Precast Concrete StructuresThis Page Intentionally Left BlankPrecast ConcreteStructuresKim S. ElliottOXFORD AMSTERDAM BOSTON LONDON NEW YORK PARISSAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYOButterworth-HeinemannAn imprint of Elsevier ScienceLinacre House, Jordan Hill, Oxford OX2 8DP225 Wildwood Avenue, Woburn, MA 01801-2041First published 2002Copyright#2002, Kim S. Elliott. All rights reservedThe right of Kim S. Elliott to be identified as the author of this workhas been asserted in accordance with the Copyright, Designs andPatents Act 1988No part of this publication may be reproduced in any material form (includingphotocopying or storing in any medium by electronic means and whetheror not transiently or incidentally to some other use of this publication) withoutthe written permission of the copyright holder except in accordance with theprovisions of the Copyright, Designs and Patents Act 1988 or under the terms ofa licence issued by the Copyright Licensing Agency Ltd. 90 Tottenham Court Road,London, England W1T 4LP. Applications for the copyright holder's writtenpermission to reproduce any part of this publication should be addressedto the publisherBritish Library Cataloguing in Publication DataElliott, Kim S.Precast concrete structures1. Precast concrete constructionI. Title693.5/22Library of Congress Cataloguing in Publication DataA catalogue record for this book is available from the Library of CongressISBN 0 7506 5084 2For information on all Butterworth-Heinemann publicationsvisit our website at www.bh.comTypeset by Integra Software Services Pvt. Ltd, Pondicherry 605 005, Indiawww.integra-india.comPrinted in Great Britain by Antony Rowe Ltdcontents Preface 1 What is precast concrete? 1.1 Why is precast diffrent? 1.2 Precast concrete structures 1.3 Why choose a precast structure? 2 Materials used in precast structures 2.1 Concrete 2.2 Steel reinforcement 2.3 Structural steel and bolts 2.4 Non-cementitious materials 3 Precast frame analysis 3.1 Types of precast concrete structures 3.2 Simplified frame analysis 3.3 Substructuring methods 3.4 Connection design 3.5 Stabilizing methods 4 Precast concrete floors 4.1 Precast concrete flooring options 4.2 Flooring arrangements vii 4.3 Structural design of individual units 4.4 Design of composite floors 4.5 Composite plank floor 1 5 Precast concrete beams 5.1 General introduction 5.2 Non-composite reinforced concrete beams 5.3 Composite reinforced beams 15 5.4 Non-composite prestressed 15 beams 19 5.5 Composite prestressed 2 1 beam design 22 5.6 Propping 5.7 Horizontal intevface shear 23 6 Columns and shear walls 6.1 Precast concrete columns 23 6.2 Column design 27 6.3 Precast concrete shear walls 34 6.4 Distribution of horizontal 41 loading 45 6.5 Infill shear walls 6.6 Cantilever walls 59 7 Horizontal floor diaphragms 59 7.1 Introduction to floor 69 diaphragms 7.2 Shear transfer mechanism 7.3 Edge profile and tie steel details 7.4 Design of floor diaphragm 7.5 Shear stiffness 7.6 Diaphragm action in composite floors with structural toppings 8 Joints and connections 229 8.1 Definitions 229 8.2 Basic mechanisms 230 8.3 Compression joints 232 8.4 Shear joints 248 8.5 Tension joints 257 8.6 Pinned-jointed connections 263 8.7 Moment resisting connections 268 213 9 Beam and column connections 287 215 9.1 Types of beam and column 216 connections 287 222 9.2 Beam-to-column connections 291 9.3 Beam end shear design 320 9.4 Column foundation 224 connections 334 10 Ties in precast concrete structures 10.1 Ties in precast concrete structures 10.2 Design for robustness and avoidance of progressive collapse 10.3 The fully tied solution 10.4 Tie forces prefaceIn 1990, the chairman of the British Precast Concrete Federation (BPCF), Mr GeoffBrigginshaw, asked me what level of teaching was carried out in British univer-sities in precast concrete construction for multi-storey buildings. The answer, ofcourse, was very little, and remains that way today in spite of considerable effortsby the BPCF and sections of the profession to broadcast the merits, and pitfalls ofprecast concrete structures. Having given lectures at about 25 UK universities inthis subject, I estimate that less than 5 per cent of our civil/structural engineeringgraduates know about precast concrete, and less than this have a decent ground-ing in the design of precast concrete structures. Why is this?The precast concrete industry commands about 25 per cent of the multi-storeycommercial and domestic building market if frames, floors and cladding (facades)are all included. In higher education (one step away from the market), precasteducation commands between zero and (about) 5 per cent of the structuralengineering curriculum. This in turn represents only about 1/8 of a civil engineer-ing course. The 5 per cent figure claimed above could indeed be an over estimate.The reasons are two-fold:1 British lecturers are holistic towards structural engineering.2 British lecturers have no information in this subject.This book aims to solve these suggestions simultaneously. Suggestion no. 2 ismore readily solved. This book is, unfortunately, one of very few text books in thissubject