Failures in Concrete Structures - · PDF fileRobin Whittle Failures in ConCrete struCtures...
Transcript of Failures in Concrete Structures - · PDF fileRobin Whittle Failures in ConCrete struCtures...
Robin Whittle
Failures in ConCrete struCturesCase Studies in Reinforced and Prestressed Concrete
A S P O N B O O K
ISBN: 978-0-415-56701-5
9 780415 567015
9 0 0 0 0
Y106141
Cover image: © Dr Jonathan G M Wood
STRUCTURAL ENGINEERING
“The book provides a fascinating insight into what can go wrong. It illus-trates the importance of approaching problems from first principles and identifying the key load paths, loads, etc. This type of material rarely sees the light of day for reasons of litigation and commercial confidentiality.” —Robert Vollum, Imperial College, London
“This book is essential reading for any experienced engineer and of-fers insight into how designers and builders accidentally create struc-tures that fail. This book both explains how things fall down and what was done or could have been done to overcome these errors. Learning from errors is an essential part of any engineer’s knowledge and expertise.” —David M. Scott, Laing O’Rourke, Connecticut
Failures in Concrete Structures: Case Studies in Reinforced and Prestressed Concrete presents a selection of the author’s firsthand experience with incidents related to reinforced and prestressed concrete structures. He includes mistakes discovered at the design stage, ones that led to failures, and some that involved partial structure collapse—all of which required remedial action to ensure safety. The book focuses on specific incidents that occurred at various points in the construction process, including mistakes related to structural misunderstanding, extrapolation of codes of practice, poor detailing, poor construction, and deliberate malpractice. The author provides this information for readers to gain an understanding of errors that can occur in order to avoid making them. The book also examines issues during the procurement process, as well as the importance of research and development to gain a deeper knowledge of materials and help prevent future failures.
Failures in ConCrete struCturesCase Studies in Reinforced and Prestressed Concrete
Y106141_Cover_mech.indd 1 8/24/12 2:11 PM
Case Studies in Reinforced and Prestressed Concrete
FAILURES IN CONCRETE STRUCTURES
Case Studies in Reinforced and Prestressed Concrete
Failures in ConCrete struCtures
Robin Whittle
A S P O N B O O K
CRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742
© 2013 by Robin WhittleCRC Press is an imprint of Taylor & Francis Group, an Informa business
No claim to original U.S. Government worksVersion Date: 20120815
International Standard Book Number-13: 978-0-203-86127-1 (eBook - PDF)
This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint.
Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information stor-age or retrieval system, without written permission from the publishers.
For permission to photocopy or use material electronically from this work, please access www.copy-right.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that pro-vides licenses and registration for a variety of users. For organizations that have been granted a pho-tocopy license by the CCC, a separate system of payment has been arranged.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.
Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.com
and the CRC Press Web site athttp://www.crcpress.com
v
Contents
Foreword ixAcknowledgements xiIntroduction xiii
1 FailuresduetoDesignErrors 1
1.1 EdgeBeamandColumnConnection 21.2 ConcreteTruss 51.3 CircularRampstoCarPark 81.4 TransferBeamwithEccentricLoading 101.5 EarlyThermalEffects 101.6 SecondaryEffectsofPrestressing 141.7 TemperatureEffectsonLong-SpanHybridStructure 161.8 LoadingforFlatSlabAnalysis 181.9 PrecastConcreteCarPark 191.10 ArchFloor 211.11 PrecastConcreteStairflights 211.12 ShearStudsonSteelColumntoSupportConcreteSlab 241.13 PiledRaftforTowerBlock 261.14 FloatingPontoonforResidentialBuilding 271.15 PrecastColumnJointDetail 27
2 ProblemsandFailuresduetoErrorsinStructuralModelling 31
2.1 ReinforcedConcreteTransferTruss 312.2 ModellingRigidLinks 312.3 AssessingModelLimitsandLimitations 322.4 EmpiricalMethods 342.5 InitialSizingofSlabs 352.6 AnalysisofFlatSlabswithFiniteElementPrograms 352.7 ScaleEffects 37
vi Contents
3 FailuresduetoInappropriateExtrapolationofCodeofPracticeClauses 39
3.1 CoolingTowers 393.2 DesignBendingMoments 413.3 PileswithHighStrengthReinforcement 423.4 ShearCapacityofDeepSections 43
4 FailuresduetoMisuseofCodeofPracticeClauses 47
4.1 FlatSlabandTwo-WaySlabBehaviour 474.2 RibbedSlabSupportedonBroadBeam 484.3 CarParkColumns 50
5 ProblemsandFailuresduetoInadequateAssessmentofCriticalForcePaths 53
5.1 HeavilyLoadedNibs 535.2 ShearWallwithHolesandCornerSupports 535.3 DesignofBootNibs 56
6 ProblemsandFailuresduetoPoorDetailing 57
6.1 ConcreteOffshorePlatform 576.2 AssemblyHallRoof 606.3 UniversityBuildingRoof 626.4 MinimumReinforcementandCracking 646.5 PrecastConcretePanelBuilding 656.6 Footbridge 67
7 ProblemsandFailuresduetoInadequateUnderstandingofMaterials’Properties 69
7.1 ChangesoverTime 697.2 RebendingofReinforcement 717.3 TackWeldingofReinforcement 737.4 HighAluminaCement 737.5 CalciumChloride 747.6 Alkali–SilicaReaction 757.7 LightweightAggregateConcrete 77
8 ProblemsandFailuresduetoPoorConstruction 79
8.1 FlatSlabConstructionforHotel 79
Contents vii
8.2 SteelPilesSupportingBlockofFlats 818.3 ShearCracksinPrecastTUnits 818.4 CantileverBalconiestoBlockofFlats 828.5 PrecastConcreteTank 838.6 CarPark 858.7 CrackingofOffshorePlatformduringConstruction 878.8 SpallingofLoadBearingMullions 908.9 Two-WaySpanningSlab 928.10 ChimneyFlueforCoal-FiredPowerStation 93
9 ProblemsandFailuresduetoPoorManagement 97
9.1 Column–SlabJoint 979.2 PlacingofPrecastUnits 1009.3 WeakAggregateConcreteinChimney 101
10 ProblemsandFailuresduetoPoorConstructionPlanning 103
10.1 PowerStationonRiverThames 10310.2 TowerBlock 106
11 ProblemsandFailuresduetoDeliberateMalpractice 111
11.1 FloorwithExcessiveDeflection 11111.2 PilesforLargeStructure 11311.3 InSituColumnsSupportingPrecastBuilding 113
12 ProblemsArisingfromtheProcurementProcess 117
12.1 EffectsofDifferentFormsofContracts 11712.2 Workmanship 11812.3 CheckingConstruction 119
13 ContributionsofResearchandDevelopmenttowardAvoidanceofFailures 121
13.1 LinksbetweenPracticeandResearch 12113.2 FlatSlabBehaviour 12113.3 SpanandEffectiveDepthRatiosforSlabs 12213.4 BeamandColumnJoints 12213.5 TensionStiffeningofConcrete 123
References 127
ix
Foreword
Errarehumanumest.…Westructuralengineersarehumanandsohavemadeanumberoferrorsovertheyearsresultinginnarrowescapes,badlyperformingstructures,andevenfatalcollapses.ButasSenecacontinues…sedperseverarediabolicum,wemustnotrepeatourerrors.
Toavoidthismeansthatwemustlearnfromourpastmistakes;wemustknowwhatwentwrongandwhy.Someofthelessonsfromourpasterrorsget embodied in clauses in codes of practice, butmanydo not, and thecollectivememoryoftheprofessiontendstofadeasthegenerationofengi-neerswholearntfromthemishapsandcatastrophesretires.
Pastbooksonthesubjectofstructuralfailurestendedtodealwiththegeneralcausesoffailuresandmethodsofinvestigation,illustratedwiththemorespectacularexamples.However,detailsofsomefailuresthathavenotmadetheheadlines,butneverthelessholdimportant lessons,arehardtofindormaynotevenbeinthepublicdomain.
In thepast,RobinWhittle and Iworked together atArupR&Donavarietyofproblemsofconcretestructures.Someofthesearosefromfail-ures,andotherswereencounteredwhenforestallingundesirableoutcomesof the enthusiasm—untempered by experience—of some of our youngercolleagues.
RobinwasalsoinclosecontactwithresearchersatthenowsadlydefunctCement&ConcreteAssociation,thePolytechnicofCentralLondon,andtheuniversitiesofLeeds,Durham,andBirmingham,andsowasprivytomuchof thebackground for the initialdraftandsubsequent revisionsofCP110.
Nowadays,apreoccupationwiththeever-multiplyingminutiaeofcodes,whetherEuroCommunityorNational,canblinddesignerstotheimpera-tives of first principles. New patterns of procurement and site manage-mentalsowiden thecommunicationgapbetweendesignandexecution,and exert pressures to adopt shortcuts that sometimes have unforeseenconsequences.
x Foreword
Withhisbackground,Robiniswellplacedtopresentaselectionofcasestudiesthathavelessonsforallofus.Thisisabookthatshouldbereadbythosestructuralengineerswhowishtobroadentheirknowledgebylearn-ingfromsomeoftheexperiencesofthelast50yearsandalsoforthosewhowouldliketorefreshtheirmemories.
PoulBeckmann,F.I.Struct.E.,M.I.C.E.,M.I.D.A.,Hon.F.R.I.B.A.
xi
Acknowledgements
TheauthorwouldparticularlyliketothankPoulBeckmannforhiscon-tributiontothisbook.Hehasprovidedthedescriptionsofseveraloftheincidentsandhiscommentsontherestofthetexthavebeengreatlyvalued.
Thecontributionsfromthefollowingpeoplearealsogratefullyacknowl-edged.
IanFelthamTerryBellAlfPerryGeoffPeattieTonyStevensTonyJonesChrisKenyonJohnHirstBrynBird
SteveMcKechnieHowardTaylorRobertVollumRichardScottAndrewBeebyRobKinchGillBrazierAndrewFraser
xiii
Introduction
Thisbookisapersonalselectionofincidentsthathaveoccurredrelatedtoreinforcedandprestressedconcretestructures.Notallhaveledtofailuresandsomeofthemistakeswerediscoveredatthedesignstage.Eachincidentrequiredsome formof remedialaction toensure safetyof the structure.Someoftheincidentswerecausedbymistakesindesignorconstructionorboth.Someinvolvedcollapseofpartofthestructure,butinsuchcasesthecausewasfrommorethanoneunrelatedmistakeorproblem.Afewoftheerrorsandincidentswerecausedbydeliberateintent.
Chapters1to11describespecificincidentssuchasstructuralmisunder-standing,extrapolationofcodesofpractice,detailing,poorconstruction,andotherfactors.Whenaparticularincidentinvolvedmorethanoneofthesecauses,itisdescribedinthemostrelevantsection.Chapters12and13discussissuesrelatedtoprocurementandresearchanddevelopment.
Carehasbeen takennot toname theparticularprojects inwhich theincidents occurred, and the intention in providing the information is toensurethatsuchmistakescanbeunderstoodandavoidedinthefuture.
Someoftheproblemswerediscoveredinassociationwithrequestsforsupportaboutadifferenttopic.Intryingtodiscoverthedetailsoftheprob-lemitbecameclearthatothermoreseriousissueswereatstake.Thisbegsthequestion,howmanyunresolvedproblemsandmistakesareouttherethat have not seen the light of day? It is fortunate that most reinforcedandprestressedconcretestructuresareindeterminateandallowalternativeloadpathstoformandpreventfailuresthatwerenotforeseeninthedesign.
Thereisaworryingtrendinboththedesignandconstructionofbuildingstructures.Materialstrengthsofbothconcreteandsteelhavecontinuedtoincreaseforthepastcentury.Atthesametime,theoverallsafetyfactorusedindesignhasbeenreducing.Thistrendislargelycausedbythepressuretoreducecosts.Inevitablythetimewillcomewhentheeffectofthistrendwillmeanthattheerrorsmadeindesignandconstructionwillnotbeabsorbedbytheglobalsafetyfactorortheabilityofastructuretofindalternativepathsfortheloads.Thisincreaseinriskisnothelpedbythechangesinthe
xiv Introduction
formofcontractsandcontractualprocedure.Healthandsafetyclausesdonotprovide,intheauthor’sopinion,thenecessarysafetynets.
Inordertoimprovethequalityofallaspectsofdesignandconstruc-tion,itisessentialtoinvolvepeoplewhoknowwhattheyaredoing.Moretrainingisrequiredattheoperationallevelsofbothdesignandconstruc-tion. More research and scientific development are required to allowbetterunderstandingofthematerialsandtheiruses.Butthemoresophis-ticatedthematerialsandtoolsbecome,themoreintelligenceisrequiredfortheircontrol.
