitionand experience.3.06 Some advice is given on distortion and settlement in apaper by Skempton and McDonald1. They refer to damage



3.07 For design purposes they suggest that a factor of safetyof 1.25 to 1.50 should be applied. Using the latter figure, anangular distortion limited to 1 in 450 would be considereddesirable. The bare frame distortion, ie without stiffeningpanels, could be double this amount, ie 1 in 225. Otherinvestigators have suggested comparable figures of 1 in 750and 1 in 150 respectively, indicating the lack of precisionpossible and therefore the desirability of erring on the safeside.3.08 In the case of contraction or expansion joints in the

length of a building, the object is to avoid damage resultingfrom the movement of sections which are restrained betweenstiffer sections. Irrespective of the length, a 150 mm thick concrete slab, if fully restrained, would develop a com-

pressive force between the restraints of about 500 kN/mwidth of the slab, with a rise in temperature of 17°C. Thesame slab, if cast in one pour between restraints wouldproduce a tension force on shrinking of the same order.The distance between restraints determines the extent of potential temperature movement. In 30 m this mightamount to 6 mm. Obviously the restraints and/or the slabwill move or crack under such conditions.3.09 Summarising the whole question of major movementand joint positioning, building or group of joined buildings

will probably crack around certain points. By predeterminingthese expected cracks in the form of joints, the problems

can be faced before the cracks occur. Some examples fromexperience are shown in 5-9.

7 Two-storey car park, approximately 7.6 m square grid with

beams running parallel to x axis, slabs spanning in Y

direction. The columns, internal 610 mm x 229 mm, and

external 610 mm x 305 mm parallel to Y axis (as a), were

successful. But when external columns were turned through

90° for planning reasons (as b), these failed. Stiffness had been

increased to four times its original value, and instead of

flexing they attempted to act as buttresses

9 Effect of layout of stiff vertical supports. In typical

multi-storey block, positioning of stiff stair and lift

complexes as in a, although desirable planning, contradicts

free movement of structure. Alternative solutioons which solve

the structural problem either b allow movement outwards from

the stiff complex, or c provide a flexible wall at the end to

accept movement

4 Seismic and mining problems

4.01 Earthquakes are the surface vibration caused eitherby earth slip along fault lines or by volcanic explosions.Vibrations occur in both horizontal and vertical directions,the former being about five to 10 times the magnitude of the latter. Tremors are occasionally experienced in the UKbut are not considered in design. In other parts of the worldthey present a serious design problem.4.02 Joints are determined in accordance with the normalprocedures outlined previously with the added isolation of building parts dissimilar in mass or rigidity. The joints insuch conditions should be much wider than normal toprevent the sections of the building pounding each otherduring an earthquake.4.03 Mining problems are found in some areas of the UK,

and with increasing scarcity of building land, the use of sites hitherto ignored because of this condition is likely toincrease. Two situations arise, one where the mining hasalready occurred and the other where it will occur after thebuilding has been built. The former becomes a settlementproblem of rather large magnitude and follows normal



settlement design and detailing procedure. The latter is

more complex and requires the help of a mining expert topredict the pattern and order of movement.4.04 Put in its simplest form, a wave movement occurs sothat a part of the building is at one time on the crest, in thetrough or riding up and down the slope. As the waveprogresses across the site, the foundations will tend to beeither torn apart from each other or compressed towards

each other. On completion of the mining operation thelong-term settlement effect will again occur, depending onthe extent to which the mined volume is replaced by fill.4.05 It is possible to articulate the building by a three -pointfoundation system, though this is very costly. The entirestructure is carried on a triangular frame incorporatinglevelling jacks at the apexes. More usually the building isdivided into small sections, and these are dealt with inde-

pendently. A shallow raft with a granular underlayer willallow the building to slide over the wave. Where the sizeis such that normal pad foundations are essential, a highbearing pressure should be employed and the bases tied toeach other. The more flexible the superstructure, the lessrepair will be necessary on completion of the movement. A

detailed and useful description by F. W. L. Heathcote2

has explained both the problems and some methods of overcoming them.

modulus of elasticity (the relationship between stress andstrain), which results in more deflection when these mater-

ials are used with improved structural efficiency.6.03 The strength of concrete in reinforced concrete struc-tures has risen from a cube strength of 2000 lb psi (13.790N/mm2) to between 3000 and 6000 lb psi (20.685 and41.470 N/mm2). The modulus of elasticity does not in-

crease linearly with the strength, therefore the use of high

strength concretes tends to produce greater deflection byvirtue of the reduced section employed. BS CP 1143 recognisesthis by reducing the allowable span to depth ratios of beams and slabs for various combinations of concrete andsteel reinforcement stress.6.04 In prestressed concrete even higher concrete strengthsare demanded, but tho deflection becomes a more complexcondition. On the one hand the section may be madesmaller, but the prestress can keep the whole concretesection uncracked so that the product of second momentof area and elastic modulus may be greater than that of a‘cracked’ reinforced concrete section of equal size, thereforehaving less deflection potential 11. On the other hand theprestressing force is an active one and will result in an axial

shortening of the member and long-term deflections upwardsor downwards resulting from further stress variations dueto creep. This condition will be examined further on.

10 Floor is jointed around pile cap. Beam settles with poor in groove in pile cap face, thus stabilising pile cups whileavoiding damage to settling poor slab

5.02 The heated floor of a boiler house can cause shrinkageif there is a cohesive soil underneath, while the cold effectfrom a cold store can cause expansion of the soil below thefloor and therefore heave.

6 Differential movement of buildingelements

6.01 The obvious movements to be considered are thedeflections of beams, floors, walls and columns due to self weight, service load and wind loads. Over the last 30 yearsa better knowledge of the performance of materials andcontrol in manufacture has led to designed membersbecoming smaller in section to carry the same load.6.02 Since the 1930s the allowable bending stress in struc-

tural steel has risen from 8 tons psi (123.5 N/mm 2) to 10.5tons psi (162.1 N/mm2) for mild steel, and to 13.5 tons psi

(208.4 N/mm2

) for high tensile steel. The allowable tensilestress in reinforcing steel has risen from 16 000 lb psi(110.3 N/mm2) to 20 000 lb psi (137.9 N/mm2) for mildsteel, 33 000 lb psi (227.5 N/mm2) for medium high tensilesteel and even 50 000 lb psi (344.75 N/mm2) for very hightensile steel. In all cases there has been little change in the

6.05 However, kept within normal limits, the movement of the structure is not harmful and it is only when it is con-sidered relative to the other members which it supports thatconcern arises. Obviously a deflection large enough to givethe impression that the building is about to collapse isundesirable, or a vibration produced by jumping on anover-flexible member can be disquieting. But the major

problems arise with other building elements supported onthe structure such as partitions, applied finishes, claddingpanels and so on. Service conditions also may be unaccept-able, such as drainage falls in a roof which become reversedby the deflections.