area aimed at students at a level which they can assimilate in their overallstructural engineering learning process. It does this by considering design both atthe macro and micro levels global issues such as structural stability, buildingmovement and robustness are dissected and analysed down to the level ofdetailed joints, localized stress concentrations and bolts and welds sizes.Suggestion no. 1 is more complex. Having been acquainted with members ofthe FIB* (formerly FIP,) Commission on Prefabrication, it has come to my notice* FIB (Federation International du Beton), born from a merger of FIP with CEB. An international, butpredominantly European organization for the welfare and distribution of information on structural concrete.,FIP (Federation International de la Prefabrication) is an international, but predominantly European,organization for the welfare and distribution of information on prefabricated concrete.the differing attitudes towards the education of students in certain forms ofbuilding construction precast concrete being one of them (timber another).In continental Europe, leading precast industrialists and/or consultants holdacademic posts dedicated to precast concrete construction. Chairs are even spon-sored in this subject. In South America, lecturers, students and practitioners holdseminars where precast concrete is a major theme. It is not uncommon for as manyas 10 Masters students to study this subject in a civil engineering department. Inthe United States of America collaborative research between consultants, precastmanufacturers and universities is common, as the number of papers published inthe PCI JOURNAL testifies.The attitude in Britain is more holistic and less direct. Firstly, basic tuition isgiven in solid mechanics, structural analysis and material properties. Students arerequired to be capable of dealing with structural behaviour independent of thematerial(s) involved. Secondly, given the fundamental principles of design (and areminder that code equations are often simulations and their data conservative)students can assimilate any design situation, with appropriate guidance. This maybe true for structural steelwork and cast in situ concrete structures where thedesigner may (if he wishes) divorce themselves from the fabricator and contractor.It is not true for precast concrete (and timber) structures where the fabricator andsite erector form part of the `design team'.Precast concrete design is an iterative procedure, linking many aspects of archi-tecture, design, detailing, manufacture and site erection together in a 5-point lattice.Major linksDetailingArchitectureManufactureDesignSiteerectionFigure iviii PrefaceMany students will be familiar with these names, but few will see or hear themin a single lecture. Some of the links are quite strong. Note the central role of`designing' (this does not mean wL2/8, etc.) in establishing relationships witharchitectural requirements, detailing components and connections, etc., manufac-turing and erecting the said components at their connections. Could similardiagrams be drawn for structural steelwork or cast in situ concrete structures?Further, there are a number of secondary issues involving precast concreteconstruction. Prefabrication of integrated services, automation of information,temporary stability and safety during erection, all result from the primary links.Some of these are remote from `designing'. The illustration reminds us of theirpresence in the total structure. The design procedure will eventually encompassall of these aspects.This book is aimed at providing sufficient information to enable graduates tocarry out structural design operations, whilst recognizing the role of the designerin precast concrete construction. Its content is in many parts similar to but morefundamental than the author's book `Multi-Storey Precast Concrete Framed Struc-tures' (Blackwell Science 1996). The Blackwell book assumed a prior knowledge ofSite erectionDesignSequence ofdeliveriesIntegrated servicesStructural zonesand facadesServices andfacade detailsManufactureDetailingArchitectureTemporarystabilityFigure iiPreface ix Preface ixthe building industry and some experience in designing concrete structures. Thispresent book takes a backward step to many of the design situations, and does notalways uphold the hypotheses given. Reference to the Blackwell book may there-fore be necessary to support some of the design solutions.The design examples are carried out to BS8110, and not EC2 as might beexpected from a text book published today. The reason for this is that the clausesrelevant to precast concrete in EC2 have yet to find a permanent location. Origin-ally Part 1.3 was dedicated to precast concrete, but this was withdrawn and itscontent merged into the general code Part 1.1. For this reason specific design datarelevant to precast concrete is not available.The author is grateful to the contributions made by the following individualsand organizations: to members of the FIB Commission on Prefabrication, inparticular Arnold Van Acker (Addtek Ltd., Belgium), Andre Cholewicki (BRIWarsaw), Bruno Della Bella (Precompressi Centro Nord, Italy), Ruper Kromer(Betonwerk Fertigteil-Technik Germany), Gunner Rise (Stranbetong, Sweden),Nordy Robbens (Echo, Belgium) and Jan Vambersky, (TU Delft & Corsmit, Neth-erlands); to Trent Concrete Ltd (UK), Bison Ltd (UK), SCC Ltd (UK), TarmacPrecast Ltd (UK), Tarmac Topfloor Ltd (formerly Richard Lees) (UK), TechcreteLtd (UK and Ireland), Composite Structures (UK), British Precast ConcreteFederation, British Cement Association & Reinforced Concrete Council (UK),Betoni (Finland), Bevlon (Netherlands), C&CA of Australia, Cement Manufac-turers Association of Southern Africa, CIDB (Singapore), Andrew Curd and Part-ners (USA), Echo Prestress (South Africa), IBRACON (Brazil), Grupo CasteloAuthor visiting one of his structures in Malaysia 1998.x Preface(Spain), National Precast Concrete Association of Australia, Nordimpianti-Otm(Italy), Hume Industries (Malaysia), Prestressed Concrete Institute (USA), Spaen-com Betonfertigteile GmbH (Germany), Varioplus (Germany), Spancrete (USA),AB Stranbetong (Sweden), Tammer Elementti Oy (Finland); to his research assist-ants Wahid Omar, Ali Mahdi, Reza Adlparvar, Dennis Lam, Halil Gorgun, KevinPaine, Aziz Arshad, Adnan Altamimi, Basem Marmash and Marcelo Ferreira, andto his secretarial assistant Caroline Dolby.Preface xi Preface xiThis Page Intentionally Left Blank1 What is precast concrete?1.1 Why is precast different?What makes precast concrete different to other forms of concrete construction?After all, the concrete does not know it is precast, whether statically reinforced orpretensioned (=prestressed). It is only when we consider the role that this con-crete will play in developing structural characteristics that its precast backgroundbecomes significant. The most obvious definition for precast concrete is that it isconcrete which has been prepared for casting, cast and cured in a location whichis not its final destination. The distance travelled from the casting site may only bea few metres, where on-site precasting methods are used to avoid expensivehaulage (or VAT in some countries), or maybe thousands of kilometres, in thecase of high-value added productswhere manufacturing and haulagecosts are low. The grit blasted archi-tectural precast concrete in Figure1.1 was manufactured 600 km fromthe site, whereas the precast con-crete shown in Figure 1.2 travelledless than 60 m, having been castadjacent to the final building.What really distinguishes precastconcrete from cast in situ is its stressandstrainresponse toexternal (=loadinduced) and internal (=autogenousvolumetric changes) effects. Aprecastconcrete element is, by definition, ofa finite size and must therefore bejoined to other elements to form aFigure 1.1: Architectural-structural precast concrete structure(courtesy Trent Concrete, UK).complete structure. A simple bearing ledge will suffice, asshown in Figure 1.3. But when shrinkage, thermal, or loadinduced strains cause volumetric changes (and shorteningor lengthening) the two precast elements will try to moveapart (Figure 1.4a). Interface friction at the mating surfaceprevents movement, but in doing so creates a force F = jRFigure 1.2: Site cast precast concrete.Figure 1.3: Simple bearing nib.RFree shrinkage etc.lbFigure 1.4a: Unrestrained movement betweentwo precast concrete elements.F R = F R = Splitting cracksFigure 1.4b: Restrained movement but withouttensile stress prevention.2 Precast Concrete Structureswhich is capable of splitting both elements unless the section is suitably rein-forced (Figure 1.4b). Flexural rotations of the suspended element (=the beam)reduces the mating length lb (=bearing length) creating a stress concentration untillocal crushing at the top of the pillar (=the column) occurs, unless a bearingpad is used to prevent the stress concentration forming (Figure 1.4c). If thebearing is narrow, dispersal of stress from the interior to the exterior of the pillarRlbPossible spallingLarge shift inposition of RFlexural rotationRSmall shiftin position of RBearingpadFigure 1.4c: Reduced bearing length and stress concentrations due to flexural rotation.Lateralburstingcracksbpb b10PPrecastVVNNShear keys in registrationLarge compression fieldPShear keys at half pitchTension cracksSmall compression fieldFigure 8.17: Shear keys for shear resistance.Joints and connections 251sion P from external sources (post-tensioning for example), or a combination of bothsuch that:As =Vcot c P0.95fy8.14Failure is generally ductile due to the warnings given by concrete cracking in thisway. When the shear key effect decreases due to dislocation, the behaviourchanges to a frictional mode with considerable shear slip along the cracked inter-face. Also, as with shear friction methods, the interfaces should be prevented frommoving apart, either explicitly by external forces, or implicitly by placingreinforcement according to Eq. 8.13 across the shear plane. Providing that this isdone BS8110, Part 1, Clause 5.3.7c states that no shear reinforcement is required ifthe ultimate shear stress is less than 1.3 N/mm2, when calculated on the minimumroot area. If this value is exceeded transverse steel must be provided and