1
Chapter 1
Failures due to Design Errors
Manyfailures,wheninvestigated,havebeenfoundtoarisefromacombi-nationofcauses.Thetraditionaldesignsequencestartswiththesizingofmembers.Thesearedeterminedfromtheloading(permanentandvariableactions)withreferencetobendingmomentsand,forbeams,shearforces.Thereinforcementisthencalculatedtocaterfortheseforces.Muchofthereinforcementisdetailedonlyaftercompletionofthecontractdocuments.Later,ifproblemsarefoundinfittingtherequiredreinforcementintoanelementor joint, it isdifficulttochangethesizeofsectiononwhichthearchitectandservicesengineersagreed.Manysuchproblemscouldhavebeenavoidedbyproducingsketchesearlyontoshowhowthejointdetailscouldworkbeforesizeswerefinalised.
Computer software is commonly used to provide design information.Thiscannotalwaysbetailoredtosuittheproblemexactly.Alltoooftenadesignerfailstoensure,withmanualchecks,thatthesoftwareprovidesareasonablesolutionfortheparticulardesign.Theeffectsofcreep,shrink-age,andtemperatureareoftennotconsideredbythesoftware,andmanualcalculationsarerequiredtocheckwhethereffectsaresignificant.
Robustnesshasbecomeanimportantconsiderationindesignandthisiscloselylinkedtothedetailingofjoints.Theelementsofaninsitustructurearemechanicallyconnectedtogetherbynormaldetailingandthisshouldprovidesufficientrobustness.However,forhybridstructurescontainingamixtureofprecastandinsituconcrete,muchmorethoughtisrequiredtoobtainreliablejointdetails.
Acode-of-practicementalitycan inhibitaholisticapproach todesign.Incodes,thewholeisbrokendownintopartsthatareanalysedseparately.Theresultisasafestructurebutnotoneinwhichthestrainsandstresseshavemuchsemblancetothecalculatedones.Buildingsaredesignedfortheloadsexpectedtobeapplied,butrarelyforthestrainscausedbyshrinkage,creep,andtemperatureeffectsontheconcrete.
Design-buildcontractshavemeantthatmoreconsiderationofthecon-structionmethodsisgivenatthetimeofdesign.Thishasoftenledtomoreefficient construction (faster and/or cheaper). Unfortunately it has also
2 Failures in Concrete Structures
meantthatthedesignprocessisdominatedbythedemandsofspeedandcost.Sometimesthishasledtoadesignernottoconsideralltheimportanteffectson thefinal structure.Thedesignof inadequatemovement jointsinbuildings(e.g.,carparks)isanexamplethathassometimesbeencom-poundedbypoorworkmanship.
1.1 EDGE BEAM AND COLUMN CONNECTION
Acollapseoccurred involving the connectionofaheavy concrete gutter(1mwide)tothesupportingcolumns.Thestructureconsistedofaseriesofreinforcedconcreteedgecolumnssupportingsteeltrussesthatspannedthewidthof thebuilding.Edgebeamsspannedbetween thecolumns tosupport the concrete gutter. Figure 1.1 shows the collapseof the gutter.Althoughtheedgebeamhadbeendesignedanddetailedtoresistthefullload fromthegutter (bendingmoment, shear,and torsion), thishadnotbeencarriedthroughtothejoint.Figure 1.2showswheretheedgebeamjuststartedtotearawayfromthecolumnandFigure 1.3showsthetopofacolumnwheretheedgebeamhasseparatedandfallenoff.
Analysis—Figure 1.4showsafailuremodelbasedonthetensionstrengthoftheconcrete.Evenifthelinksthathadbeendetailedtopassthroughthejointhadbeenconstructedthisway,theywouldhavebeeninadequatetosupporttheloading.
Figure 1.1 Collapse of concrete gutter.
Failures due to Design Errors 3
Areasonablehandcalculationtochecktheresistanceistoassumethatthe tensile strengthof concrete is abouta tenthof the cube strength. Inthiscase,theconcretestrengthwas20MPasothetensilestrengthmaybetakenas2MPa(nosafetyfactorsapplied).Figure 1.4showshowthetensilestrengthoftheconcreteprovidesthetorsionalresistance.
Figure 1.2 Edge beam starting to sepa-rate from column.
Figure 1.3 Column where edge beam has broken off.
Cracks form
Gutter slab
Column
Compression
T1Appliedtorque
T2
Tension
Tension
Figure 1.4 Failure model relying on tension strength of concrete.
4 Failures in Concrete Structures
Spacingofcolumns 7.0mSupportingbeamheight,hb 0.6mSupportingbeamwidth,bb 0.3mWidthofcolumnhead 0.6mWidthofbeamsupport 0.1mTorquefromselfweightofgutter 25×7×(0.2×1×(0.9–0.3/2)+0.2 ×(0.8–0.3)×(0.8–0.3)/2+0.3/2) =56.88kNmDensityofsandandbricks 18kN/m3
Volumeofsand 1m3
Loadfromsand 18kNVolumeofbricks 0.6m3
Loadfrombricks 10.8kNTorquefromsandandbricks (18+10.8)×(0.8–0.3/2)=18.72kNm
Totalappliedtorque(unfactored) =56.88+18.72=75.6kNm
T1=0.5×2×0.6/2×0.6×1000 =180kNTorqueresistanceofT1=180×0.6×2/3 =72kNmT2=0.5×2×0.3/(2×3)×0.6×1000 =30kNTorqueresistanceofT2=30×0.1×2/3 =2kNm
Totalresistancefromtensileconcrete =72+2=74kNm
Applied load exceeds resistance
Detailing — The reinforcement for the edge beam–column joint wasbuiltasshowninFigure 1.5.Thetoptwolinksweredetailedtoenclose
T10s@100
T20s
2T8s2T20s
2T16s
Steel truss.
T8 links@130 T8 links @ 300 (stopped off at beam cage)
T8 links @ 300
Figure 1.5 Reinforcement detailing for edge beam and column joint.
Failures due to Design Errors 5
allthecolumnmainbars.Evenifthelinksthathadbeendetailedtopassthrough the joint had been constructed this way, they would have beeninadequatetosupporttheloading.Ifthedetailhadbeendrawntoalargescale it would have become apparent that the column links would havebeendifficulttofitthroughthebeamcage,andthisshouldhavetriggeredconcernabouttheconnection.
Construction—The fabricator reduced thedimensionof the top twolinks(seeFigure 1.5)inordertoeasethedifficultiesofconstruction.
Thecontractorhadbeenusingtheguttertopilebricksandsand.Thisloadexceededthedesignloadandfailureoccurred.
Comment — Although the edge beam had been designed and detailedto resist the full load from thegutter (moment, shear, and torsion), thishadnotbeencarriedthroughtothejoint.Thejointreliedonthetensionstrengthoftheconcretewhichwasnotsufficient.Thiswasaseriousdesignerror,butitisunlikelythatthefailurewouldhaveoccurredifthegutterhadbeensubjecttojusttheloadfromrain.Ifthedetailingandconstruc-tionhadbeencarriedoutthoroughlyandcorrectlyitisunlikelythatthecollapsewouldhaveoccurred.
The collapse resulted from combined errors in design, detailing, and construction.
1.2 CONCRETE TRUSS
Aprojectteamrequestedasecondopinionaboutthedesignofthebottomboom of a 19 m span reinforced concrete truss. The project team con-sidered that the tensionwould causeunsightly cracking.Adrawing (seeFigure 1.6showingthedetailofpartofthetruss)andtheanalysisofthetrusswereprovided.Muchattentionhadbeengiventothisdesignandspe-cialcarehadbeentakentoensurethatthestressesintheconcretewerelow.
The requestwas to check thedesignof thebottomboom.This checkwascarriedoutanditbecameapparentthattheshearstrengthoftheendverticalpostconnecting the topandbottombooms (seeFigure 1.7)wasinadequate.Thesectiondimensionswereb=230mmandd=460mm.
fy =460MPaAppliedshearforce,VE =488kNDesigntoBS81101(Cl3.4.5.2),fcu =25MPa
Maximumconcretestrutshearcapacity,Vmax=230×460×0.8×√25/1000 =423kN(notOK)
DesigntoBSEN1992(EC2)2(Cl6.2.3):fcd =13.33MPaαcw =1
6 Failures in Concrete Structures
Figure 1.6 Detail of part of truss.
Compression
Tension
Shear failure
Figure 1.7 Shear in vertical post at end of truss.
Failures due to Design Errors 7
z =0.9dν1 =ν=0.552cotθ=1
Maximumshearcapacity,VRd,max=1×230×0.9×460×0.552×13.33/(2×1000)=350kN(notOK)
Itwasalsoapparentthatthedetailingrequiredsomechanges.Figure 1.6showsthemainT32bars,drawnasiftheycouldbebentwithsharpcornerswheninfacttheminimuminnerradiusofbendrequired(3.5×diameterofbar)is112mm.Whendrawntoscale,itbecameclearthattheconcretewouldfailintwoplacesasshowninFigure 1.8.
Failureofconcreteat support (1 inFigure 1.8)—Thesupportwidthwasonly150mmandthedrawingdidnotshowanyreinforcementintheconnectiontothetruss.Ifreinforcementwasintendedinthejoint,thenitwouldbedifficulttofitthisbetweenthereinforcementofthetruss.Ifnot,theeffectofthecoverandtheradiusofbendoftheT32barsatthecornerofthetrusswouldleadtospallingoftheconcreteandthesupportwouldfail.
Failureofconcreteatinnercorner(2inFigure 1.8)—Sincethebottomboomwasatensionmember,thetopreinforcementinitwouldbeinten-sion.Thewaythatitwasdetailedmeantthatacrackintheconcretewoulddevelopontheinsideofthebendwhereitjoinedtheverticalpostattheendofthetruss.
Section through bottom boom
230
600
1. Spalling of concrete
Compression
Tension
2. Cracking of concrete
Figure 1.8 Concrete failure points.
8 Failures in Concrete Structures
Comment—Theseproblemswerenoticedwhilstcheckingsomethingquitedifferent.Forthedesignstrengthofconcrete,theshearreinforcementwasinsufficientandtheconcretestrutstrengthinadequate.Ifthecubestrengthhadbeenincreasedfrom25to35MPa,thesectionwouldhavebeenstrongenough.
Theshapeofthetrusswasinherentlyunsuitable;thecentrelinesoftherafterandthetiedidnotintersectoverthesupport.Itmayhaveledtodif-ficultdetailingiftheyhaddoneso,buttherewouldhavebeenagoodsolid‘lump’ofconcretetotaketheshear.
Ifthedetailhadbeendrawntoalargescalethedetailingproblemswouldhavebecomeobviousand simple changes to thedesign couldhavebeenmadetoavoidoverstressingoftheconcrete.
1.3 CIRCULAR RAMPS TO CAR PARK
The construction of this car park (see Figure 1.9) was nearly complete.Theparapet and slabof the circular rampswere supportedon two col-umnsdiametricallyoppositeeachother.Thetorsionandcantilevereffectsoftheloadscausedwidecracksinthesupportingcolumns.Althoughthesecolumns were damaged with shear cracks, it was considered that if thecantileverandtorsionaleffectsfromtherampcouldbereduced,thenthecolumnscouldberepairedwithoutriskthatthecrackswouldreturn.Thiswasachievedbyinsertingsteelcircularcolumnshalfwayaroundateverylevelthroughouttheheightofthestructure(seeFigure 1.10).Inordertoensurethatthesenewcolumnswouldtakethecorrectload,flatjackswereintroducedateachfloorlevel(seeFigure 1.11).
Figure 1.9 Car park with circular ramps.
Failures due to Design Errors 9
Figure 1.10 Additional steel columns.
Figure 1.11 Flat jacks fitted under each new column.
10 Failures in Concrete Structures
Alltheflatjackswereconnecteduptoasinglepumpandpressuregauge.Epoxyresinwaspumpedintoeachoftheflatjacksatthecorrectpressureandallowedtoharden.Thisensuredthatthecorrectloadhadtransferredtothenewsteelcolumnsateachlevel.
Comment—Therehadbeenadesignerror from lackofunderstandingof the structural behaviour effects of the shear forces on the columns.Althoughtheexistingcolumnshadcrackedasaresultofashearfailure,inthiscaseitwasnotconsideredtobeultimatelimitstateandtheexist-ingcolumnswerestillcapableofsupportingareducedloadwithoutriskoffurtherdamage.Assoonastheerrorwasrecognisedbythedesigner,asimplesolutionwasprovidedandexecutedquicklywithoutcausingadelaytotheoverallprogramme.