7 Partitions

7.01 Despite the advice of BS CP 52346, partitions still remaina constant source of distress and are a frequent battle -

ground of claims for professional negligence. This need notbe so, for partitions can be designed to be compatible withthe structure, or vice versa. Unfortunately old traditions die

hard despite new materials and design concepts, and it isnot unusual to find heavy rigid partitions with hard plasterfaces being placed on flexible floors.7.02 In the 1930s, columns in reinforced concrete invariablyhad a beam framing into the head on each face as a measureof stability. Relative economics of concrete, formwork and




steel indicated a deep beam solution and with this a con-

siderable degree of rigidity. In the early 1950s there arrived

the now commonplace concrete slab with 'beam strip'

within its depth. Relative economics had changed and a flat

slab requiring minimum finishing treatment led to minimum

building height and minimum decoration costs. It also led

to a more flexible, floor both from direct load and wind-

loading effects, a property which is still largely unappre-

ciated. Some indication of the cracking which can occur is

shown in 12 and 13, while 14 illustrates the transfer of loadfrom a beam to a non-loadbearing partition.

12 Effect of deflection-prone floors on rigid partitions, with or

without doors in them: a arching action of partitions as

supporting floor deflects; b resultant cracking,

downward movement and rotation of cracked areas

upper floor

partition

lower floor

cracking pattern

support preventing

rotation

b

d o o rd o o r

door door

door door

load transferred from upperfloor into partition

deflected pattern of lower floor

deflected pattern of upper floor

13 Partition damage by load transfer from floor above. Here

partition spans as deep beam, with load transferred from

differential deflection between upper and lower floors

roof

bui lding frame

first floor

14 Partition damage by load transfer from beam over. Here

partition is constructed too early in building programme, and

pinned to roof beam. Roof beam deflects more than first floor

beam and transfers load to partition

82

8 Other deflection problems arising fromapplied load

8.01 Cantilevers are often calculated as though they spring

from an infinitely stiff structure and no consideration is

given for the rotation at the support, which increases or

decreases the end deflection 15. Even when the correct

calculation is made, distress can still occur in supported

elements 16, if allowance for the movement is not made.

Continuous beams develop tension over the supports, and joints are needed in supported elements 17.

a

b

15 16

15 In a, dotted line is simple cantilever deflection.

Rotation at support produces real deflection as shown in

solid line. In b, dotted line shows cantilever lifting as load is

applied to next span16 If edge cladding is too stiff or too rigidly connected to the

cantilevers it may be damaged, or even fall out. A wall built

along cantilever may crack as shown.

concrete parapet

17 Cracks occur in parapets over column supports to bridge

deck. In this case joints had to be sawn in parapet

8.02 Large panel construction or heavy precast beam

erection requires the use of levelling bolts or shims. Unless

the shim or bolt is released, or unless a packing capable of

carrying the erection load and capable of settling uniformly

with the finished structure is employed, 'stress raisers' will

occur which will split the supported member 18. Cladding

panels should not be supported in such a manner that they

become a prop between floors unless specially designed to

do so. It is preferable to have support on one member, and

flexible attachment elsewhere 19. Changes of grid can be

another source of cracking 20. A long span floor can be

damaged by the intervention of an extra supporting system

and this happens frequently at the ramps in multi-storey

car park construction. Problems which arise with the

rotation of simply supported beams at support points are

shown in 21, 22 and 23.

nylon or plastic washer

18Nylon or plastic washer carries initial erection loads, but

it deflects more than the concrete member when further load

is applied



83 Technical study Safety 2 para 9.01 to 9.02

19 Cracking, caused by hard joints

20 Cracking caused by change of grid

21 Underground structure, with large span over a theatre

or swimming pool. Change in slope in main girder will cause

cracking in finishes at x, unless slip surface is

introduced

22 Change in slope at end of heavy girders on brackets or

bearings can break the edge. This can be avoided by raising

beam on bearing with flexible pad such as neoprene

23 Prestressed barns with high pressure under bearing can

pull bracket away as creep occurs

9 Movement resulting from change inenvironment

9.01 While all materials react to temperature change,concrete, masonry and timber react also to shrinkage andmoisture migration. Despite the use of modern lightweightinsulation materials of great efficiency, it is unlikely that allparts of a building will be at the same temperature. Where

a structure can be contained entirely within the insulation,the best movement control is effected, but exposed columnsand edge beams create transverse deformations whichresult in the cracking of internal walls or floors 24. Claddingmembers, inevitably outside the insulation barrier, shouldbe given an adequate allowance for movement, dependingon the size of the member, both in terms of gaps betweenmembers and the form of the fixings to the structure.

24 Cracking effects of temperature difference on exposed

columns and edge beams: a ,floors; b internal walls

9.02 Roof beams and slabs will tend to deflect upwards whensubjected to heat; this can be a very sudden movement insunshine where there is poor roof insulation. The roof as awhole can expand and damage the supporting structure, or

itself if the supporting structure is stiff. Parapets can bepushed out of place and non-loadbearing partitions can bepulled over laterally or cracked along their length 25. Theeffect of direct sun heat on a large concrete panel is shownin26.

25 Dark faced aggregate concrete panel bowed 13 mm in

sunshine between cross walls 5.5 m apart



84

9.03 BS CP 1115 proposes vertical joints in facing brickwork at intervals not exceeding 15 m and where an external faceof a cavity wall exceeds 9 m in height; the wall should besupported from the internal structure to avoid disruption

of the wall ties. Combination of brickwork panels in aconcrete structure is complicated by the addition of shrink -age and moisture movement. An actual occurrence whererigid brick panels expanded within a frame which wascontracting is illustrated in 27. This resulted in diagonalcracks in the corners of the floor slabs. Supporting nibs ingable walls of concrete have been sheared off by the mois-ture movement of the brickwork when a soft joint has notbeen left below the nib, as 28.9.04 Shrinkage and creep in concrete will be considered inmore detail later in the handbook (section 5: CONCRETE).One effect often encountered is where an extend concretemember has tiles, mosaics or precast cladding. Unlessadequate horizontal joints are made, the cladding will beforced off as the concrete moves downwards under elasticcompression, creep and shrinkage.