theinterface shear capacity is based on the shear strength of the reinforcement alone.Example 8.7Figure 8.18 shows the detail of the castellations along a 3000 mm long 200 deepjoint between two precast units subjected to a shear force and possible normalcompression. At the ends of the units there is provision toplace transverse tie steel. Calculate the ultimate shear forcecapacity, the average ultimate shear stress, and the maxi-mum ultimate shear stress across the root of the castellationsif: (a) There is no tie steel, but the units are otherwise pre-vented from moving apart; (b) Tie steel comprises 2 no. T16bars at each end of the joint; and (c) Case (b) plus an externalcompressive stress of 0.25 N/mm2.Check the local compressive stress across the castellations.Use fcu = 25 N/mm2for the infill and fy = 460 N/mm2.Ignore dowel action.SolutionPitch of castellations = 200 mmNumber of castellations = 3000/200 = 15From geometry c = tan160/35 = 59.7 and x = 22 mm(a) tmax by code limitation = 1.3 N/mm2Root shear area = 70 200 15 = 210 000 mm2V = 1.3 210 000 103= 273 kNtave = (273 103)/(3000 200) = 0.45 N/mm2(b) Equation 8.4 with P = 0, As = 452 mm2V = 0.95 460 452 103/cot 59.7 = 338 kNtave = 0.56 N/mm2and tmax = 1.61 N/mm2Transverse the steel(at both ends)In situ infill15 20 20200 pitch130105010xFigure 8.18: Detail to Example 8.7252 Precast Concrete Structures(c) P = 0.25 3000 200 103= 150 kNV = 338 150/cot 59.7 = 585 kNtave = 0.98 N/mm2and tmax = 2.83 N/mm2Compression strut force per castellation = 59515 sin59.7 = 45.9 kNfc =45.9 103200 22 = 10.4 N/mm2< 0.6fcu = 15 N/mm28.4.4 Dowel actionWhere reinforcing bars, bolts, studs, etc., are placed across joints, shear forces maybe transmitted by so-called `dowel action' of the bars. In this context the bar iscalled a dowel. (This subject was introduced in Section 7.4.) Where it is used todetermine the shear capacity of a joint, dowel action acts alone, i.e. shear frictionand shear key effects are ignored. The `dowel' is loaded by a shear force acting inthe concrete in which the dowel is embedded, as shown in Figure 8.19a. Failure canoccur by local crushing of the concrete in front of the dowel, which may lead to anincrease in the bending arm of the embedded dowel, as shown in Figure 8.19b. Thismay lead to a plastic (=ductile) bending failure in the dowel a brittle shear failureis extremely unlikely unless the separation gap, w in Figure 8.19b, is kept small byexternal compression. The length of embedment should be the lesser of 30 doweldiameter c or 300 mm, including hooks and bends. Splitting reinforcement, typi-cally R8 loops, may be placed around the dowel to increase dowel resistance,although the code of practice does not recognize its presence in the followingequation. The shear capacity of a dowel which is loaded without eccentricity e(w 0 in Figure 8.19b) is given as:Vd = 0.6fyAs cos c 8.15If a dowel is loaded in shear and bending such that e c/8, bending action willcause yielding of the dowel somewhere along the embedded length. The resultantbearing stress of the concrete beneath the dowel has a maximum value of around2fcu. An empirical equation, which is not included in BS8110 but has been wellproven in tests,4gives the dowel capacity Vd as:Vd = 1.15 c0.67fcu12e20.95fyc20.67fcu 4e c0.67fcu 8.16Example 8.8Calculate the shear capacity of a 16 mm dowel embedded into a precast concreteelement. The dowel is connecting a steel section, 8 mm thick. The gap between thesteel section and the face of the concrete is 10 mm. Check the bearing capacity ofJoints and connections 253the dowel in the steel section if the edge distance to the hole is 50 mm. Usefcu = 40 N/mm2, fy = 460 N/mm2, fbs = 460 N/mm2.Solutione = 10 8/2 = 14 mmVd = (1.15 16 0.67 4012 1420.95 460 1620.67 40_(4 14 16 0.67 40) 103= 15.8 kN (using Eq. 8.16)Vd = 0.6 460 201 103= 55.5 kN (using Eq. 8.15)Perpendiculardowel barVdInclined dowel barCrack of width wVdVdVdCracking ontrailing sideCrushing on leading faceDiameter w(a)(b)eFigure 8.19: Dowel action for shear resistance.254 Precast Concrete StructuresBearing capacity of dowel.BS5950, Part 1, Clause 6.3.3.3.Pbs = 16 8 460 103= 58.9 kN or Pbs = 0.5 50 8 460 103= 92.0 kNLimiting capacity = 15.8 kN8.4.5 Mechanical shear devicesShear transfer may be achieved locally using mechanical shear joints. The designmust be very carefully considered because to ensure high shear stiffness the jointis made either by site-welding embedded plates, or by tightly clamping usingfriction-grip bolts. Thus, there is no inherent flexibility in a joint which cannottolerate out-of-plane forces. The most common form of mechanical connection isthe welded plate or bar shown in Figure 8.20. The effects of thermal expansion ofthe embedded plate must be considered to prevent cracking in the surroundingconcrete. A small slit (e.g. made by diamond tip wheel) at either end of the platewill suffice. Steel angle sections anchored with headed studs are often used. Thetop leg of the angle should contain air bleed hole(s). Bolted connections are rarelyused, except for friction-grip (or similar) bolts, because of the potential for slidingin the oversized hole