1.4 TRANSFER BEAM WITH ECCENTRIC LOADING
Adesignerwasconsideringalargetransferbeamforpickingupaneccentricload.Thetorsiondesignwasleadingtoaverycomplicatedreinforcementdetail. It was discovered by a chance comment that the support for thetransferbeamwasonmasonryandthatnostepshadbeentakentotransferthetorsiontothesupport.Fortunately,thiserrorwasdiscoveredatanearlystageandthewholestructuraldesignwasrevised.
Comment—Thisisasimpleexampleinwhichadesignerhadnotcheckedhowtheflowofforceswastakenthroughthewholestructure.Itisunfor-tunatethatthisisnotanuncommonfailing.
1.5 EARLY THERMAL EFFECTS
The design of a five-storey car park incorporated long post-tensionedbeams.Theseformedpartofanunbracedconcreteframesupportedby600msquarecolumns;floor-to-floorheightwas2.7m.Thebeamsincludedsix15.6mspansthatwerecastinonepour.Theywerestressedat3daystothefullrequiredprestress.Thebeamswere575mmdeepand2000mmwide,withrebatesinthetopcornerstotake150mmdeephollowcoreunits.Thehollowcoreunitsspannedbetweenthebeamswhichwereat7.2mcentres.Figure 1.12showsthearrangement.
Theconstructionperiodforthecarparkwasextremelyshort(8months)and hence all the prestressing work was carried out under a very tightschedule.Figure 1.13showsatypicalbeamunderconstruction.
Unfortunately,5to6daysafterthetransferofprestressforthefirstsetofbeams,cracksappearedinmanypartsoftheframe.Thecrackingwas
Failures due to Design Errors 11
In-situ concrete tie beam (providingmoment frame in transverse direction)
Post-tensioned in-situ concretespine beam (1800 wide × 575 deep)
75 mm concrete �oor screed
150 mm thick precast pre-tensioned concrete hollow-core �oor slabs (supportedon formwork and cast inwith the spine beam)
In-situ concrete columnsDucts for post-tensioning cables
7200
Tableformsystem
Openingsfor tiebars15600
Figure 1.12 Layout of structural members.
Figure 1.13 Typical beam under construction.
12 Failures in Concrete Structures
widespreadinboththecolumnsandbeamsandexceeded0.7mminplaces.Figure 1.14showsthedifferenttypesofcrackingthatoccurred.
Ittookalongtimetounderstandwhatcausedthecracking.Whilsttheprob-lemwasdebated,inordertoproceedwithconstructionwithminimumdelay,theprestressingwasalteredtoatwo-stageprocess—only50%appliedattrans-ferandtheremainingprestressappliedafter2weeks.Aftermuchdiscussion,itwasconcludedthatwhenearlythermaleffectswereincludedwiththeothershorteningeffects,thetotalshorteningwassufficienttocausethecracking.Atthattime(early1990s)itwasnotcommontoincludeearlythermaleffectsinthedesignsofconcreteframes(forreinforcedorprestressedconcrete).Thecal-culatedvalueofearlythermalmovementattheouterendsofasix-spanbeamwasover8mmwhich,whenaddedtotheelasticshorteningfromprestressof7mm,providedsufficientmovementtocausethecrackingthatoccurred.
Atthattime,thecodeofpracticestated‘unlessthelessersectiondimen-sionisgreaterthan600mmandthecementcontentisgreaterthan400kg/m3thereisnoneedtoconsidertheearlythermaleffect.’AlthoughCIRIA3Report 91 covering early thermal crack control in concrete, provided ameansofcalculatingtheeffectforsituationsofvariousrestraints,itdidnotindicatewhatvalueoftherestraintfactorshouldbetakenforsuchabeaminastructural frame.Furthermore it introducedamodification(‘fudge’)factor,Kandsuggestedthatthisbe0.5.
Therestrainttoshorteningofthebeamsbythecolumnswasnotgreatforthisproject.Theearlythermalmovement,includingframeaction,was8mmthatcomparedwiththefreemovementof10mm.However,itwouldhavebeenincorrecttobasethemovementonthefulltemperaturefallfrom
End elevation
(a) Column �exure (b) Column shear (c) Beam �exure
(d) Beam/column tearingSo�t
(e) Tension in transverse beamBeam 1
Beam 2So�t
Tension�is crack also
appeared intop of beam
Figure 1.14 Different types of cracking.
Failures due to Design Errors 13
peaktemperaturesincethebeamstriedtoexpandwhileheatingup.Atypi-calcurveforoneofthebeams,givingthetemperatureriseandfallovertime,isshowninFigure 1.15.
From a layman’s view, as the concrete started to set, the temperaturerose.Duringthisperiod,theconcreteremainedsomewhatplastic.Thestiffcolumns prevented any expansion of the beams (they just bulged a bit).Whenthetemperaturestartedtofall,theconcretehadhardenedandthebeamshortened,causingthecracks.
Comment—Thisisanexamplewheretheeffectsofearlythermalmove-mentshouldhavebeencheckedbecauseofthelongcontinuousmulti-spans.Thebeamshappenedtobeprestressedbutthesameeffectsapplytorein-forcedconcretestructures.Typically,fora300mminternalfloor,anallow-anceof100×10–6 forearly thermalcontractionstrainshouldbemade.Thisprojectprovidedthefirstclearrecordofearlythermaleffectsinacon-creteframestructure.Theresultsemphasizedtheimportanceofincludingtheseeffectsindesign—notcommonpracticebefore1995.
Thisparticularcontractwascomplicatedbythefactthatitwasadesign–buildtype.Thedesignofthebeamswascarriedoutbyaspecialistsubcon-tractor,buttheresponsibilityfortheframedesignwaswiththestructuralengineeringconsultant.Thissplitresponsibilitycontributedtotheconfusionanddelayinresolvingtheproblems.Insuchcontractsit is essential to name a single designer or engineer who retains overall responsibility for the stabil-ity of the structure, the compatibility of the design, and details of the parts and components, even where some or all of the design including detailing of those parts and components are not carried out by this engineer.
ReferenceshouldbemadetoDesignofhybridconcretebuildings.4
25
20
15
Tem
p. °C
Abo
ve A
mbi
ent
10
5
1 2 3 4Time in Days
5 6 7
Figure 1.15 Typical early temperature rise and fall in concrete beam.
14 Failures in Concrete Structures
1.6 SECONDARY EFFECTS OF PRESTRESSING
Aconcreteframewithatwo-spanprestressedbeamhadbeendesignedforserviceabilitylimitstate(SLS).Whencheckingtheframefortheultimatelimitstate(ULS), thedesignerdiscoveredthatthemoments inducedintotheedgecolumnsexceededtheircapacity,850kNm.Figure 1.16showsthelayoutofthestructuralmodeloftheframe.ThemomentsfromtheULSanalysisare shown inFigure 1.17.Toreduce the transfermomentat theedge,thedesignerconsideredcreatinghingesbetweenthecolumnandslabasshowninFigure 1.18.Thisproposalwasnotfavouredbecause:
3 m
3 m
15.6 m 15.6 m
Figure 1.16 Layout of structural model of concrete frame.
1350 kNm 1730 kNm
Figure 1.17 Moments from ULS analysis.
Provide recess tocreate hinge e�ect
Figure 1.18 Proposal for providing hinges.
Failures due to Design Errors 15
• Reinforcementmustbeplacedcentrally toallowahingeeffect tobecreated.
• Compression in the concrete section would automatically createmomenttransfer.
Thequestionraisedwaswhethersecondaryeffectshadbeen included intheULSanalysis.Toanswerthisquestion, it is importanttounderstandthemethodofanalysis.ForprestressedstructuresintheUK,itiscommontocarryouttheprimaryanalysisatSLS.Oftentheequivalentloadmethodisused.Forthistheprestressingeffectsaremodelledasanequivalentload(e.g.,auniformlydistributedloadmodelstheprestressingeffectofapara-bolicdrapeofthetendons).Thistypeofanalysistakesaccountofsecondaryeffects.Forthosenotfamiliarwiththistypeofwork,secondary(parasitic)effectscanbesummarisedbythediagramsshowninFigure 1.19.
AcheckisthencarriedoutatULS.Forthislimitstate,theprestressingactionisoftenconsideredasaffectingonlytheresistanceandnotaspartoftheloading.Thesecondaryforcesandmomentsmustbecalculatedsepa-ratelyandaddedtothoseoftheprimaryanalysis.Forthisparticularframe,thesecondarybendingmomentsareshowninFigure 1.20.Theseeffectshadnotbeen included in theULSanalysis check for thisproject.Whenaddedtotheexistingresults,themomentsbecameacceptableasshowninFigure 1.21.Onceitwasagreedthatthiswasthecorrectanalysis,itwasrealisedthatthecolumnwasnotoverstressedandthemomentsinthespanofthebeamshadincreased.Therewasplentyofroominthesectiontoaddmorereinforcementtotakeaccountofthis.
Unstressed element in structure
Unstressed isolated element
Stressed isolated element
Secondary forces for element
Figure 1.19 Example of prestress secondary effects.
16 Failures in Concrete Structures
Comment—ThisproblemwaspartlyduetothedesignmethodusedintheUK.Forprestressedbeams,theprimarydesign/analysisisoftencarriedoutattheSLS.Theprestressisconsideredaload(oractioneffect)withintheequivalentloadmethod.
ItisnotpossibletousethisapproachfortheULScheckastheprestress-ingtendonsresisttheappliedactionsinthecriticalpartsofthebeamandthus do not provide an equivalent load. The secondary effects must beconsidered separately at such parts. The debate about the interaction ofsecondaryeffectsandmomentredistributionisongoing.However,inthissituationtherewasnoreasontobelievethatmomentredistributionwouldorcouldtakeplacebeforedamageoccurredtothecolumn.
1.7 TEMPERATURE EFFECTS ON LONG-SPAN HYBRID STRUCTURE
Thegroundlevelofatwo-storeyundergroundcarparkslabwasnotcov-ered.Thestructureconsistedof16mspanninghollowcoreunitsbearingonprecastconcretebeamnibs.Movementjointshadbeenshownonthedrawingsbutthesedidnotfunctioncorrectlyforavarietyofreasons.Theuppersurfaceoftheslabwasexposedtotheweatherandinparticulartolargevariationsintemperature.Thelattercausedmovementandrotationoftheunitsandtheirsupports.Thisresultedinseverecrackingofthesup-portingnibsand in someplacescrackingat theendsof thehollowcoreunits. Even after repair, the cracks reappeared each subsequent year for
1350 kNm1730 kNm
737 kNm
Figure 1.21 Moments from ULS analysis adjusted for secondary effects.
Structural Concrete Failures
613 kNm 270 kNm
Figure 1.20 Secondary effects for frame.
Failures due to Design Errors 17
more than5years. Figure 1.22describesdifferentmechanisms that canoccurevenwhereastructuraltoppinghasbeenused.
Ingeneral,ifthebearingmaterialcreateslargefrictionforces,thesecanleadto largetensionstresses inboththesupportandtheprecastslaborbeam(seeFigure 1.22a).Neoprenebearingsorsimilarshouldbeusedtoavoidthis.
Ifthespacebetweentheprecastslaborbeamandthefaceofthesupport-ingmemberisnotadequatefortherequiredmovementoritfillswithhardmaterialovertime,crackingwilloccur(seeFigure 1.22b).Iftheeffectsofmovementand/orrotationcausethelineofactiontomovetooclosetotheedgeofthesupport,localspallingcanoccur(seeFigure 1.22c).
Afurtherproblemariseswherehollowcoreunitsareused.Anycrackingthatoccursclosetotheirendsislikelytocauseanchoragebondorshearfailure of the unit (see Figure 1.23). Anchorage bond failure may occurbecausethecrackingclosetothesupportdoesnotallowthefullanchorageresistancetodevelopandtheprestressingstrandsstarttoslip.Thiscausesthecracktogrowuntiltheunitfails(seeFigure 1.23).Hollowcoreunitsareinherentlyvulnerabletotheeffectsofcrackingclosetothesupportastheirshearresistancereliesonthetensionstrengthoftheconcrete.Unlikeasolidsection,acrosssectionavailableforshearresistanceoftheseelementsismuchreducedduetothepresenceofthecores.
(a) Effect of high friction (b) Effect of hard material in joint (c) Effect of rotation/Movement
Friction cancause cracking
Movement
Hard material canprevent movement
Rotation Rotation
Rotation cancause spalling
Figure 1.22 Examples of potential failures at movement joints.