10 Conclusion

10.01 The purpose of this study has been to examine thegeneral aspects of movement but not to solve the detail of movement joints. In normal circumstances these are costly;and even more costly if keeping out the weather. Fine

judgment is therefore needed to balance the risk of crackingagainst the cost of the joints, and this is an area in whichthere should be full understanding between architect andengineer.

References

1 SKEMPTON, A. w. and MCDONALD, D. H. Acomparisonof cal-culated and observed settlements. Stress Conference:

London, 1955, Institution of Civil Engineers [(2-

) (L4)]p318

2 HEATHCOTE, F. w. L. The movement of articulated buildingson subsidence sites. ICE Journal, vol 30, 1965, February[(2-) (L4)] p347-368

BRITISH STANDARDS INSTITUTION

3 BS CP 114: Part 2 1969 Structural use of reinforced concretein buildings4 BS CP 121: Part 3 1973 Brick and block masonry5 BS CP 111: Part 2 1970 Structural recommendations forload bearing walls6 BS 5234: Code of Practice for internal non -load bearing

partitioning, 1975



85

Technical studySafety 3

Section 3 Structural safety

Fire protection

Technical studySafety 3 para 1.01 to 2.03

ALLAN HODGKIN SON'sfinal technical study on safety

discusses the effects of flame and heat on structural

materials. He considers the general principles of protection.

and in four information sheets which follow, adds tables of

'deemed to satisfy' conditions for protected structural

members in concrete, steel, timber and masonry. This

information is the most up to date available on the subject,

now largely published for the first time. The technical study

is concerned only with general aspects, and protection of

structural members

1 The general problem

1.01 Various statutory regulations classify the use of

buildings according to fire risk. They also define fire resis-

tance of structural elements and the resistance of finishes to

spread of flame; this is related to the purpose, size and

degree of separation of buildings or parts of buildings. The

main concern is safety of life but further fire resistance may

be necessary for reasons of insurance premium.

1.02 A fire spreads because the radiation and convection

from its flames and hot gases heat other combustible

materials so that they also ignite. Burning doors and break-

ing windows may increase ventilation and so help to spread

the fire. With smoke and toxic gases a hazard both to

escaping occupants and firemen, it is important that this

spread should be retarded. The structural material, or

finishes applied to it, can help to prevent this spread.

1.03 The temperature may reach 1200°C if a fire continues

unchecked. Heat can then be transmitted through walls,

floors or roofs, igniting other materials or causing structural

members to crack or collapse If a very hot member is

hosed by firemen, the sudden cooling may further impair

its strength.

1.04 There are tests that make allowances for these effects

and can be used to allot fire resistance gradings to the main

structural elements. In addition many commercially

sponsored tests on specific products have been carried out

by the Joint Fire Research Organisation. The JFRO reports,

published from time to time, give guidance on other forms

or combinations of materials. From their experience, the

officers of JFRO may be able to assess gradings of new

structural elements and their advice is usually accepted as

the basis of a successful waiver of Building Regulation.

1.05 BS 4761 defines fire resistance, incombustibility and

non-inflammability of building materials and structures,

and describes all the requirements for testing. The heating

curve given in the standard represents a rapidly growing

fire with a smooth temperature profile of maximum dura-

tion six hours and maximum temperature 1200°C. Fire

resistance is related to the 'standard fire' (as defined in

BS 476). Building elements are tested in a furnace where

temperature is controlled so that particular temperatures

are reached after periods ranging from ½ hour to 6 hours.

Then, for a particular time resistance required, relative to

the severity of fire expected, a fire should not be more

severe in its effect than the BS 476 test.

1.06 For many years there has been a movement towards

basing fire requirements on the actual fire load, ie the

calorific value of the buildings' contents. But this would

require a special grading procedure and more basic design

data. Two JFRO tests, one on multi-storey car parks and the

other a two-storey steel-framed building forming part of a

multi-storey block of flats, have provided valuable informa-

tion. The second test established that the position of a

stanchion inside the fire compartment has a marked effect

on the temperature it reaches. It also showed that the size

of the steel section in a casing affects the temperature it

reaches and that the degree of protection needed can be

calculated. Building Regulations now recognise the effect of

the mass of the protected member by relating deemed to

satisfy requirements to a minimum weight of steelwork.

2 Choice of structure

2.01 Structure provides support for floors (for habitation)

and roofs (for cover). With medium or large spans, the roof

is invariably of lightweight construction, but where there

are suspended floors, the construction is often repeated at

roof level.

2.02 Floors are usually made of reinforced concrete, timber

or sheet steel screeded over, roofs have been built in almost

every material. Fire hazards exist for all types of structure,

and, for those materials lacking the in-built protection

of concrete, a careful consideration of protection method

is essential.

2.03 Long-span structures—like hangar roofs—are tradi-

tionally built in steel or aluminium. But the extra costs of

drencher systems, sprinklers and insurance premiums

should be assessed before making a final choice. (An aircraft

hangar has been built with prestressed precast space frame

roof and industrial buildings have employed roof trusses of

lightweight concrete.)




3 Reinforced concrete

3.01 Reinforced concrete has the best fire resistance of

common structural materials and indeed concrete is used to

protect other structures. It does not burn or give off suffi-

cient inflammable vapour to ignite and so may be regarded

as incombustible.

3.02 The fire resistance depends largely on the type of

aggregate. Siliceous aggregates are the poorest; the class 2

reference of the Building Regulations couples flint-gravel,granite and all crushed natural stones other than limestone.

Aggregates which have been subjected to heat during their

manufacture give a better performance; the class 1 refer-

ence couples foamed slag, pumice, blast furnace slag,

pelleted fly ash, crushed brick and burnt clay products,

well-burnt clinker and crushed limestone. This group con-

tains the commercial lightweight aggregates.

3.03 Concrete fails in fires because of the differential expan-

sion between exposed hot surface layers and inner cooler

layers. The movement of cement, as it shrinks with loss of

moisture, compared with the continued expansion of the

aggregate as temperature increases, creates another differ-

ential and so a further stress.