reducing the initial stiffness. There is some difficulty inachieving the correct torque in every bolt in a bolt group owing to the flexibility ofthe embedded plate.Typical dimensions for the welded plate detail are 100 100 6 mm mild steelsite plate, and 150 75 10 mm mild steel embedded plates. Plates larger thanthis should contain air bleed holes to prevent air pockets forming. The holdingbars are typically T10 or T12, and are welded to the underside of the embeddedplate for a distance of 6070 mm. Cast-in angles are typically 75 50 6 rolledsection 100150 mm long. The studs are typically 100 mm long 10 or 12 mmdiameter headed studs, attached using the semi-automatic welding process.The ultimate shear capacity of the welded plate joint is the least of: (a) the pullout resistance of the embedded plate; (b) the weld capacity of the holding bars tothe embedded plate; or (c) the shear capacity of the intermediate plate or bar.The strength of the bar must be down-rated by a factor of 2 to allow for possibleeccentric bending due to the inclined position of the bar relative to the plate. Thisfactor of 2 assumes that the bar is welded as close to the start of the bend aspossible and that u and ~ 20. Referring to Figure 8.20 the pull-out capacity (a)is given as:V = n 0.95As 0.5fycos u cos 8.17where n is the number (typically 1) and u and are the inclinations (typically2030) of the holding bars to the horizontal and vertical. The embedment resist-ance of the plate itself is ignored.Joints and connections 255The weld capacity (b) of the holding bars is given as:V = npwlwtw 8.18where pw is the strength of the weld, taken as 215 N/mm2for grade E43 electrodesand mild steel bars, lw is the actual weld length 2tw, and tw is the throatthickness ( =weldsize/2_ ). The weld size is usually between 3 and 6 mm for barsupto 25 mm in diameter.The plate capacity (c) is determined as follows. The site weld is subject toa shear force V plus a horizontal moment M= Ve, where e is the distanceCast-in plateIntermediate sitebar or plateSite weldAnchor barsAnchor barsSite weld of leg lengthintermediate site platetwlwelwFigure 8.20: Welded plate for shear resistance.256 Precast Concrete Structuresbetween welds and is equal to the width of the intermediate plate (see Figure8.20). Shear deformations and bending of the plate are negligible. If the net lengthof the weld is lw and leg length tw, the maximum ultimate stress in the weld owis given as:ow = Vlwtw 4Vel2wtw< pw 8.19If ow = pw in the limit, thenV =pwtwlw1 4elw8.20The ultimate shear capacity of the studded angle joint is, in addition to the above,given by the shear capacity of the headed stud embedded in concrete. BS8110 (andmost other `concrete' codes) does not give data on this. It is found in BS5950, Part3.1, Table 5.Example 8.9Determine the ultimate shear capacity of the embedded plate joint shown inFigure 8.21.SolutionV = 10.952010.5250cos 20cos 20103= 21.1 kN (using Eq. 8.17)V = 1215(4/2_ )2(60 run-outs 12)103= 58.4 kN (using Eq. 8.18)V =215 4.24 88 1031 4 6088= 21.5 kN (using Eq. 8.20)Minimum shear resistance V = 21.1 kN8.5 Tension jointsLapping of reinforcement bars or loops is often used to connect precast membersas shown in Figure 8.22. The precast units have projecting bars, which areembedded in situ after erection. A full anchorage length is provided for theembedded bar, and this is calculated according to the same rules as in situconcrete. The projecting bars are usually hooked a full 180, as shown in Figure8.22a, otherwise the lap becomes unacceptably large. The length of the overlap is2r 3c c. If r = 8c, according to Eq. 9.32, then the overlap length is 20c. AllowJoints and connections 25720 mm clearance from the face of the precast unit to the tip of the loop. If the loopscannot nestle together ( =touching) the maximum distance, above one another,between the loop should be 4c to enable a compressive strut to form betweenneighbours. The transverse component of the diagonal strut must be resisted bytransverse bars which have a force of 0.2Ny, where Ny is the axial force in theloops. The transverse bars must themselves be anchored this often causesproblems in shallow joints where the loops are situated near to the bottom (ortop) of the section. In this case loops may be orientated in a direction perpendi-cular to the smaller dimension of the joint.20CFW to site plate150 40 10mmcast in plates100 60 6mmsite plateAnchor bars(ditto other side)Weld to cast in platePlanSection20Figure 8.21: Detail to Example 8.9.258 Precast Concrete StructuresWhere a loop of bend radius r is embedded over a length lp = 20c 20 mm, thepull-out resistance of the loop Ny is given by the following empirical equation(this does not appear in BS8110 but is validated in tests4):Ny = (1.2rlp0.7l2p)(0.6on1.1ft)orNy = 2 0.95fyAs 8.21where on is a normal stress and ft is the tensile strength of the concrete taken asft = 0.24fcu_ .Despite the full anchorage provided for the bars embedded in the precast and insitu, concrete bond stresses quickly break down close to the interface and the twohalves of the joint may be considered separately. The flexibility of each half of