(a) Anchorage bond failure (b) Shear tension failure
Anchorageslip
Shear tension crack
Large crackclose tosupport
Figure 1.23 Types of end failure of hollow core units.
18 Failures in Concrete Structures
Comment—Theremedialworktocorrectthissortofproblemislikelytobeveryexpensive.Makinggoodthecracks isnotasatisfactorysolutionasthecracksreappearonanannualcycle.Eventuallythedangeroffallingconcretetousersofthecarparkresultsintemporaryworksthatmaybesoextensive thata rebuildof thewhole structurecanbecomeasensiblesolution.
Whereprecastconcreteelementsformamajorpartofastructure,care-fulcalculationoftolerancesisessential.Thiswasparticularlysoforthisprojectasthedesignofthemovement jointsdidnottaketolerances intoaccountsufficiently.Thisaspectofdesignisnotsocriticalforinsitucon-cretestructuresasmanyofthetolerancesgetabsorbedsatisfactorilybytheconstructionprocess.
Thisprojectwasanotherexampleshowingit is essential to designate a single designer or engineer who retains overall responsibility for the sta-bility of the structure and the compatibility of the design and details of the parts and components, even where some or all of the design including detailing of those parts and components may not be carried out by this engineer.Referenceshouldbemade toDesignofhybridconcretebuild-ings4 and the Concrete Society’s report titled Movement, restraint, andcrackinginconcretestructures.5
1.8 LOADING FOR FLAT SLAB ANALYSIS
Aflatslabspansareasbetweencolumnsandfailurecanoccurbytheformationofhingesalongthelinesofmaximumhoggingandsaggingmoments.ThiscanbemosteasilypresentedusingthefoldedplatetheoryasshowninFigure 1.24.Acomplementarysetofyieldlinescanformintheorthogonaldirection.
Column supports
Hoggingyield lines
Saggingyield lines
Figure 1.24 Simple yield line mechanism for flat slab.
Failures due to Design Errors 19
Onemisconceptionofsomeengineers is toconsiderareducedloadingwhenanalysingaflatslab.Eachmomentappliedineachorthogonaldirec-tionmustsustainthetotalloadingtomaintainoverallequilibrium.Thereisnosharingoftheloadbypartialresistanceineachdirection.
Comment—Oneengineer,whohadworkedintheU.S.,foundthatthiserroneousbeliefwentbacktotheearlydaysofflatslabconstruction,whenthe promoters measured the strains in the reinforcement of newly con-structed slabs. They ignored the contribution of the concrete in tensionandpromulgatedtheidea.Theengineerhad,infact,designedsomeware-houseslabsbythatmethod.Manyyearslater,anewowneraskedwhetherthe imposedfloor loading couldbe increased, andoneofhis colleaguescheckeditandfoundthatitwasoverloadedunderthedeadload.
Althoughtheauthorcannotquoteotherinstanceswherethisoccurredin designs, there have been many comments by engineers to this effect.It is clear thatmanyengineersbelieve that it is reasonable toconsiderareducedloading.ReferenceshouldbemadetotheConcreteSociety’stech-nicalreporttitledGuidetothedesignofreinforcedconcreteflatslabs.6
1.9 PRECAST CONCRETE CAR PARK
Thelayoutofthecarparkdeckincludedprecastspinebeamssupportingdoubleteeunits.Figure 1.25showsasectionthroughthedeckclosetotheedgeofthestructure.Astructuralscreedwasplacedoverthewholedeck.Themethodof construction for the cantileverunitswasunconventionalandincludedon-sitewelding.Figure 1.26showsanenlargeddetailthroughthesection.
Thestructuralscreedlaidoverthewholedeckwasdesignedwithastepovertheweldedplateabovethespinebeam.Thefinalscreed,incorporatingawaterproofmembrane,waslaidtofalls.Thedrainagepointsweresitu-atedattheendsofthespinebeams.Figure 1.27showsthefinalarrange-mentthroughasection.Thestepinthescreedcreatedacrackinducerforthetensionstressesatthetopofthecantileverandwaterdrainingoffthe
Structural screed
Double teecantilever
Spinebeam Double tee
Figure 1.25 Section edge of car park deck.
20 Failures in Concrete Structures
carparkwas likelytopermeate it. Inwinter, thiswaterwas likelytobecombinedwithde-icingsalts.Theriskofcorrosionofanimportantpartofthestructurewashigh.Bythetimethisproblemwasrealized,itwascon-sideredtoolatetochangethedesignandformofconstruction.Insteadthedepthofthewaterproofingmembranewasincreased.
Comment—Thedesignincludedfourmajorerrors:
1.Site welding for the main cantilever reinforcement is not recom-mended.Whencheckedonsite,someoftheweldswerefoundtobeofverypoorstandardandhadtobecondemned.
Site weld
Structural screed
Plan of Plate
720 × 100 × 10 thk plt.
Figure 1.26 Detail through section.
Salty water
Crack inducer
Salty water
Figure 1.27 Section through completed deck.
Failures due to Design Errors 21
2.Thedesign includedfilling the spacesbetween the spinebeamanddoubleteeunitswithmortar.Thiswasverydifficulttoachieveandresultedinpoorcompactionthatallowedwaterpassage.Thedesignshouldhaveprovidedabettersolution.
3.Thesharpcornerinthestructuralscreedformedacrackinducer. 4.There was a high probability that salt water would penetrate the
structureandcausecorrosionofthereinforcement.
Ifthemembranewasasphaltorsimilar(madetocarrywheel loads)andfullybondedtothescreed,itwouldwithtimebecomesobrittlethatthecrackinthescreedwouldpropagateintothemembrane.
1.10 ARCH FLOOR
Theconstructionofahotelwasnearingcompletion.Thefirst levelfloorspanning10mhadbeendesignedasaninsituconcreteflatarch.Thedesignkeptthefloorthicknessatmidspantoaminimum(100mm)increasingtowardtheendsto275mmthickatthesupport.Theformworkforpartsofthefloorhadnotbeensupportedsufficientlyanddeflectedtocausethefloorthicknesstobe75mmthickerthandesigned.Theslabwassupportedonmasonrywalls.Somemonthsafterconstruction,crackswerenoticedinthefloorandwereconsideredtohavebeencausedbyshrinkage.Theclient,forotherreasons,complainedabouttheconstructionandanindependentengineerwasaskedtogiveasecondopinion.Duringthisinspection,itwasdiscoveredthatnodesigntiestotheflatarcheshadbeenprovided,althoughnosignificantmovementhadoccurredatthesupports.
AnumberofremedieswereconsideredandtheonethatappearedtobemostsuitablewastoinsertMacalloybarsandanchorthemtotheoutsidesofthesupportingmasonrywalls.Specialfireprotectionfromboxesmadeoffire-resistantmaterialwasrequired.
Comment — It was by pure chance that the structural integrity of thebuildingwascalledintoquestionanditisquitepossiblethatitwouldnothavefailedorcollapsedformanyyears,ifever!
1.11 PRECAST CONCRETE STAIRFLIGHTS
The1968collapseofastaircaseduringconstruction,killingtwopeople,causedarethinkindesign,detailing,andconstruction.Theprecaststair-flightsweredesignedwithhalfjointstositoninsituconcretelandings.Oneoftheupperfloorstairflightshadbeenplacedinpositionbyacranebutitneededtobeshiftedintoitscorrectposition.Temporary‘acro’propswere
22 Failures in Concrete Structures
usedfromtheflightbelowtosupportandliftitasitwasleveredintoitsfinalposition.Thestairflightbelowwassupportedonaninsituconcretenibthathadbeencastonlyafewdaysearlier.Thisnibdidnothaveitsfulldesign strength and was loaded with almost twice its design load whenfailureoccurred.Thethicknessofthelandingslabwasonly200mmwhichmeantthatthedepthofthenibs,100mm,didnotallowsufficientroomforreinforcement.
Sincethattime,precastmanufacturershavedevelopedavarietyofdiffer-enttypesofjointsusingprecaststairflights.Whenusingsuchaproprietarysystem,itisessentialthatadesignerconsider:
• Themethodofadequately tying the stairflight toadjacentpartsofthestructure
• Thesequenceofconstruction• Thetemporaryworksinvolved• Thechainofresponsibilityinachievingthefinalstructure(Oftenthe
temporaryloadsduetopropsandotherfactorscanprovidethecriti-caldesigncondition.)
Manycurrentsystemsdonotincludeadequatetiesbetweenthestairflightandtheadjacentstructure.Thefollowingexamplesshowhowtyingrein-forcementcanbeprovided.
Half-joints—Figure 1.28showsatypicallayoutofreinforcementwherehalfjointsareused.Thetyingreinforcementthatprojectsfromtheprecastunitsprovidescontinuitywiththereinforcementinthestructuralscreed.
Doweljoints—Toprovidesufficientroomforadowelhole,thedimen-sionsofthenibmustbeincreasedtothoseshowninFigure 1.29.Figure 1.30showsapreferredarrangementofreinforcementforadowelconnection.
375
375
15
15
Tie reinforcement instructural screed
Tie reinforcement instructural screed
Figure 1.28 Typical layout of reinforcement for half joints.
Failures due to Design Errors 23
Proprietarysystemusingsteelangles—Figure 1.31showshowapropri-etarysystemcanbeadaptedtoallowcontinuityofties.
Comment—Thisfailurehasbeenincludedwithinthischapteralthoughitisclearthatthedetailingandconstructionfaultswereimportantcontribu-tors.Greatcareisrequiredwhenconstructingstaircaseswithprecaststair-flights.Theimportanceofthiscannotbeoveremphasised.Ahybridform
120
120
35
70
40
Figure 1.29 Half-joint with dowel.
15 375
375 15
Figure 1.30 Reinforcement arrangement for dowel connection.
Structural screedStructural screed
Structuralreinforcement
Structuralreinforcement
PrecastStairflight
PrecastStairflight
Reinforcement welded toangle & lapping with
structural reinforcement
Reinforcement welded to angle& lapping with structural reinforcement
Insitu structure Insitu structure
Figure 1.31 Stairflight system using steel angles.
24 Failures in Concrete Structures
of construction is commonly used and the dangers get easily forgotten.Although safe construction largelydependsonworkmanship,adesignerhas an important obligation to provide a robust and buildable solution.ReferenceshouldbemadetoDesignofhybridconcretebuildings.4
1.12 SHEAR STUDS ON STEEL COLUMN TO SUPPORT CONCRETE SLAB
Thedesigner’sproposalwastouseshearstudsweldedtoasteelcolumntotransfertheloadfromtheconcreteflatslabtothecolumn(seeFigure 1.32).However,withthisconfiguration,apunchingshearfailurecouldoccurwithonlythebottomrowofstudsprovidinganyshearresistance.Alltheothershearstudswouldremainintheconeofconcreteattachedtothecolumn(seeFigure 1.33).Toavoidsuchafailure,twooptionsprovidesensiblesolutions:
Option1:Providelinkstotransfertheloadbacktotopofslab—Links,inadditiontothoserequiredtoresistpunching,wouldberequiredtotrans-ferthefull loadtothetopoftheslab.Struts intheconcretewouldthentransfertheloadontotheshearstuds(seeFigure 1.34).Thisisunlikelytoprovidethemostpracticaloption.
Punching shearfailure
Figure 1.33 Punching failure of slab.
Steel column
Reinforced concrete slab
Figure 1.32 Junction between steel column and concrete slab.
Failures due to Design Errors 25
Option2:Provideshearkey(insteadofstubs)atbottomofslab—Usingashearkeywithfullstrengthweldsatthebottomoftheslabwouldpro-videadequate resistance.Theshear linkswouldensureagainstapunch-ing failure and the resulting forceflowwould engage the shear key (seeFigure 1.35).Thesizeoftheshearkeywoulddependonthethicknessofslabandtheshearforce.Theupperlimittothissolutionwouldbethecom-pressionstrengthoftheconcreteatthecolumnface:
BS81101:Maximumcompressivestress=0.8√fcubutnotmorethan5MPaBSEN19922:Maximumcompressivestress=0.8×0.6(1–fck/250)MPa
Comment—Thisisanexamplewherethinkingstrutsandtiescanbehelpful.
Extra links to liftload to top of slab
Figure 1.34 Use of links to transfer load to shear studs.
Forceflow
Shear keys welded to column
Figure 1.35 Use of strut-and-tie model to transfer load to shear key.