3.04 Steel reinforcement, exposed by cracking, conducts theheat rapidly and increases the temperature differential. The

concrete cracks and spalls and the reinforcement loses

strength as its temperature rises. Ultimately the element

fails. The insulation value of reinforced concrete, as a

structural element or as a casing to steelwork is obviously

important; from this point of view lightweight aggregate

concretes offer the better value.

3.05 As concrete with sand, ballast, sandstone and lime-

stone aggregates (but not aggregates of igneous rocks) is

heated, the colour changes from pink or red at between

300°C and 600°C, to grey between 600°C and 900°C, and

to buff at higher temperatures. These colour changes are

permanent and so help to identify the extent of the damage.

Strength begins to fall rapidly at about 250 °C and althoughthe structure may appear sound at 600 °C, the strength will

have dropped to 40 per cent.

3.06 Repair of a damaged structure is often easy. The tem-

perature is unlikely to have exceeded 800°C if less than one-

quarter of the steel surface is exposed by spalling. In the

ease of mild steel, this represents a permanent loss of

strength of 20 per cent of yield strength and 15 per cent of

ultimate strength. Work-hardened steels are more seriously

affected and it is essential that test pieces are cut from the

bars, which is obviously more difficult with prestressing

tendons. The steel may revert to its unworked condition,

depending on the temperature attained. When the structure

is under both superimposed load and fire attack, the critical

temperature for ordinary reinforcing steel will be about550°C and for prestressing tendons about 400°C. At these

temperatures the steel retains about half its normal am-

bient temperature strength.

3.07 A given thickness of concrete protects the reinforce-

ment for a specific time under the worst fire conditions; a

total thickness of member controls the temperature rise

on the unexposed face and reduces cracking. Different shapes

react differently to fire. Variations are needed with different

materials, eg less protection thickness for lightweight

aggregate, more protection thickness for hard-drawn steel

used in prestressed concrete.

3.08 Finishing materials such as plastics, renderings and

suspended ceilings add to the protection. Recent investiga-

tions showed that end restraint against thermal expansion

can substantially increase the fire resistance of a structural

element. Until more research results are available, it is

86

proposed that for beams and slabs, built into a structure

with restraints to thermal expansion provided at opposite

ends, the amount of protective cover to reinforcement may

be reduced to that for the next lower period in tables of fire

resistance requirements.

3.09 The surrounding structure can be assumed to provide

thermal restraint if there are no gaps between it and the ends

of floor or beam, no combustible materials used to fill these

gaps, and if the surrounding structure can withstand

thermal stresses induced by the heated floor or beam.

beam attacked on 3 sides

Concrete structures: 1 rib beams; 2 prestressed beams;

3a exposed column; 3b column in wall

4 Structural steelwork

4.01 Steelwork is incombustible. When heated it expands at

a known rate and its strength decreases 4. Mild steel is not

really affected until 300°C but decreases rapidly in yield

strength to about 50 per cent at 550°C and to 10 per cent at

800°C. On cooling it will recover about 90 per cent of its

initial strength; this is also true of alloy steel. Work-hard-

ened steels, usually cold-worked bars or prestressing wire,

deteriorate more rapidly and equivalent yield strength drops

to half at about 400°C. On cooling, this steel reverts to the

original unworked form. There is considerable permanent

loss of strength, and elongation characteristics are affected.

4 Graph shows changing strength of steel with temperature



4.02 With a thermal conductivity of 42 W/m °C, there israpid heat transmission through the element. Hut there is ahigh heat capacity, so the temperature of the steel lagsbehind the environmental temperature. Unprotected mem-

bers in standard fire tests have reached failure conditionsin 10 to 15 minutes, showing that some protection is

4.03 The protection system chosen must provide overall

value; relative costs of a number of treatments are shownin table I. These are costs of protection per foot of member,assuming conventional I-shaped sections. A protection thatfollows the profile of the steel should be cheaper with anRHS section.

necessary for even the lowest fire grading.

4.04 The Building Regulations suggest a variety of protec-tion methods for stanchions of minimum weight 44.64 kg/m,and beams of minimum weight 29-76 kg/m. These areillustrated in information sheet SAFETY 2.4.05 The 44.64 kg/m limit excludes the three lowest-weightsections in the universal column range and the eight lowest-weight sections in the universal beam range if the beamsare employed as columns. The 29.76 kg/m limit excludesonly the lowest-weight section in both the universal beamrange and universal column range, and all the six joists in the

quickly with aluminium but because it melts at 650°C the

section would melt before the end of a 30-minute fire test.

6 Timber

6.01 Unlike the other materials discussed, timber actuallyburns. But though combustible, the ease of ignition is

related to the density and moisture content of the timberand the size of the member.6.02 In a fire, moisture (which may be from 10 to 20 per centof the dry weight) is driven from the surface layers of thetimber. Little chemical change occurs until the temperature

reaches 270 to 290°C when exposed surface layers begin todecompose and the liberated gases can ignite. Flaming

continues as long as there is a heat input; without this, heatradiated back towards the wood by the flames is notsufficient to maintain the decomposition process.6.03 With continued flaming, a layer of charcoal is pro-

duced. This shields the inner timber from the effects of thefire. The charcoal is a better insulator than the naturaltimber, so strength is lost only from the outer layers whichare consumed. This charcoal layer is inert up to tempera-tures of about 500°C, glowing combustion then starts andthe charcoal is gradually consumed. A state of steadycombustion is achieved and the charring area advances intothe unburnt timber at a steady rate 5.

a circuit is formed. This can work as a radiator heatingsystem in reverse when columns are subjected to heat.The heated water moves upwards and is replaced by coolerwater, thus keeping the shell of the column at a lowertemperature. A header tank can be used, as in a radiatorsystem, to ensure that the system is kept full.4.08 This might seem to have rather limited applicationbecause the bare steelwork is the structural member whichis exposed to corrosion. But there are now weathering steelsthat develop their own protective oxide coating and which,in external situations or on unclad structures, may hotreach critical temperature in a fire because the heat is sorapidly dissipated.