theinterface may be determined using the data in Table 8.36 this is not specified inBS8110 as this type of analysis is not a requirement for design. However, it enablesdesigners to calculate the total flexibility of a tension lap and determine crackwidths, etc.A tensile crack resulting from elastic deformation in the bar and slippage isformed in the interface and the joint's tension deformability may be calculated inthe same manner as for the compression joint. The main problem with verticallapping is to ensure that the in situ concrete forms a full and positive bond withthe steel bars. Pressurized grout is inserted through a hole beneath the level of thelap, and the appearance of the grout at a vent hole above the top of the lap is usedas an indication of complete filling as illustrated in Figure 8.22b. The annulusshould be at least 6 mm clear on all sides of the bars. The grout should be non-shrinkable and be sufficiently flowable to allow pressure grouting through a20 mm diameter nozzle using a manually powered hand pump. A 2:1 sand cementmix containing a proprietary expanding agent is used to give a 24-hour strength of20 N/mm2and a 28-day strength of around 60 N/mm2.Bolting is used extensively to transfer tensile and shear forces. Anchoragessuch as bolts, threaded sockets, rails or captive nuts attached to the rear of platesTable 8.3: Deformability `o of different joints in tension (mm/N) 105Type of joint Diameter (mm) of deformed bar Diameter (mm) of plain bar6 8 10 16 6 8 10 16Straight bar 4.0 2.5 2.0 1.3 5.2 3.2 2.6 1.7180 U-bar or loop 3.2 2.0 1.6 1.0 4.2 2.6 2.1 1.490 hooked end bar 6.4 4.0 3.2 2.1 8.4 5.2 4.2 2.8Joints and connections 259are anchored in the precast units. Tolerances are provided using over-sized orslotted holes in the connecting member. The tensile capacity of bolted connectionsshould be governed by the yield strength of the bolt, as this gives a ductile failure.In most types of bolted joints tension is accompanied by shear. Shear capaci-ties are governed by the local bearing strength of the concrete in contact withthe shank of the threaded socket. Shear bolt failures are brittle and should beavoided.Welding is used to connect elements through projecting bars, fully anchoredsteel plates or rolled steel sections, etc. The joint can be made directly between theprojecting plates or bars as shown in Figure 8.23a, but is more commonly madeNyNyNyNyR R31 2(behind the extreme loop)Istu2Ny2NyMax 4Transverse reinforcement(a)Figure 8.22a: Tension joint using direct lapping loops.260 Precast Concrete Structuresindirectly using an intermediate bar or plate. Figure 8.23b gives some guidance asto sizes, and Table 8.4 is used to obtain the relationship between bar diameter andweld size.DuctDe-aerationGroutProtruding bar Mortar jointlstu(b)Figure 8.22b: Tension joint using bond resistance.(a)Figure 8.23a: Welded connection between rebar and plate.Joints and connections 261Post-tensioning is used to resist tension and shear forces by the application ofclamping forces across the joint. Cable ducts are inserted into the precast concreteelements, or in the spaces around the elements, and, after erection, the cables areplaced in the ducts and post-tensioned. Tensile capacities are computed from thestate of stress in the post-tensioned elements, and shear resistance is calculatedusing the shear friction hypothesis.Example 8.10Calculate the pull-out resistance and displacement at maximum force of a 10 mmdiameter 180 loop embedded into a cast in situ joint. All external forces on thejoint may be ignored.Use fcu = 25 N/mm2, fy = 460 N/mm2.Flare-bevel-groove welds++twtw= 0.2dbtw= 0.2dbdblw(b)Figure 8.23b: Dimensions for welded rebar to plate or angle.Table 8.4: Minimum weld lengths to develop full strength in lapped barsBar diameter (mm) Weld depth (mm) Weld length (mm) Nominal length (mm)12 3 25 2516 4 33 3520 5 42 4525 6 52 55Note:fy = 250 N/mm2; ms = 1.15; pweld = 215 N/mm2for grade E43 electrode.262 Precast Concrete StructuresSolution(a) Strengthr = 8 10 = 80 mmlp = 10 80 30 80 10 20 = 230 mmft = 0.24 25_ = 1.2 N/mm2Ny = (1.2 80 230 0.7 2302) 1.1 1.2 103= 78.0 kN (using Eq. 8.21)Ny = 2 0.95 460 78.5 103= 68.6 kNResistance = 68.6 kN, i.e. the bars will yield first.(b) FlexibilityDeformability coefficient = 1.6 105mm/N (from Table 8.3)Ultimate force in bars = 68.6 kNc = 1.6 10568.6 103= 1.1 mm8.6 Pinned-jointed connectionsPinned connections are used extensively in precast structures as they may beformed in the simplest manner by element-to-element bearing. The very natureof precast construction lends itself to forming simply supported connections inorder to avoid flexural continuity across the ends of individual elements. For thisreason they are often referred to as `joints' as they tend to involve one bearingsurface only. Although they are much criticized by adventurous engineersattempting to create structural frame continuity, pinned connections are seen bymost precast manufacturers as safe and economical. In many cases, for example,in prestressed beams, there is no advantage in providing continuity of hoggingmoment as the beam is already fully stressed in that mode. In