26 Failures in Concrete Structures
1.13 PILED RAFT FOR TOWER BLOCK
Thedesignof the raftassumed that thewallsof the two-levelbasementcarparkwouldactwiththeraftoverthepilestotransmittheshearandbendingforcestotheouterpiles.Thewallsofthebasementhadalmostfullheightopenings,placedoneabovetheother,andcontainedonlynominalreinforcement.Thecombinedstrengthofthesewallsplusthe1.5mthickslabwasinadequatetotransmittheloads(seeFigure 1.36).
The mistake was discovered whilst the tower block was being con-structed.Theremedialworkrequiredanewrafttobeconstructedbeneaththeexistingone(seeFigure 1.37).Thenewdesignreliedonthecompositeactionofthenewandoldrafts.Alltheexistingsurfacesofconcretewerescabbledand further reinforcementwas laidbetween the existingpiles.Placing of concrete for the lower part of the new raft was carried outconventionally.
Toachievegoodbondwiththebottomoftheexistingraft,theupperpartofthenewraftwaspackedwithsinglesizedaggregateandthengroutedwitharetardedandfluidcementpaste.Thegroutwas introducedunderpressure to thebackof thepour throughacomplicated systemofmetalpipespinnedtotheundersideoftheexistingraft.Themethodproducedawallofgroutthatextendedfromtoptobottomofthepourandflowedforward toward the peripheral shutters with the top surface behind thebottom.Pipesweresoplacedtolettheairoutinfrontofthegroutsurface,thenindicatewhereandwhenthegroutarrived,andthenallowgroutingtocontinuefromimmediatelybehindtheadvancingwallofgrout.Groutingwascontinuousuntiltheworkwascomplete.
Columns Core walls
Piled raft
Possible line ofshear failure
Basement car parks
Figure 1.36 Arrangement of piled raft and basement car parks.
Failures due to Design Errors 27
Comment—Thiserrorwasdiscoveredasafollow-uptoacheckofsomeofthereinforcementdrawingsofthetowerblockthatwerefoundtocontainmistakes.
1.14 FLOATING PONTOON FOR RESIDENTIAL BUILDING
Thedesignandconstructionofafloatingpontoon tocarrya residentialblockwerecomplete.Thepontoonwasafloatinthemarina,readyforworkto start on thebuilding.As the load increased, it becameapparent thatthepontoonwasnotsufficientlybuoyant.Thedesigncheckthatfollowedshowedthatthedensityofconcreteassumedwastoolow.Thiserrorwascompoundedbyerrorsinconstruction(oversizingofthewallsandbaseofthepontoon),allofwhichresultedinaseriousreductioninbuoyancy.Thesituationwasresolvedbyattachinglargeblocksofpolystyrenetothecon-cretestructureofthepontoon.
Comment—During thedesignandconstructionof thepontoon, insuf-ficientthoughthadbeengiventotheessentialrequirementofensuringthatthestructurewasadequatelybuoyant.
1.15 PRECAST COLUMN JOINT DETAIL
Aprecastslopingcolumn,400mmindiameter,wasdesignedtotaketheloadfromseveralfloorsabove(seeFigure 1.38).Ashortsteelstubwascast
Existing raft
Piles scabbled totake new concrete
New supplementaryraft to take shear
3.5 m
Figure 1.37 Schematic arrangement of new raft.
28 Failures in Concrete Structures
intothebottomoftheprecastelementtosimplifytheconnectiondetailtothebeambelow.Duringconstruction,itwasrealisedthatthedesignofthiscon-nectionwasunsafeduetoinsufficientbondcapacityandanti-burstingsteel.
Theshortstubconsistedofa254×254UCsectionthatextendedintothecolumnabout300mm.Thestubhadfour19×100shearstudsweldedtoeachfaceoftheweb(seeFigure 1.38b).ThelinkssurroundingthestubcolumnwereT12hoopsat250mmpitch.Nootheranti-burstingreinforce-menthadbeenprovided.
Thecolumnswerealreadyconstructedandsupportedfourfloors.Checkcalculationsshowedthatthefactorofsafetyagainstcollapsewasabout1.1andthiswouldreducetolessthan1whenthenextfloorwasadded4dayslater.Workwasstoppedinthatpartoftheprojectandtemporarypropsinstalledintheareatopreventpossiblecollapse.
Theremedialactionwastoencasetheexistingcolumnwithasteelcir-cularhollowsection,457mmdiameter,ofthesamelength.Thiswascutintotwohalvesalongitslengthandthenweldedinplacewithfullstrengthweldsalongthefull length.Thespacebetweenthesteelandprecastcol-umnswasthengroutedup(seeFigure 1.39).
Nocalculationshadbeencarriedouttodeterminetherequiredembed-ment lengthof theUCstubcolumn.Bondandendbearing shouldhavebeenconsidered.
(a) Elevation of column
Precast column
(b) Bottom detail of column
T12 hoops @ 250
300
(c) Section through column
203 × 203 UC
19 × 100shear studs
Figure 1.38 Sloping precast column.
Failures due to Design Errors 29
Bond—If the forcewas tobe transferredusingbondstress,adesignshearstrengthof0.6MPa independentofconcretestrengthshouldhavebeenused.ThisisthevaluegiveninBSEN1994-1-17forthedesignshearstrengthdue tobondandfrictionforacompletelyconcrete-encasedsec-tion.Thedesignvalueof studsand shear connectorsmayalsobe takenfromthatstandard.TheconcretearoundshearconnectorsfixedtothewebofHsectionsispartiallyconstrainedbytheflanges,andthiscanresultinhigherdesignresistances.
Endbearing—Someofthecolumnforceistakeninendbearing.Thebearingstressshouldbelimitedtothreetimesthedesignconcretecompres-sivestrength,e.g.,3×0.85fck/1.5=1.7fckfordesignstoBSEN1992-1-1.2
Links—Linksmustbeprovidedovertheembeddedlengthtopreventsplitting.The force tobe resistedby the links (one leg) in aunit lengthshouldbeequaltotheshearforcetransferredinthatlengthdividedby2π.Thelinksabovethestudmustresistaforceequaltotheendbearingforcedividedby2π.
Comment—Itwasafortunatechancethatthisfaultwasfound.Theengi-neerwhodiscovereditwasintendingtocopythedetailforasimilarnearbystructure.
Fullstrength
welds
CHS
Grout
Figure 1.39 Remedial work.
31
Chapter 2
Problems and Failures due to Errors in Structural Modelling
2.1 REINFORCED CONCRETE TRANSFER TRUSS
Amulti-storeybuildingconstructedaccordingtoadesign–buildcontractincluded adeep transfer reinforced concrete truss.The client required asecondopiniononthedesignofthisfromanindependentconsultingengi-neer.Thereportofthisengineercondemnedthedesignasunsafe.Thecon-tractorthenemployedanotherconsultingengineertocarryoutafurthercheckand,ifitconfirmedthefindingsofthereport,tofindasolutionthatprovidedtheleastdisruptiontotheexistingworks.
Thesecondcheckrevealed that the initialdesignof theconcrete trussinvolvedtheuseofacomplicatedfiniteelementprogram.Theendsupporttothetrusswasastiffwallandthishadbeenmodelledwithplateelementsthathadnolateralstiffness(seeFigure 2.1).Thesecondconsultingengi-neerthencarriedoutseparateframeanalyseswithsimplerprogramsthatincludedmorerealisticpropertiesfortheedgewall.Theresultsshowedthatthedesignofthetrusseswassafe—just.Thereinforcementdetailsofthenodesrequiredsomeminorchanges.
Comment—Thisexampleshowsthattheoriginaldesignerhadnotunder-stoodtheeffectoftheendwallstiffnessordidnotunderstandthelimita-tionsofthemodellingsoftware.Bothcasesreflectedsomeincompetence.Thisisanexampleinwhichthetwocheckingengineersactedforpartieswithoppositeviewsaboutthesafetyofthestructure.Itshowshowvulner-ableengineerscanbeintryingtoprovewhattheyareaskedtodoratherthanexaminingtheinformationobjectively.
2.2 MODELLING RIGID LINKS
Itispossibleinsomestructuralframeprogramstosetuprigidlinksinthedata.Theseallowtheuser tospecify thatpartof thestructure thatwillbehaverigidlybetweenstatednodes.Arigidlinkmayincludeanynumber
32 Failures in Concrete Structures
ofelementsfromasinglepairtoalargegrouprepresentingastiffpartofthestructure;forexample,allnodesonaparticularplanemaybelinkedtorepresentthein-planestiffnessofafloorslabwithoutexplicitlymodel-lingit.Itisoftenpossibletolinkonlysomeofthedegreesoffreedom;forexample,withthefloorslab,theout-of-planetranslationandthein-planerotationsmustnotbelinked.
Forthedesignofoneparticularbuilding,rigidlinkswereusedtomodelfloorplates.Atalevelwherethewallarrangementwithinindividualcoreschanged significantly, the rigid links had to translate very large forcesthrough the floor slabs (transferring forces from one core structure toanother). The program did not provide any output of the forces withinthe rigid linksand thedesignerassumed that loadscouldbe transferredthroughthefloorssafely.Duringadesignreview,itwaspointedoutthatthese forces couldbevery large. In fact, itwas thendiscovered that thedesignforcesexceededthedesignresistanceofthematerialsandaredesignhadtobecarriedoutduringconstructionofthebuilding.
Comment—Thisexamplehighlightstheneedtoknowwhatlevelofinfor-mationisrequiredandwhatlevelofdetailwillbeprovidedbyastructuralmodel.Inthiscase,thedesignerhadnotgiventhoughttotheproblemandthestructuralmodellingsystemdidnotsupplyadequateinformation.
2.3 ASSESSING MODEL LIMITS AND LIMITATIONS
Most analyticalmodels for reinforced concrete, including those forulti-matelimitstate(ULS),arebasedontheelasticpropertiesoftheconcretesection,uncrackedandwithoutreinforcement,i.e.,ahomogeneousmate-rial.Thisallowsa simpleanalysisofa frameor structurewithuniformstiffnessforeachelement,andformostsituationsprovidesanacceptable
RC transfer trussStiff edge wall - analysis assumed aplate element without lateral stiffness
Figure 2.1 Diagrammatic view of concrete truss.
Problems and Failures due to Errors in Structural Modelling 33
solutionfordesign.Thereasonthisisnormallyacceptablecanbeexplainedbyconsideringasimpleindeterminatestructure—aproppedcantileverslabwithUDL.Figure 2.2showstheflexuralresultsofanalysisforseveraldif-ferentsituations.
Thefirstanalysisassumessimpleconcretesectionproperties—ahomoge-neoussectionthroughout.ThisgivesamaximumsupportmomentofWL/8andamaximumspanmomentofWL/14.Theengineer thendesigns thereinforcementforthesemomentsand,ofcourse,thereinforcementrequiredatthesupportisnearlytwicethatrequiredinthespan.Theeffectofinclud-ing this reinforcement (givinggross sectionproperties) is to increase thestiffnessoftheslabnearthesupport.
Ifafurtheranalysisisthencarriedoutwiththerevisedstiffnessproper-ties (notnormallydone), thebendingmoment isaltered to that showingGrossSection(1).Thiswouldcausetheengineertoincreasethereinforce-mentatthesupportandreduceitinthespan,causingafurtherincreaseofstiffnessatthesupportandreductioninthespan.ThenextanalysiswouldresultinmomentsshownasGrossSection(2).Thisiterativeprocesswouldleadtheengineertodesigntheslabasacantilever.However,inreality,theslabwillcrackundertheultimateloads.Thisreducesthestiffnessatthecrackedsections.Acrackedanalysisofthesectionwiththereinforcementrequired by the analysis using simple concrete section properties wouldresultinmomentsshownasCrackedSectionwithTensionStiffening.Thisresultsinasimilarcurvetothatshownfortheanalysisusingsimplecon-cretesectionproperties
If,asaresultofthefirstanalysiswithconcretesectionproperties, theneutralaxisdepthatacriticalsectionisgreaterthanthatforabalancedsection(i.e.,thereinforcementdoesnotyieldattheultimateload),andthe
Cracked sectionwith tension
stiffening
WL/8
WL/14
Gross section (2)
Gross section (1)
Concrete section
W
L
Figure 2.2 Design analysis of propped cantilever.
34 Failures in Concrete Structures
appliedmomentislessthanthecrackingmoment,thereisapossibilityofaprematurebrittlefailure.Boththeseconditionsshouldbecheckedtoensurethatthecriticalsectionsarecapableofmomentredistribution.Otherwisethedesignmaybeunsafe.
Many reinforced concrete continuous beam and slab and frame pro-gramsmodeltheelementswithhomogeneouspropertiesandtheengineerinputsthesewithjusttheconcretesectionwithoutreinforcement.Itisthusimportant that the software, after it designs the reinforcement, containschecks for neutral axis depth and cracking to ensure against prematurebrittlefailure.