5 Aluminium

5.01 Aluminium is not widely used in building structures.Table II compares three properties of steel and aluminium.In a fire, heat would be conducted away from a point more

6.04 Laboratory tests have established that for the majorityof timber species this rate is 0.64 mm/min, with low-density woods charring quicker than high-density ones.The rate is scarcely influenced by the severity of the fire,and this makes reasonably accurate prediction of the fireresistance of timber members possible.

Sacrificial timber 6.05 A timber member can be designed by the usual methodsand then increased in size to allow for the loss of timberexpected in a particular period of fire. For example a timber

beam 6 bearing its design load, is exposed to fire for 30minutes on the soffit and two sides. The resulting sectionfor calculation purposes will be (D - 30 x 0.64) mm deepand (W - 2 x 30 x 0-64) mm wide. That is, the sectionis reduced on each exposed face by the product of theelapsed time (30 min) and the charring rate (0.64 mm/min).



6 Timber beams showing ‘sacrificial’ material

6.06 For timber columns, heat can usually be applied onall sides; an arbitrary charring rate of 0.83 mm/min is usedbecause of the more rapid temperature rise possible undersuch conditions.6.07 Beams or columns laminated from smaller timbersusing structural adhesives of the resorcinal or phenol-resorcinol type, have charring and strength loss character-

istics equal to those of the solid section. But this does not

apply to mechanically fastened laminated members, unlessthe fixings remain within the undamaged timber at the endof the period of required fire resistance. Walls and floorscan be calculated in the same way but are generally of

such small sections that it would be more economical toconsider the protective properties of an applied finish.

7 Masonry

7.01 Masonry, in the form of solid brick, cellular brick, solidconcrete blocks, hollow concrete blocks (with either heavyor lightweight aggregate) and aerated concrete blocks,provides considerable resistance to fire.7.02 Bricks and concrete blocks with hollow cores notexceeding 25 per cent by volume can withstand the four-hourfurnace test of about 1100°C face temperature withoutfusion or spalling from the exposed face. Blocks with larger

cavities can have thin internal webs; here the high thermalstresses across the section could lead to fracture. Aeratedconcrete blocks provide better insulation but as the materialloses more strength than other blocks at high temperatures,extra thickness is required.7.03 As temperature rises, the heated face not only losesstrength but, in an axially-loaded wall, creates a conditionof eccentricity and thus a further reduction in ultimate load-

bearing capacity due to greater instability.7.04 As the result of a large number of tests which havebeen carried out on loadbearing walls, the Building Regula-tions give good guidance in ‘deemed to satisfy’ form.However the latest information will be published in the nextrevision of CP 1212.

References

1 BS CP 476 Part 8: 1972 Test methods and criteria forthe fire resistance of elements of building construction£5.60

2 BS CP 121: Part 1: 1973 Brick and block masonry(CP121.201 Masonrywalls:CP121.202Masonry: rubble walls)



89

Information sheet

Safety 1Section 3 Structural safety

Information sheet Safety 1

Fire resistance of concretestructures

A Reinforced concrete walls

When using lightweight concrete a reduction of thickness is possible but this

must be confirmed by a test Concrete cover to the reinforcement should not

be less than 1 5 mm for fi re resistance up to 1 h and not less than 25 mm for

higher periods Walls containing less than one per cent vertical

reinforcement are considered as plain concrete for fire purposes unless a test

shows otherwise

Table I Fire resistance of reinforced dense concrete walls

exposed to fire on one side only

Description of applied finish

Minimum thickness of concrete in

mm to give fire resistance of

4 3 2 1 ½ 1 ½ hours

none 180 150 100 100 75 75

cement or gypsum plaster on one

or both sides 180 150 100 100 75 75

vermiculite/gypsum plaster* at

least 15 mm thick on both sides 125 100 75 75 65 65

*Vermiculite/gypsum plaster should have a mix ratio in the range from1½ 1 to 2 1 by volume

Walls exposed to fire on more than one face should be regarded as columns

B Plain concrete walls

From the limited data available the fi re resistance of plain concrete walls can

be taken as

1 h for concrete 1 50 mm thick 1½h for concrete 175 mm thick

C Reinforced concrete beams

Table II Fire resistance of reinforced concrete beams

Description

Dimension of concrete in mm to

give a fire resistance of

4 3 2 1½ 1 ½ hours

1 dense concretea concrete cover to mam

reinforcementb beam width

65* 55* 45* 35 25 15

280 240 180 140 110 80

2 as 1 with cement or gypsum

plaster 15 mm thick on light mesh

reinforcement

a concrete cover to mamreinforcement

b beam width

50* 40 30 20 15 15

250 210 170 110 85 70

3 as 1 with vermiculite/gypsum

plaster or sprayed asbestos 15 mm

thick

a concrete cover to mam

reinforcement

b beam width

25 15 15 15 15 15

170 145 125 85 60 60

4 lightweight aggregate concrete


reinforcement

b beam width50 45 35 30 20 15

250 200 160 130 100 80

*Supplementary reinforcement consisting of either a wire fabric not lighterthan 0 5 kg/m

2(2 mm diameter wires at not more than 100 mm centres) or a

continuous arrangement of stirrups at not more than 200 mm centres must beincorporated in the concrete cover at a distance not exceeding 20 mm fromthe face

Note• Vermiculite/gypsum plaster should have a mix ratio in the range from

1½ 1 to 2 1 by volume Sprayed asbestos should conform to BS 3590

This sheet consists of tables of 'deemed to satisfy'

conditions for concrete walls, beams and floors

The fire is assumed to attack the soff it and two sides of the beam

When the reinforcement is used in more than one layer the value of the

total protective concrete cover is the arithmetic mean of the nominal cover to

the tensile reinforcement in each layer The value of the minimum cover to any

bar should not be less than half the value shown in table II f or different

periods of fi re resistance and never less than the value shown under the

i hour periodAlternatively the average concrete cover may be determined by summing