other cases, suchas hollow core floors, there is no provision for continuity reinforcement and veryspecial construction details have to be contrived. However, this is not to say thatpinned connections should only be considered each connection should bejudged on its merits bearing in mind all manufacturing and erection aspects.Pinnedconnections transfer shear andaxial forces, bothfor the (dominant) gravityforces and possible uplifting forces due to overturning. By definition they cannottransfer moment and torsion, although in reality there is no such thing as a pinnedjoint. Small moments of resistance may develop due to the interlocking effects ofinfill grouting, shear friction, etc., but these will be rapidly exceeded in service, thusrendering the connection `pinned'. Recent research has attempted to harness thisstiffness and strength and develop design methods for semi-rigid connections (Ref.[3.3]),7,8,9but the results have been largely ignored by precast frame designers.Joints and connections 263Pinned connections occur mainly where vertical and horizontal elements arejoined. Figure 8.24 shows where pinned connections may be used in a skeletalframe and explains the primary function of pinned connections. They shouldallow for the rotation of the bearing element without spalling or cracking to eitherof the joining elements. The connection should also enable relative horizontalmovement without initiating large interface forces which cannot be resisted byreinforcement (or similar) in the elements. The dimensions of the connectionshould allow for construction tolerances without jeopardizing the strength of theconnection and/or encroaching into the cover concrete at the edges of elements.Therefore, the detailing of the interface and of the contact/end zones in both thesupported and supporting elements is important.8.6.1 Pinned connections between vertical and horizontalelementsThe most common situation occurs at the top of a column where one or two beamsbear directly as shown in Figure 8.2510. The bearing may be made either dry or bywet mortar bedding the former being preferred to speed up erection. If a bearingFloor slab to beamColumn to column(braced frame only)Beam to column headBeam to column faceColumn to foundation(braced frame only)Flexural rotationsColumn headNo tensionNo tensionSlab-to-beamFigure 8.24: Positions of pinned-jointed connections in skeletal structures.264 Precast Concrete Structurespad is used in accordance with Section 8.3.1 the net concrete bearing stresscalculated on the area of the pad should not exceed 0.4fcu, where fcu is for theweaker concrete element. Bearing pads are typically 100 mm to 150 mmsquare 6 mm thickness. Wet bedding area is usually equal to the area of theinterface and therefore the limiting stress is 0.6 f/cu, where f/cu is the strength of themortar. Bedding thickness is harder to control but is usually around 35 mm.a amortarSection a-aaMortara abLoopsFigure 8.25: Reinforcement details for beam-to-column head connection.Joints and connections 265Some form of mechanical joint is provided, e.g. projecting dowel(s), for hori-zontal restraint. Corresponding holes in the beam are site filled using flowablegrout. The shear capacity of the dowels should be capable of resisting horizontalforces due to friction, jV, as defined in Section 8.3.3. Horizontal reinforcement ofarea As is placed around the dowel hole in the form of small diameter loops, suchthat As = jV/0.95fy. The bending strength of the dowel should resist any over-turning moment due to construction eccentricities, e.g. unequal loading on beams.In beam design, reinforcement is provided to resist tension across an obliqueshear crack at 45 to the axis of the beam, commencing at the edge of the bearing, asshown in Figure 8.2610. The ultimate force in the bottom reinforcement is given as:Fh = Vtan0jV 8.22The bars resisting this force must extend a full bond length beyond the face of thesupport. The bar may be formed into a horizontal loop providing that the internalradius of the bar is according to Eq. 9.32, and the start of the loop does notcommence beyond the face of the support.This bond length is not possible in prestressed beams where the pretensioningsteel is cut off at the end of the beam. However, compressive forces are exerteddue to the pretensioning steel such that the bottom of the beam is in a state ofNsuFhuChamferFvuFvuFhuNsuFigure 8.26: Structural mechanism at reinforced beam end connection.266 Precast Concrete Structurescompression. Even though the full prestressing force may not develop at thispoint it is not possible for the oblique crack to develop in this region. AlthoughBS8110 offers no guidance as to this design, it is found by testing that thetransmission length is sufficiently developed to act as a bond length, and thatno additional anchorage reinforcement is necessary. This design must not beconfused with ultimate shear where vertical shear links (or inclined bars) arerequired.The vertical force V gives rise to a lat-eral horizontal bursting force Hbst = V,where is obtained from Table 8.1. Hor-izontal hairpins are provided across theend face of the beam such