Thisdemonstratesthatdesignisheavilyreliantonamaterial’scapacitytoredistributetheforcesachievedbytwoindependentmeans:(1)crackingoftheconcrete,and(2)yieldingofthereinforcement.Theeffectofthesecanoftenprovideaverydifferentultimateresistanceofastructurethanthatpredictedfromtheelasticresults.
Although elastic results are normally conservative, in some modellingsituationsthisisnotso.Atypicalexampleisintheuseofaplaneframeprogramforanalysingflatslabs.Themodellingsimplifiestheflatslabasthough it is a continuous slab spanning on continuous supports (walls).Thebendingmoments from this analysispeakover the supportsbutdonotvaryacrossthewidthoftheslab.Infact,thebendingmomentspeakovertheactualcolumnsupportssothetotalbendingmomentcalculatedforthefullwidthofslabshouldbedistributedacrossthelineofsupport,unevenlypeakingoverthesupport.Thecodeprovidesrulestocompensateforthiseffect(usingcolumnandmiddlestrips).Anincorrectdistributionofmomentswillalsocauseincorrectdistributionofshearforces.
2.4 EMPIRICAL METHODS
Thedevelopmentofgoodpracticeshasreliedonempiricalandmorerigor-ousanalyticalmethods.Manyempiricalmethodshavebeenhoned fromexperience to provide very efficient (cost effective) solutions. Sometimesthishascausedthesemethodstobelessconservativethanamorerigorousapproach.
Onereasonforthisisthatthematerialpropertiesoftheconcreteandreinforcementhavechangedsincetheempiricalmethodswereintroducedand the original assumptions for them are no longer valid. This hascausedsomeconcernanddiscussionamongstthecodewriters.Generallyitisfeltthattheempiricalsolutionsshouldhaveanoverallbuilt-insafetyfactorthat isgreaterthanthatformorerigorousmethods.Overrecentyearsthishasbeenimplementedforultimatelimitstatesbyeditingthecodeclauses.
Problems and Failures due to Errors in Structural Modelling 35
2.5 INITIAL SIZING OF SLABS
During the past 50 years, the reduction of partial safety factors andincreases instrengthsofmaterialshaveresulted inthethicknessofslabsoften being controlled by serviceability limit state (SLS; deflection andcracking).However,determiningaccuratedeflectioninformationofrein-forcedconcreteslabsandbeamsatthedesignstageisalmostimpossible.Itrequiresknowledgeofthefollowingfactorsand/orproperties:
1.Materials used (type of aggregate, type of cement, and amount ofwater)
2.Weather conditions at timeof construction (hot or cold, humidordry)
3.Mechanical properties of hardened concrete (compression and ten-sionstrength,modulusofelasticity)
4.Mechanical properties of reinforcement and characteristics of itsbondwiththeconcrete
5.Actualloadingontheelement,notonlyatthepointintimerequired,butalsotheloadhistoryuptothatmoment
6.Effectofcontinuitywithadjacentstructuralelement 7.Exactconstructionprocessandsequence
Whereaccurateinformationexists,forexample,whencheckinganexistingstructure,itispossibletomakeanexcellentcalculationassessmentofthedeflection(certainlywithin5mm).
Incontrast,atthetimeofdesign,verylittleaccurateinformationexistsabout concrete properties or loading. Nevertheless, for the purposes ofdesign, codesofpracticemake reasonableassumptions forall theabovefactors and properties and provide simplified methods to aid the designengineer.Themostimportantoftheseisthespan/effectivedepthmethod.Howeveritisimportanttorealisethatthismethodcannotbeconsideredmorethanaroughguideandthosewhorelyonittoparethedesignthick-nessofaslabover7minspantoanaccuracyoflessthan25mm,withoutthebenefitofexperienceorspecificinformation,arelivingin‘cloudcuckooland’!Itisthuswisetoerrontheconservativesidewhenusingthespan–effectivedepthmethodtoassessthedesigndepthofaslab.
2.6 ANALYSIS OF FLAT SLABS WITH FINITE ELEMENT PROGRAMS
When using finite element programs to analyse and design flat slabs, itis important to realise that to reinforce for the peak moments over the
36 Failures in Concrete Structures
supports is normally considered overly conservative. Common practiceassumessomelateralredistributionofthepeakmoments.Figure 2.3showsthemomentprofileresultsacrossthefullwidthofatypicalslabatthesup-portfromthefiniteanalysis.Oftenthemomentsaroundthesupportnodeareinconsistent.Itisreasonablewithinhalfthecolumnstrip(thecolumnstripisdefinedashalfthetotalwidthofslab)toassumemeanmomentsacrossthiswidth.Thismaycausecrackingunderworkingconditionsandoneortwoofthereinforcingbarsoverthecolumnmayevenreachtheiryieldstress.Thisisnormallyconsideredacceptablebutadesignershouldbecarefultoensurethattheanalysisissatisfactoryfortheparticularsituation.
Thisisoneexampleinwhichadesignerreliesontheductilitypropertiesofaslab.Thisductilityresultsfrombothcrackingoftheslabandyieldingofthereinforcement.Thecrackingoftheslabcausesitsstiffnesstoreduceandhencetheredistributionofmoments.Theyieldingofthereinforcementcausesaplastichingewithsimilareffects.
Section
Plan
Middle strip Middle strip
Column strip, CC/2
C/4
M/2
Mean of node momentsacross half column
strip width
Inconsistent node momentacross face of column
Width of column
Moment of resistanceMoment diagram
Calculate mean ofthese nodes results
Column centrelineCentreline of panel Centreline of panel
Column strip
Figure 2.3 Bending moment results from typical finite element program.
Problems and Failures due to Errors in Structural Modelling 37
ThechoiceofthestiffnessforaflatslabandsupportingcolumnsforULSanalysisdependsonengineeringjudgement.Takinghalftheuncrackedcon-cretesectionpropertiesfortheslabandthefulluncrackedconcreteprop-ertiesforthecolumnsisconsideredreasonableformostsituations.ThisisdiscussedinmoredetailintheConcreteSociety’sGuidetothedesignofreinforcedconcreteflatslabs.6
2.7 SCALE EFFECTS
Designengineerssometimesfindthattheyareinvolvedinverylargesizestructural projects (e.g., multi-storey blocks, heavy transfer slabs, deepraftsonlargepiles,etc.).Thereisatemptationtocontinuetousetherulesofthumbandempiricalmethodsthatarecommonforsmallandmediumsizedstructures.Oftentheseortheassumedsimplereinforcementdetailinglayoutarenotapplicableandmayleadtounsafedesigns.
Itisimportanttothinkmorefundamentallyhowtheforcesaretransmit-tedand ensure that reinforcement isprovidedand fully anchoredwheretensionforcesneedtoberesisted.Atypicalexamplewouldbeapilecapthathas3mdiameterpilesandrequiresathicknessover5m.Thestrutforcesmustbeconsideredcarefullyandthenodesreinforcedtoensuretheconcretecancontaintheforces.Thiscanleadtodesignreinforcementinadditiontothenormalbottombarsthatenclosethecaptoresisttheburst-ingforces.
Strut-and-tie methods are very useful in helping explain the flow offorcesbuttheyareoftendifficulttoapplyquantitatively.BSEN1992-1-12providessomehelpfulrulesforsuchapplications.Whendealingwithdeepsections,itisimportanttorememberthattheleverarmforcalculatingthetension in thebottomreinforcementmustnotexceed0.6 times thespan(maximumheightofthearch).
39
Chapter 3
Failures due to Inappropriate Extrapolation of Code of Practice Clauses
Duringthe1960sand1970s,therewasastronginclinationamongstsomeengineerstoextrapolateclausesinthecodesofpracticebeyondreasonablelimits. This may have been a result of the growing pressure to increasespans without reducing the depth of floors and without adding furthercoststodesigns.Rule-of-thumbmethodshadbeendevelopedandrefinedforfloorspansupto7mbutincreasingdemandforlongerspansmadeitalltooeasyforengineerstoextrapolatethecurrentmethodswithoutensuringthattheywerestillvalid.
Codesofpracticegiverulesthatcanatbestprovideapproximationsandwillworkinmost,butnotallcases;theydonotnecessarilygivefactualinformation.
3.1 COOLING TOWERS
InNovember1965,threeoutofeightcoolingtowerscollapsedingalesofover85mph(seeFigure 3.1).Eachtowerwas375feethighandtheyhadbeenconstructedclosertogetherthanusualandhadgreatershelldiametersandshellsurfaceareasthananyprevioustowers.Thehighwindswerecon-sideredtohavetriggeredthecollapse,butaninquiryfound theexactcausetobeanamalgamationofseveralotherfactorsinthetowerdesign:
• BritishStandardwindspeedshadnotbeenusedinthedesign;asaresult,designwindpressuresatthetopsofthetowerswere19%lowerthantheyshouldhavebeen.
• Basic wind speed was interpreted and used as the average over a1-minuteperiod,whereas, inreality thestructuresweresusceptibletomuchshortergusts.
• Thewindloadingwasbasedonexperimentsusingasingleisolatedtower.Thegroupingofthetowerscreatedturbulenceontheleewardtowers—theonesthatdidactuallycollapse.
• Safetymarginsdidnotcoveruncertaintiesinthewindloadings.
40 Failures in Concrete Structures
Based on the findings, wind loading in the initial design was seriouslyunderestimated.
ThecodeofpracticeusedatthattimewasCP1148.Itusedoverallsafetyfactorsanddidnotprovidefactoredloadswithspecialcombinations.Thecollapse occurred a month before the publication of a paper by Rowe,Cranston,andBestonNewconceptsinthedesignofstructuralconcrete.9Thisnewapproachtodesignwouldhaveensuredthatthedesignloadingcombinationwouldhaveresultedinasafestructure.
Figure 3.2showsasimplifiedviewoftheoveralldesignforces,whereWisthewindload;Gistheselfweightofthecoolingtower;hwistheheightofthecentreofthewindforce;andbisthewidthofthebaseofthecool-ingtower.Thetwodesignapproachesgiveverydifferentrequirementstoensurethatnooveralltensionforcesarecreatedinsuchstructures.
PermissiblestressCode(CP 114)approach:Appliedwindloadmoment=W×hw
Restoringgravitymoment =G×b/2G×b/2≥W×hw
Figure 3.1 View of collapsed cooling towers. (Courtesy of Gillian Whittle.)
Failures due to Inappropriate Extrapolation of Code of Practice Clauses 41
Newlimitstateapproachwithfactoredloads:Appliedwindloadmoment=1.4(W×hw)Restoringgravitymoment =1.0(G×b/2)
1.0(G×b/2)≥1.4(W×hw)
Comment—This incidentshouldreallybeexplained inachapterof itsownasthefailurewasaresultofaphilosophybuiltintothethencurrentcodeofpractice,CP114.Althoughthereweresomedefectsinwallthick-ness,themaincauseofthecollapsewasbecausethedesignvaluechosenforthewindloadwastoosmall.ThiscollapseensuredtheearlyadoptionofalimitstatecodeofpracticeintheUK,resultinginacompletelynewapproachtodesign.Thefirstdraftoftheunifiedcodeappearedin1968andin1972itwaspublishedasCP110.10Thiswasthefirstcomprehensivelimitstatecodeofpracticeeverpublished.
3.2 DESIGN BENDING MOMENTS
Itiscommonforengineerstousesimplifiedcoefficientstoobtainbendingmoments for continuous beams and slabs. Although it is generally con-servativetodesigncontinuousbeamswithpinsupports(i.e.,nomomenttransfer to the supports), itmaybeunsafe toassume thatnomoment istransferredtothecolumnsorendsupportsofthebeams.Asanexample,designers using BS 8110,1 Clause 3.4.2 and Table 3.5, often incorrectlyassumepinnedsupports.
The coefficients given in Table 3.5 of the standard are unsuitable forsituationswheremomentscanbetransferredtothesupports.Thisispar-ticularlysoforendsupports.Fortunately,thedetailingrulesrequiresomereinforcementtoresistnominalhoggingmomentsattheendsofbeams.Itisnotuncommonforengineerstoignoreanymomenttransfertosupportsevenwhentheadjacentspansdifferbymorethanthelimitationsgiveninthe codes (e.g., variations in span length should not exceed 15% of thelongest).Evenmasonrysupportsprovidesomemomentresistance.
W
b
hw
G
Figure 3.2 Equilibrium requirement.