the product of the cross sectional area of each bar or tendon and the distance

from the nearest exposed face and dividing it by the total area of the steel

provided to resist tensile stresses induced by the imposed loads

Average concrete cover

where AS1 — cross sectional area of the steel bar or tendon

and c1 — its distance from the nearest exposed face

D Reinforced concrete columns

Table III Columns with all faces exposed

Type of construction

Dimension of concrete in mm to

give fire resistance of4 3 2 1½ 1 ½hours

1 dense concrete

a without additional protection 450 400 300 250 200 150

b with cement or gypsum plaster

15 mm thick on light mesh

reinforcement 300 275 225 150 150 150

c with vermiculite/gypsum plaster*

15 mm thick 275 225 200 150 120 120

d with supplementary reinforcement

in concrete cover or limestone

aggregate concrete 300 275 225 2CO 200 150

2 lightweight aggregate concrete 300 275 225 200 150 150

*Vermiculite/gypsum plaster should have a mix ratio in the range from 1½ 1 to

2 1 by volume Sprayed asbestos should conform to BS 3590

Note

The minimum dimension of a column is a determining factor in the fire

resistance it can provide The dimensions given in the table relate to columns

which may be exposed to fir e on all faces when subjected to characteristic

loads The use of limestone or other calcareous aggregates or the use of

supplementary reinforcement in the concrete cover would reduce spalling

and allow a reduction in the size of the section The supplementary

reinforcement should consist of steel fabric of not less than 2 mm diameter

wire and of mesh not greater than 150 mm or an equivalent material and

should be placed at mid cover not more than 20 mm from the face The

concrete cover to the main bars should not exceed 40 mm without the use ofsupplementary reinforcement The data in table III are based on a rectangular or

a circular cage reinforcement

Table iv Columns with only one face exposed

Type of constructionDimension of concrete in mm togive a fire resistance of4 3 2 1½ 1 ½ hours

dense concrete

a without additional protection 180 150 100 100 75 75b with 15 mm vermiculite/gypsumplaster* on exposed face 125 100 75 75 65 65

*As footnote to table III

Note

Columns with their full height built into fi re resisting walls may beexposed to fir e on one face only Data in table IV apply when the face of the

column is flush with the wall or when that part of the column embedded inthe wall is structurally adequate to support the load provided that anyopening in the wall is not nearer to the column than the minimumdimension for the column specified in table IV




E Reinforced concrete floors

Table V is not exhaustive and the performance of types not shown can either be assessed by analogy or determined by testing

Table v Fire resistance of reinforced concrete floors

Floor type

90

Dimension of concrete in mm to givefire resistance of

4 3 2 1½ 1 ½hours

1 solid slabs cover to reinforcement

overall depth*

25 25 20 20 15 15

150 150 125 125 100 100

2 cored slabs, area of cores less than50 per cent of solid area, cores

higher than width

cover to reinforcement

thickness under cores

overall depth*

25 25 20 20 15 15

50 40 40 30 25 20190 175 160 140 110 100

3 hollow box sections with one or morelongitudinal cavities

cover to reinforcementthickness of bottom flange

overall depth*

25 25 20 20 15 15

50 40 40 30 25 20230 205 180 155 130 105

4 inverted T-section beams with

hollowinfillblocks of concrete or

clay having not less than 50 per

cent of solid material


width of T-flange

overall depth*

25 25 20 20 15 15

125 100 90 80 70 50

190 175 160 140 110 100

5 ribbed floor with hollow infill blocks

of clay having less than 50 per cent

of solid material and with a 15 mm

plaster coating on soffit


width of T-flange

overall depth*

25 25 20 20 15 15

125 100 90 80 70 50

190 175 160 140 110 100

6 upright T-sections bottom cover to reinforcementside cover to reinforcementwidth of web

depth of f lange

65** 55** 45** 35 25 1565 55 45 35 25 15150 140 115 90 75 60150 150 125 125 100 90

7 inverted channel sections, radius atintersection of soffits with top of

leg not exceeding depth of section

bottom cover to reinforcement

side cover to reinforcement

width of webdepth or thickness at crown*

65** 55** 45** 35 25 15

40 30 25 20 15 1075 70 60 45 40 30

150 150 125 125 100 90

8 inverted channel sections orU-sections, radius at intersection of

soffits, top of leg exceeding depth of

section



width of web*

depth or thickness at crown*

65** 55** 45** 35 25 15

40 30 25 20 15 1070 60 50 40 35 25

150 150 100 100 75 65

* Non-combustible screeds and finishes may be included in these dimensions** Additional reinforcement is necessary to hold the concrete cover in position

Note• In estimating the thickness of concrete, non-combustible screeds or finishes can be taken into account. The effect of the ceiling finish is shown in table VI.

Table vi Effect of ceiling finish on fire resistance of structural

suspended floors

Ceiling finish

Thickness of finish in mmto

give an increase in fireresistance of3 2 1½ 1 ½ hours

1 vermiculite/gypsum plaster* or sprayed

asbestos applied to the soffit of floortypes 1, 2 or 3 25 15 15 10 10

2 vermiculite/gypsum plaster* or sprayed

asbestos on expanded metal as asuspended ceiling to floor types 4 or 5 15 10 10 10 10

3 gypsum/sand or cement/sand on

expanded metal as a suspended ceiling

to any floor type 25 20 15 10 10

*Vermiculite/gypsum plaster should have a mix ratio in the range of 1 i :1 to2:1 by volume. Sprayed asbestos should conform to BS 3590

F Prestressed concrete beams

Table vii Fire resistance of prestressed concrete beams

DescriptionDimension of concrete in mm togive a fire resistance of

4 3 2 1½ 1 ½ hours

1 dense concrete:

a concrete cover to main

reinforcement

b beam width

100* 85» 65* 50* 40 25

280 240 180 140 110 80

2 as 1 with vermiculite concrete

slabs, 15 mm thick, used aspermanent shuttering:


reinforcementb beam width

75* 60 45 35 25 15

210 170 125 100 70 70

3 as 2 with 25 mm thick slabs:


reinforcement

b beam width

65 50 35 25 15 15

180 140 100 70 60 60

4 as 1 with 15 mm thick gypsum

plaster with light meshreinforcement:

a concrete cover to mainreinforcement

b beam width

90* 75 50 40 30 15250 210 170 110 85 70

5 as 1 with vermiculite/gypsum

plaster, or sprayed asbestos*15 mm thick:

a concrete cover to mainreinforcement

b beam width

75* 60 45 30 25 15

170 145 125 85 60 60



91

DescriptionDimension of concrete in mm to

give a fire resistance of4 3 2 1½ 1 ½ hours

6 as 5 with 25 mm thick coating


reinforcement 50 45 30 25 15 15

b beam width 140 125 85 70 60 60

7 lightweight aggregate concretea concrete cover to main

reinforcement 80 65 50 40 30 20

b beam width 250 200 160 130 100 80

*Supplementary reinforcement consisting of either a wire fabric not lighterthan 0 5 kg/m