thatAbst = Hbst0.95fy8.238.6.2 Simply supportedslabs on beams or wallsSlab connections using hollow core anddouble-tee floors are designed as simplesupports despite the presence of rein-forced in situ concrete strips cast in theends of the units. Hollow core floors areusually laid dry directly onto the shelfprovided by the boot of the beam, butneoprene bearing pads or (less fre-quently) wet bedding onto grout is alsoused in certain circumstances, e.g. dou-ble-tee floor slabs. Wet bedded bearingsare sometimes used on refurbishedbeams with uneven surfaces.Hollow core units are laid directlyonto a dry precast beam seating. (Figure8.27). The design ultimate bearing stress0.4fcu is rarely critical. A nominal bear-ing length of 75 mm results in a netlength of 60 mm after spalling allowan-ces have been deducted. Rigid neoprenestrips or wet mortar bedding, whichhave been used in special circumstances,Longitudinal tieDirect anchorageProjecting tie-bar from beamL-shaped tie-barsinto floor unitsDowel action between loops and barsProjecting loop from beamProjecting loops from beamDowel action between loopsLoop tie-bars into floor units(a) Hollow-core unitsFigure 8.27: Floor slab to external beam connections usinghollow core slabs.Joints and connections 267e.g. in refurbished buildings or on masonrybearings, ensure that a uniform bearing ismade between them. Openings are made inthe top flanges of the units (during manufac-ture) to permit the placement of structural(grade C25 minimum) concrete on site. Con-tinuity of reinforcement is achieved either bydirect anchorage between the precast beamand in situ strips, dowel action betweenloops, or between loops and other bars. Barsmay be placed in the longitudinal gapsbetween the slabs providing that the widthof the gap is at least twice the size of the baror more than 30 mm (Ref. 4.5). The lengthof embedment is taken as the greater of oneanchorage bond length or the equivalent of the transfer length of the prestressingforce in the precast unit.A welded connection is made at the end of double-tee floors, as shown in Figure8.28. (See also Figure 5.2 where a steel plate is waiting to be welded to a plate inthe ends of double-tee slabs.) These bearings require greater considerationbecause it is vital that the loads should be equally shared between the four bearingpoints. A nominal bearing length of 150 mm minimum is recommended and theunits should always be seated on rigid 100 100 mm neoprene (or similar) pads ofabout 6 mm to 10 mm thickness. No design guidance is given in BS8110, butaccording to the PCI Manual,1which considers a potential shear failure crackinclined at 2025 to the vertical free edge of the bearing, the minimum bearingcapacity (based on a design stress of 7 N/mm2for neoprene) is in the order of170 kN/m length of bearing. This is rarely critical.8.7 Moment resisting connections8.7.1 Design philosophy for moment resisting connectionsA moment resisiting connection (MRC) is capable of transferring, to somedegree, in-plane bending moments. Although torsional moments ( =out-of-planemoments) often accompany bending moments, this book does not address tor-sional connections torsion requires very specialized considerations such that itis difficult to generalize the approach in a book such as this. The basic concept ofa MRC is shown in Figure 8.29. Continuity of moment is effected by the transfer ofa couple of axial forces. Because precast connections are usually erected as pinnedjoints, it is only end moments from imposed loads which are carried by the MRC.Bar welded to plates fully anchored in unitsBearing padb) Double-tee unitsFigure 8.28: Floor slab to external beam connectionsusing double-tee slabs.268 Precast Concrete StructuresGreat care must be taken in detailing and constructing connections to be momentresisting, and the site operative should not be given the choice of whether to insertor not a vital element required to make the connection. For example, if additionalbolts to those required for temporary stability are necessary to form a MRC, it ispossible the site operative may omit them or insert under strength bolts. A similarsituation arises where welding is required to form an MRC.The locations where such connections may be made are summarized in Figure8.30. These connections are used mainly to:1 Stabilize and to increase the stiffness of portal and skeletal frames;2 Reduce the depth of flexural frame members;3 Distribute second order moments into beams and slabs, and hence reducecolumn moments; and4 Improve resistance to progressive collapse.LapProjecting barsfrom beamSelf weight (gravity)In situ infillConnecting dowelProjecting loopsImposed loadsTensionCompressionMomentFigure 8.29: Principles of moment resisting connections.Joints and connections 269Not all precast frames lend themselves to having moment resisting connections.They may be considered only where there are a sufficient number of columnssharing the loads. It is also expected that the foundation will be encastre, whichmay not always be possible, especially on recycled land. The sway stiffness of theframe should not violate the drift criterion (usually sway deflection