42 Failures in Concrete Structures
Thedesignofaparticularconcreteframeincludedatwo-spanbeamforwhichonespanwastwicethelengthoftheother.Theanalysisforverti-calloadsassumedthatmomentswerenottakenintotheinternalcolumn.Thiswasquestionedatadesignreview.Acheckshowedthatthecolumnmoment capacity was not sufficient to take the moments caused by thebeamrotationatthatsupport.Itwasfortunatethatthedesignloadscouldbereducedsufficientlysothataredesignofthecolumnwasnotrequired.
Comment — Although it is not possible to cite particular failures fromthiscase, it isanotherexamplewhere theductilityof reinforcementandtheredundantcapacityofmoststructuresallowalternativeloadpathstopreventfailure.
3.3 PILES WITH HIGH STRENGTH REINFORCEMENT
Anengineerwashopingtoreducethenumberofpilesrequiredforabuild-ingbyprovidingreinforcementwithmuchhigherstrengththanrequiredfornormalhighyieldbars.Figure 3.3showsasectionthrougha225mmdiameterpilewithaproposedDywidagthreadedbar36mmindiameter(DywidagSystemsInternational,Munich).Theyieldstressfyofsuchabaris1200MPa.
Beforegoingintoproductionitwasdecidedtochecktheloadcarryingcapacitywithatestpile.Theresultwasthatthepilefailedatasimilarloadtothatofapilewithnormalreinforcementwithfykof460MPa.
After some thought, the reason for the unexpected low pile strengthbecameapparent.Theactualconcretecompressionstress–straindiagramissimilartothatgiveninPart2ofBS81101andFigure 3.2ofBSEN1992(EC2)2anddiffersinshapefromthatusedinULSdesigninthat,insteadofhavingaplateauofmaximumstress,itpeaksatabout0.002strain.Thisstrainiscloseinvaluetothatoftheyieldstrainofnormalreinforcement(500/200000=0.0025).Thismeansthatthemaximumresistanceoftheconcreteoccursatthesamestrainwhenthereinforcementreachesitsyieldstress(maximumresistance).
Dywidag threaded bar36 mm diameter
fy = 1200 MPa
225 mm dia. pile
Figure 3.3 Proposed pile with Dywidag bar as reinforcement.
Failures due to Inappropriate Extrapolation of Code of Practice Clauses 43
TheyieldstressofaDywidagbarisabout1200MPawhichcorrespondstoayieldstrainof1200/200000=0.006.Thiscompareswiththeultimatestrainoftheconcreteof0.0035,sotheconcretewillfailinacompressiontestlongbeforetheDywidagbarreachesitsyieldstrainorstress.Figure 3.4showsthisgraphically.
Comment—Thisisanexamplewhereextrapolatingfromthecodeclauses(BS 8110, Table 3.1) can lead to unexpected results. It is important tounderstandthelimitsassumedinthecode,iftheyarelikelytobeexceededinaparticulardesign.
3.4 SHEAR CAPACITY OF DEEP SECTIONS
Thedesignofaprestressedbridgesectionfollowedacodeofpracticethatsomeconsideredtooverestimatetheshearcapacitiesofprestressedbeams.Soonafterconstruction,cracksappeared.Therewasalongdebateaboutthecauseofcrackingand,untilremedialworkwascarriedout,temporaryadditionalexternalshearreinforcementwasputinplace(seeFigure 3.5).
0.00350.002
Stress
Strain
Stress
Strain
Yield stress of normal high yield reinforcement
Yield stress of dywidag bars
Steel stress/strain curve
Concrete stress/strain curve
0.67fcu
0.8fcu
Actual curveULS design curve
Figure 3.4 Stress–strain curves for concrete and steel.
44 Failures in Concrete Structures
ThecodeinquestionhadsimilarclausestothoseofBS81101thatpro-vide an expression that increases the shear resistance according to howmuchmoment is required tonullify the effects ofprestress.The follow-ingexamplecomparingcalculationsinBS81101andBSEN1992(EC2)2showshowsuchananomalycanoccur.
Designinformation:ShearforceV=9000kNM0/M =0.4Prestress =0.8MPafpe/fpu =0.5d =2700mmb =800mmfck =30MPafcu =37MPafy =500MPa
Mainreinforcement:12 bars (32 mm diameter) to simulate both pre-stressingtendonsandreinforcement
Links:16mmdiameter(sixpersection)at300mmspacing
BS 81101:(Cl.3.4.5.4)vc=0.79{100As/(bd)(fcu/25)}1/3(400/d)1/4/1.25=0.55MPa(where(400/d)1/4≥1)
Vc=bdvc /1000 =1189kN(Cl.4.3.8.5)Vcr=(1–0.55fpe/fpu)bdvc+VM0/M=4462kN
Figure 3.5 Temporary repair work on bridge.
Failures due to Inappropriate Extrapolation of Code of Practice Clauses 45
VcrandVcaretheconcreteshearresistanceswithoutshearreinforcementwithandwithoutprestress.Thelargedifference,theshearresistancefromprestress,3255kN,isquestionedbysome.
Theshearresistanceoftheshearreinforcementwas
Vs=(Asv/s)0.87fyd =4723kN
Thetotalshearresistanceisthus
Vt=Vcr+Vs =9185kNOK
EC22: The calculation of the concrete shear resistance without shearreinforcement,VRd,c, is similar to thatofVcr inBS81101, except that itincludestheeffectsofprestressbyfactoringtheaxialprestress.
(Cl.6.2.2)VRd,c=[0.12(1+√(200/d))(fck100As/bd)1/3+0.15σcp]bd/1000=1042kN
Thereductionfactorforthissectioncomparedwithasection200mmdeepis0.64.
Thecalculation for shear resistancewith shear reinforcementuses thevariabletrussmethod.Choosingcotθ=2forthisexampletheshearresis-tanceofthelinksis
(Cl.6.2.3)VRd,s=(Asv/s)zfywdcotθ/1000 =8497kNNotOK
andtheshearresistanceoftheconcretestrutis
VRd,max=αcwbzν1fcd/{1000(cotθ+tanθ)} =8540kNNotOK
ThetotalshearresistancepermittedbyBS81101isthesumoftheresis-tancesoftheconcreteandshearreinforcement.Noreductionisrequiredfor thedeepsection.This is incontrast tocalculation toEC22whicha)reducestheshearresistanceoftheconcretewithoutshearreinforcementbyalargefactorandb)onlypermitstheresistanceoftheshearreinforcementorconcretestrut.
Comment—Althoughthisdoesnotrepresentaveryunsafesituation, itdoesemphasizetheimportanceoftheincreasedreductionfactorfordeepsectionsinEC22.
Thisexamplealsodemonstratestheverydifferentapproachesofthetwocodesofpracticetoshearresistanceandespeciallyshearresistanceofpre-stressedsections.
47
Chapter 4
Failures due to Misuse of Code of Practice Clauses
4.1 FLAT SLAB AND TWO-WAY SLAB BEHAVIOUR
Aflatslabisdefinedasaplatesupportedonindividualcolumns.Atwo-wayslabisaslabsupportedbybeamsateachedge.TheUKcodesofpracticedifferentiatebetweenflatslabsandtwo-wayslabs.
Acarparkwasdesigned(basedonadesign–buildcontract)asacofferedslabsupportedbycolumns.Thecofferswereomittedaroundthecolumnssothatthesolidsectioninthisregionprovidedpunchingshearresistance.Thedesignerclearlyunderstoodthatthesystemwouldbehaveasaflatslabbutdecidedtousethesimplifiedappliedmomentcoefficientsforatwo-wayslab,ignoringanybeameffects.
Themaximummomentsforatwo-wayslab,takenfromTable 3.14ofBS8110,1are:
Hogging 0.031nl2
Sagging 0.024nl2
Themaximummomentsforaflatslab,takenfromTable 3.12ofBS8110are:
Hoggingandsaggingforinteriorspans 0.063Flor0.063nl2
Hoggingandsaggingforouterspan 0.086Flor0.086nl2
Thereinforcementdetailedwasthuslessthanhalfthatrequiredforaflatslab! For some reason, the contractor fitted only half the reinforcementdetailedforthesupports.Theslabfinishedupwithonlyaquarteroftherequiredreinforcementinsomeareas.
After10yearsofservice,crackswereappearingintheslabandadecisionhadtobemadetodeterminewhatremedialworkwasrequired.Althoughthe car park did not appear to be about to collapse, the remedial workrequiredtoensureitssafetywasconsideredtooextensiveandthestructurewasdemolished.
48 Failures in Concrete Structures
Comment—Thiswasaclearexampleofadesignerrorcompoundedbyfurthererrorsonsite.Onereasonthatthestructuredidnotcollapsewasthattheactualloadingwasmuchlessthanthedesignvalueof1.5kN/m2.Theactualloadonacarparkmaybeaslittleashalfthisvalue.Anotherreasonfortheapparentstrengthislikelytobemembraneactioneffectsnotincludedinthedesign.
Thisisanexampleofthebenefitsofanindeterminatestructure.Thereexistedalternativeloadpathstothatconsideredindesign,whichpreventedcollapseofthestructure.
4.2 RIBBED SLAB SUPPORTED ON BROAD BEAM
Several buildings of a university included ribbed floor construction (seeFigure 4.1).Theribbedslabwasdesignedasasimplysupportedelementspanningbetweenedgesofabroadbeam.Thecurtailmentofthelongitudi-nalreinforcementintheribswasinaccordancewiththecodeclausestatingthat,‘eachtensionbarshouldbeanchoredbyoneofthefollowing:…b)aneffectiveanchoragelengthequivalentto12timesthebarsizeplusd/2fromthefaceofthesupport….’Inthiscase,thecurtailmentwasmeasuredfromtheedgeofthebroadbeam.Topreinforcementintheslabwasprovidedintheformofastructuralfabric.
Thedepthofthebroadbeamwasthesameasthetroughslabandthedesignwronglyassumed that it couldbe consideredaone-way slabandthereforedidnotrequirelinks(seeFigure 4.2).
Inorderfortheedgeofthebeamtobeconsideredinthedesignasthesupportface,theverticalforcefromtheloadontheribbedslabhadtobe
Trough slab
Broad beam with samedepth as trough slab
Figure 4.1 Plan view of trough slab.
Failures due to Misuse of Code of Practice Clauses 49
transferredtothetopofthebeam.Verticalreinforcementshouldhavebeenprovidedforthis.Linksinthebroadbeam(inadditiontoanylinksneces-saryforshearresistance)couldhaveprovidedsufficientresistance.
Failureoftheribbedslaboccurredatthesupportwiththebroadbeam(seeFigure 4.3).Completecollapsewasavoidedbyimmediatetemporarypropping.Thesequenceofthefailurewaslikelytobe:
• Yieldingofthefabricreinforcementinthetopflangeoftheslab• Largeflexuralcracksopeningnearthesupport• Redistributionofmomenttransferredtensionstressestothebottom
reinforcement• Combinationofanchorageandshearfailure
Assumed width of beamSingle wayribbed slab
Figure 4.2 Section through broad beam.
Flexure cracks
Single way ribbed slab
Bond failure resultingin shear failure
Assumed beam
Figure 4.3 Section through ribbed slab at failure.
50 Failures in Concrete Structures
Comment—Failurecouldhavebeenavoidedif(1)thespanoftheribbedslabhadbeentakenasthecentre-to-centredistanceofthebroadbeams,or(2)linkshadbeenprovidedinthebroadbeam.Abetterdesignwouldhaveincludedbothfeatures.
4.3 CAR PARK COLUMNS
Figure 4.4showsthebeamsonthesidesoftheinternalcolumns.Theyaresteppedtoallowrampingbetweenlevels(asisoftenthecase).Theinternalcolumnsweremuchstifferthantheedgecolumnsandinconsequencethesupportmomentsandshearforceswithinthecolumnwerehighcomparedwiththeedgecolumns.Thecolumnshearforcescausedlargeshearcracks(upto2mm)withinmanyofthecolumns.
At first sight, this might be considered as an ultimate limit state.Howevertherewasstillaloadpathfortheverticalforcesthroughthecolumn and when the columns cracked, the moment in the adjacentbeamwasredistributedtothespan.Fortunately,thespanresistancewasadequate.
Althoughafailuremechanismwasnotpresent,aproblemoccurredfromrepairingthecracks.Assoonasthecarparkfilledwithvehiclesaftertherepair,thecracksreopenedandeventuallythedeteriorationofthecolumnconcretecouldhaveledtoacollapse.
Thedesignhadbeencarriedouttotheexistingcodeofpracticeofthetimewhichdidnothavespecificclausesforshearincolumns.Bythetimeremedialworkwascarriedout,newcodeclausesinplacerequiredshearchecksofcolumns.Thedesignofthisparticularjointwasinadequatetothenewclauses.Theinsurancecompanyrequiredremedialworktobecarriedoutsothatthebuildingwouldcomply.Thiswasconsideredtobesocostlythatthedecisionwasmadetodemolishtheexistingstructureandreplaceitwithanewdesign.