2(2 mm diameter wires at not more than 100 mm centres) or a

continuous arrangement of stirrups at not more than 200 mm centres must beincorporated in the concrete cover at a distance not exceeding 20 mm fromthe face

Notes

Vermiculite/gypsum plaster should have a mix ratio in the range from

1½ 1 to 2 1 by volumeSprayed asbestos should conform to BS 3590The protective cover to the tendon for different periods of fire resistance

should not be less than in table VII, in no case should the minimum cover toany tendon be less than half the value shown in the table for differentperiods of fire resistance it should never be less than the value shown

under the i hour periodWhen the prestressing tendons to resist tensile stresses due to imposed or

working loads are provided in a number of layers, the value of the protective

concrete cover is the arithmetic mean of the nominal cover for each layerI section beams with web thickness of half or less the lower flange

breadth require web stirrups amounting to 0 15 per cent of the web areaon plan

G Prestressed concrete floors

Table viii Fire resistance of prestressed concrete floors

Floor types—illustrated as Table V

Dimension of concrete in mm to give fire resistance of4 3 2 1½ 1 ½ hours

1 solid slabs cover to reinforcementoverall depth*

65" 50" 40 30 25 15150 150 125 125 100 90

2 cored slabs, area of cores less than 50 per centof solid area, cores higher than width

cover to reinforcementthickness under coresoverall depth*

65" 50** 40 30 25 15

50 40 40 30 25 20

190 175 160 140 110 100

3 hollow box sections with one or more

longitudinal cavities

cover to reinforcementthickness of bottom flange

overall depth*

65" 50" 40 30 25 15

65 50 40 30 25 25

230 205 180 155 130 105

4 inverted T-section beam with hollow infill blocks

of concrete or clay having not less than50 per cent solid material

cover to reinforcementwidth of T-flange

overall depth*

65** 50** 40 30 25 15125 100 90 80 70 50

190 175 160 140 110 100

5 upright T-sections bottom cover to reinforcement

side cover to reinforcementwidth of web

depth of T-flange*

100** 85" 65" 50" 40 25

100 85 65 50 40 25

250 200 150 110 90 60

150 150 125 125 100 90

6 inverted channel sections, radius atintersection of soffits, top of leg notexceeding depth of section



width of legdepth or thickness at crown*

100" 85" 65** 50**

50 45 35 25

125 100 75 55

150 150 125 125

7 inverted channel or U-sections, radius atintersection of soffits, top of legexceeding depth of section


side cover to reinforcementwidth of leg*depth or thickness at crown*

100** 85** 65** 50" 40 25

50 45 35 25 20 15

110 90 70 50 45 30

150 150 125 125 100 90

• Non-combustible screeds and finishes may be included in these dimensions"*Supplementary reinforcement is necessary as in table VII or consisting of equivalent expanded metal lath

NotesThe average cover at a section is assessed as the arithmetic mean of the nominal cover of each equal tendon of prestressing steel in the member below the

neutral axis; but the minimum cover to any tendon should not be less than half the value shown under different periods of fire resistance and in no case should

it be less than the value shown under the period of ½ hourIf the thickness of concrete cover for floor types 4 and 5 exceeds 40 mm, mesh reinforcement must be incorporated in the cover to retain the concrete in

position This is not necessary when ceiling protection of the types shown in section E of this sheet (reinforced concrete floors) is usedSimilarly the fire resistance of a given form of construction can be improved by using an insulating finish on the soffit or by a suitable suspended ceiling—as

described for reinforced concrete floors





Information sheetSafety 2


Fire resistance of steelstructures

A Fire resistance of protected steel columns

Table i Fire resistance of protected steel columns; stanchion weight per metre not less than 44-6 kg

Construction and materials

Solid protection

(unplastered)

The casing is bedded close

to the steel withoutintervening cavities and all

joints in the casing made

full and solid

Minimum thickness (in mm) ofprotection for a fire resistance of

4 2 1½ 1 ½ hours

Concrete not leaner than 1 :2:4 mix

with natural aggregates:

a Concrete assumed not loadbearing

reinforced* 50-8 25-4 25-4 25-4 25- 4

b Concrete assumed to be loadbearing

reinforced in accordance with BS 449 76 -2 50 -8 50-8 50 -8 50-8

Solid bricks of clay, composition sand

or sand lime 76- 2 50-8 50-8 50- 8 50-8

Solid blocks of foamed slag or

pumiced concrete reinforced* in every

horizontal joint 63- 5 50-8 50- 8 50-8 50- 8

Sprayed asbestos 144 to 240 kg/m3

44-5 19- 1 15-9 9 -5 9- 5

Sprayed vermiculite cement — 38-1 31- 8 19-1 12-7

Solid bricks of clay, composition orsand lime reinforced in every

horizontal joint unplastered 114-3 50 -8 50 -8 50 -8 50'8

Solid blocks of foamed slag or

pumice concrete reinforced** in every

horizontal joint unplastered 76-2 50-8 50-8 50-8 50-8

Metal lath with gypsum or cement

lime plaster of thickness — 38-1 25-4 19- 1 12-7

Metal lath with vermiculite or perlite

gypsum plaster of thickness 50-8 19-1 15-9 12-7 12-7

Metal lath spaced 25 mm from flanges

with vermiculite gypsum or perlite

gypsum plaster of thickness 44 -5 19-1 12-7 12-7 12-7

Gypsum plasterboard with 16 swg

binding at 100 mm pitch:

a 9-5 mm plasterboard with gypsumplaster of thickness — — — 12- 7 12-7

b 19 mm plasterboard with gypsum

plaster of thickness: — 12-7 9 -5 6-4 6- 4

Plasterboard with 16 swg binding at

100 mm pitcha 9 -5 mm plasterboard with

vermiculite gypsum plaster of thickness — 15-9 12-7 9-5 6-4b 19 mm plasterboard with

vermiculite gypsum plaster of thickness 31-8* * 9 -5 9-5 6- 4 6- 4

Metal lath with sprayed asbestos of

thickness 44 -5 19-1 15-9 9-5 9- 5



93

Table I continued



Minimum thickness (in mm) of

protection for a fire resistance of4 2 1½ 1 ½ hours

Vermiculite cement slabs of 4:1 mix

reinforced with wire mesh finished

with plaster skimSlabs of thickness 63-5 25- 4 25- 4 25-4 25-4

Asbestos insulating boards of density513 to 881 kg/m3

(Screwed to 25 mm asbestos battens for

½ hour and 1 hour periods) — 25-4 19-1 12-7 9 -5

*Reinforcement of steel binding wire 13 swg or a steel mesh weighing not less than 0-542 kg/m2. Minimum spacing in concrete not less than 150 mm**Light mesh reinforcement required 12 to 19 mm below surface unless special corner beads are used

B Fire resistance of protected steel beams

Table II Fire resistance of protected steel beams; joist weight per metre not less than 30 kg


Solid protection

(unplastered)The casing is bedded closeto the steel without

intervening cavities and all joints with casing made full

andsolid

floor

concrete

Concrete not leaner than 1 -2:4 mix

with natural aggregate:a Concrete not assumed to be

loadbearing reinforced*

b Concrete assumed to beloadbearing reinforced in

accordance with BS 449


protection for a fire resistance of4 2 1½ 1 ½hours

63-5 25-6 25-4 25-4 25-4

76-2 50-8 50-8 50-8 50-8

floor Sprayed asbestos 144 to 240 kg/m2 44-5 19-1 15-9 9- 5 9-5

Sprayed vermiculite cement — 38 1 31-8 19-1 12-7

floor

metal lath

void

plaster

Metal lathing:

a with cement lime plaster of thickness — 38-1 25 -4 19-1 12- 7b with gypsum plaster of thickness — 22-2 19-1 15-9 12-7

c with vermiculite gypsum or perlite

gypsum plaster of thickness 31- 8 12-7 12-7 12-7 12-7

floor-

plasterboard

void

wire binding

Gypsum plasterboard with 16 swg wirebinding at 100 mm pitch:

a 9•5 mm plasterboard with gypsum

plaster of thickness — — — 12-7 12-7b 19 mm plasterboard with gypsum

plaster of thickness — 12 7 9-5 6-4 6-4

Plasterboard with 16 swg wire bindingat 100 mm pitch:

a 9 -5 mm plasterboard nailed to

wooden saddles finished with gypsum

plaster of thickness — — — — 4-8b 9 -5 mm plasterboard with

vermiculite gypsum plaster of thickness — 15-9 12-7 9 -5 6-4c 19 mm plasterboard withvermiculite gypsum plaster of thickness 38-1 9-5 9- 5 6-4 6- 4

d 19 mm plasterboard with gypsum

plaster of thickness — 12-7 — — —

floor

metal lath

void

sprayed asbes tos

Metal lathing with sprayed asbestos •

144 to 240 kg/m3of thickness 44-5 19-1 9-5 9-5 9-5



Information sheet Safety 2 94


protection for a fire resistance of4 2 1½ 1 ½ hours


asbestos insulatingboardscrew to battens

asbestos battens

Asbestos insulating boards of density

513 to 881 kg/m2 (screwed to 25 mm

asbestos battens for ½ hour and1 hour periods)

floor

floor

slabs reinforced with

wire mesh

vermiculite cementslabs

plaster skim coat

Vermiculite cement slabs of 4 1 risereinforced with wire mesh and

finished with plaster skin slabs of

thickness

25 4 19 1 12 7 9 5

63 5 25 4 25 4 25 4 25-4

50 8 38 1 38 1 38 1

Gypsum sand plaster 12 mm thick

applied to heavy duty (type B)

woodwool slabs of thickness

woodwool

slobs

gypsum sand

plaster

floor

'Reinforcement of steel binding wire 13 swg or a steel mesh weighing not less than 0 542 kg/m2 Minimum spacing in concrete not less than 150 mm



95 Information sheet Safety 3



This sheet consists of two tables of ‘deemed to satisfy’

conditions for timber floors and structural stud partitionsFire resistance of timberstructures



96






This sheet consists of two tables of 'deemed to satisfy'

conditions for brick and block wallsFire resistance of masonrywalls

A Single leaf walls

T a b l e I E i r e re si st an ce of s in g le le af m a s o n r y wa l l s

* the number of cells in any cross-section through the wall thickness** suitable for 75 mm brick -on-edge construction with a completely solid unit with plane faces

Note1 Class I aggregates f or concrete blocks can be limestone, aircooled blast furnace slag, foamed or expanded slag, crushed brick, well burnt clinker. expandedclay or shale, sintered pelleted flyash, pumice.Class II aggregates f or concrete blocks include all gravels and crushed natural stone except limestone.2 The finish shall be not less than 13 mm plaster or rendering on each face of a single sk in wall and on the exposed face of a cavity wall.

sc = sand cement plaster with or without limesg = sand gypsum plaster with or without limesc/s g may be replaced by plaster board of equivalent thickness forf ire resistance upto two hours.

vg - vermiculite gypsum plaster in proportions 13 : 1 or 2:l by volume. (Purlite may be substituted in fired clay brickwork or other materials wi th similarsurfaces).3 Solid brickwork is of bricks without frogs orfrogs up to 20 per cent of the brick volume with no through holes or perforations. (This definition differs fromBS 3921).



Information sheet Safety 4 98

B Cavity walls

Table II Fire resistance of cavity walls



AJ Handbook of

Building Structureedited by Allan Hodgkinson

In its first edition, this Handbook became a standardreference for both students and practitioners. Re-cent changes to British Standards, Codes of Practice

and Building Regulations have generated demandfor a new, updated edition; and unlike the reprints of

dated version of the original 1974 Handbook.

The principle changes are in the sections on Mas-onry (totally rewritten to take account of the 1976Building Regulations) and on Timber (substantiallyrevised to take account of new timber gradings). Inaddition to many minor improvements, the oppor-tunity has also been taken to bring up to date all thereferences quoted.

1976 and 1977, this is a radically revised and up-

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book' to take him through many years of use . . . itdeserves widespread circulation"

Paper edition ISBN 0 85139 282 2

Guide to the Building Regulations

1976 (Seventh Edition)

A J Elder

This new 1982 edition of the Guide to the 1976 Build-

tary of State's long-awaited Command Paper, in-corporates two new appendixes: on the ProposedSecond Amendment, and on The Future of BuildingControl in England and Wales.

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ISBN 0 85139 850 2

ing Regulations, coming on the heels of the Secre-

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