Edge columns Internal columnsShear cracks
in column
Figure 4.4 Structural elevations of beams and columns of car park.
Failures due to Misuse of Code of Practice Clauses 51
Comment—Althoughtheexistingdesigncodedidnotcoverthisparticu-larsituation,thedesignershouldhavebeenawareoftheproblemandtakenaction in thedesign topreventoverstressingwithin the joint.This is anexampleofcommonpracticeinthe1960sand1970swhenmanyengineersextrapolatedtheexistingclausesofthecodesofpracticeforsituationsout-sidethescopesofthecodes.
Typicallycontinuousbeamsweredesignedassumingmomentswerenottransferredtocolumns.Thisensuredsafedesignsforthebeamsbutmeantthatcolumnscouldbecomeoverstressed,asoccurredinthissituation.
53
Chapter 5
Problems and Failures due to Inadequate Assessment of Critical Force Paths
5.1 HEAVILY LOADED NIBS
Itispossiblethataheavilyloadednibmayrequiremorethanoneloadpathtotransferaloadsafely.Strut-and-tiemodelscandemonstratealternativemethodsforreinforcing.However,itshouldberealisedthattheleastdirectpathswillcausethemostdistortionandcracking,andshouldnotbeusedfortheserviceabilitystate.
Figure 5.1showsthemostdirectstrut-and-tie(primary)model.Theforcepathsareclosesttothatofanelasticmodelandwillcreatetheleastinter-naldistortiontoachieveequilibrium.Figure 5.2showsasecondarystrut-and-tiemodel.Thismaybeaccompaniedbydistortionandcrackingoftheconcretebeforeitcanachieveequilibrium.
Iftheforcesonthenibaretoogreatfortheprimarymodel,itisreason-able to superimpose the secondarymodel toprovide sufficient resistancefor the total ultimate loads.However, it is important to ensure that theprimary model is sufficient to resist the serviceability loads and providecrackcontrol.
Comment—Thisapproachwasusedforamajorviaduct.Thecombinationofthetwomodels(seeFigure 5.3)enabledallthereinforcementtofit—just!
5.2 SHEAR WALL WITH HOLES AND CORNER SUPPORTS
Amulti-storeyshearwallrequiredsomanyopenings(windows,doors,etc.)that the loadpathbecamevery complicated.Thedesigner assumed thattheloadwouldflowtothecornersandthentrackverticallydowntheedgeofthewall(seeFigure 5.4a).Infact,sincethewallwasbuiltinsituasahomogeneousstructure,straincompatibilitycausedtheloadtoflowbackintothefullwidthofthewall.Theresultwasthatseveralstoreysofload
54 Failures in Concrete Structures
weresupportedbyadeepbeamthattransferredtheloadtoitsendsupports(seeFigure 5.4b).
The limitingheightof thenaturalarchofadeepbeam(0.6×span)wasnot considered (seeFigure 5.4b)and this resulted in theomissionin thedesignofmuchof the reinforcementneeded for thebottomtie.Constructionhadreachedseveralfloorsupbythetimethemistakewasrecognisedandthisledtoaredesignofthewallduringconstructionandheavyremedialwork.Eachpartofthewallrequiredcarefulre-appraisal.This led to the requirementofmuchmore reinforcementat eachfloorlevel.Thebottomcornerreinforcementdetailsrequiredspecialattentiontoensurethatthejunctionbetweenthetieandcompressionstrutswasadequatelydesigned.
Figure 5.5 shows in a simplediagrammatic formhow the forcepathsautomatically flow out and back again. The assumed force path downtheedgeswouldnotrequiretiesattopandbottom,butwithoutthesetheactual forcepathwouldcause largecracks toopenupfromthe topandbottomsurfaces.Evenaftercracking,theanglestrutswouldstillexistandsowould theconsequentialhorizontal component.Without sufficient tieforcetoresist,thesupportjointwouldmoveoutwardandeventuallyfailurewouldfollow.
Figure 5.3 Example of use of the two strut-and-tie models. (Courtesy of Gill Brazier.)
Figure 5.1 Primary strut-and-tie model.
Cracking
Figure 5.2 Secondary strut-and-tie model.
Problems and Failures due to Inadequate Assessment of Critical Force Paths 55
Comment—Theconsequenceofmissingthissimpleprincipleofdeepbeambehaviourbeforeconstructionreachedsuchanadvancedstatemeantthatitrequiredtheredesignofthestructureandreprogrammingofconstructionwhichwereextremelycostly.
(a) Incorrect simple modelling (b) Correct simple modelling
Figure 5.4 Multi-storey shear wall.
Without tie rein-forcement large
cracks form
Assumedforce path
Actualforce path
Tie
Tie
Figure 5.5 Modelling deep beams.
56 Failures in Concrete Structures
5.3 DESIGN OF BOOT NIBS
Wherenibsareattachedtothebottomofabeamitisimportanttounder-standtheloadpathoftheforces.Figure 5.6showsatypicalsectionofsuchanib.
Theconventionalassumptionforashortcantileverofdcandzc(showninredinFigure5.6)isunsafeforsuchanib.Thedesigncompressionzoneforsuchamodelwouldbeclosetothebottomfaceofthebeamandlikelytofalloutsidethebeamreinforcement(boththelinksandmainreinforce-ment).Strut-and-tiemodellingishelpfultoexplainwhythisisso.Thestrut(showninredinFigure5.6)wouldjustcausethecovertothereinforcementtospalloff.Thestrutmustbesupportedmechanicallybythereinforcementofthesupportingbeam(showninblackinFigure5.6).Theeffectiveleverarmbecomesmuchsmallerandthetensionforceinthenibtopreinforce-mentmuchlargerthanassumedbytheshortcantileverapproach.
Itshouldalsobenotedthattheforceinthesupportinglinksofthebeam,Ft2d,islikelytobemuchgreaterthantheappliedloadonthenib,FEd,tosatisfyequilibrium.ForthesituationshowninFigure 5.6,itisconservativetoassumethecompressionactsatthecentroidofatriangularcompressionstressblock.Hencetheforceinthelink,inadditiontoanyshear,maybecalculatedasfollows:
Fc=FEd×ac/zb
Ft2d=Fc+FEd=FEd(1+ac/zb)
Comment—There are probablymanynibs of this type that have beendesignedincorrectlyandsurvivebecauseofbuilt-insafetyfactorsandthefactthattheloadassumedinthedesignhasnotoccurred.
Theerrordescribedinthiscasestudywasfoundinthedesignofanibforaveryprestigiousproject.Itwasveryfortunatethatitwasdiscoveredbeforeconstructionstarted.
db
FcFEd
Ft1d
Ft2dHEd
zn
ac
zcdc
zb
Figure 5.6 Nib attached to bottom of a beam.
57
Chapter 6
Problems and Failures due to Poor Detailing
Poordetailingisoftenconnectedwithalackofsufficientdesignthoughtandaccompaniedbypoorworkmanshipinconstruction.Thecombinationcanleadtostructuralfailure.Manyofthecasestudiesinthischapterhaveresultedfromextrapolationsfrompreviousjobs.Smallmodificationsweremadetosaveconstructiontimeandcost.Thiswasnotaccompaniedbysuf-ficientcheckstoensureasafestructure.
6.1 CONCRETE OFFSHORE PLATFORM
TheplatformincludedalargecellularconcretestructurebelowthethreetowersasshowninFigures 6.1and6.2.Duringconstruction,theplatformunderwentsubmergingfordeckmatingafterwhichtheplanwastoraiseitagainandtowittoitsfinalpositionintheoilfield.Itwasduringthesub-mergingpriortodeckmatingthatoneofthetri-cellsfailed.Thiscausedfloodingofthestructureandfurtheruncontrolledsinkingthat ledtoanimplosionofthestructureandcompletecollapse.
Figure 6.3showsadetailofthetri-cellwallthatwasdesignedtoresistthewaterpressurewhenthecellularbasewassubmerged.Figure 6.3bshowstheoriginalformofthecellswithcylindricallyshapedwalls.ThenaturalarchactionprovidedbythatgeometrywasnotpresentinthemodifiedformshowninFigure 6.3a.
Theanalysisofthecellstructurewascarriedoutusingafiniteelementsoftwarepackage.Theaccuracyoftheanalysiswasreducedasthearrange-mentofthequadrilateralelementsmeantthatthoseintheregionofthetri-cellcornersweredistortedfromtheidealsquareshape.Thisledtoerrorsintheresults.Itwasrealisedafterthefailurethattheshearstressresultsfromtheanalysiswereinerrorontheunsafeside.
ThecriticalshearsectionwasreinforcedwithT-headedbars.Thesoft-ware package gave the required areas of reinforcement, and the designrequiredthatthelengthoftheT-headedbarswouldextendacrossthefull
58 Failures in Concrete Structures
See detail
Figure 6.2 Plan section of cell structure.
Figure 6.1 Concrete offshore platform during construction. (Courtesy of Gillian Whittle.)
Problems and Failures due to Poor Detailing 59
widthofthesection.Astheyweredifficulttofixthroughtheouterlayerofreinforcement,itwasdecidedtoreducetheirlength(seeFigure 6.4).
Acrack formedatacornerof thecellandspreadto theendof theTbar.Thewaterpressurebecameactiveinthecrack,makingthesituationworse.Ashearcrackdevelopeduptothecompressionzoneofthesectionandthis failed inabrittlemanner.Theresultingmassive leak ledtothestructuresinkingfurtheruntiltheincreasedwaterpressurecausedprogres-sivefailureofthewholecellularstructure.Itfinishedupontheseabedasamassofrubble.
(a) As built (b) Original form
5800
800
550
Waterpressure
Figure 6.3 Detail of tri-cell.
Compressionfailure
Initial cracking
Water pressure
‘T’ headed baras required
‘T’ headed baras fixed
Figure 6.4 Section through tri-cell where failure occurred.
60 Failures in Concrete Structures
Comment—Thiscatastrophicfailurewastheresultofanumberoferrors:
• Theanalysisprogramwassetupwithafiniteelementmeshthatwastoocoarsetoprovideaccurateshearresults.
• TheT-headedbarsweretooshortandallowedtheshearresistancetobecomeunsafe.Thiswasprobablytheprimarycauseofthefailure.
• There was minimal checking of the design and detailing, possiblybecausethiswasatypeofstructurethatwaswellestablished.Inthesameperiod,thesamedesignerwasinvolvedinthreequitedifferentandmorecomplicatedplatforms.
• In previous designs, the geometry of tri-cells had been formed byintersectingcylinders.Althoughsomecrackingoccurred,thegeom-etryensuredthatsufficientarchingactiontookplacewithoutinduc-inglargeshearstresses.Thegeometryoftri-cellswasalteredonthisproject to make the formwork simpler to construct. Unfortunatelythenewformdidnotallowarchingactiontotakeplaceandthesharpcornersactedascrackinducers(thesewerealsopresentintheoriginaldesign).
• The rebuild retained the cylindrical geometry in the tri-cells andthe reinforcement was detailed to ensure mechanical linkage. TheT-headedbarswereextendedtotheouterreinforcement.
6.2 ASSEMBLY HALL ROOF
Thisdisastercouldalsobecalledthemiracleofthedecade.On13June,1973,lateintheevening,theroofofanassemblyhallcrashedtotheground.Inthewordsofthecaretaker,heheardaloudrumble,wenttoinvestigatebytorchlight,andfoundthewholeroofweighingmanytonshadcollapsed(see Figures 6.5 to 6.7). Twenty-four hours before this event, some fivehundredparentshadattendedameeting in thehallandthechairswerestillinplace.
The principal cause of the collapse was inadequate bearing for beamseatingsanddeteriorationofconcreteatbeamends.Thehallwasoneofthefirstbuildingsfoundtohavesufferedfromtheeffectsofhighaluminacement(HAC;seeSection7.4).
Comment—Thiswasanexampleofinadequatedesignandpoordetail-ingoftheendbearingnibsbuiltintothesupportingbeamfortheprecastbeams. The reduction in strength caused by HAC left no margin fortemperature effects.The combinationwas likely to have triggered thecollapse.
Problems and Failures due to Poor Detailing 61
Bearings for precast beams
Figure 6.5 Assembly hall showing edge beam that supported the precast beams. (From Scott, G.A., Building Disasters and Failures, Construction Press Ltd., Lancaster, 1976. With permission.)
Figure 6.6 Part of roof that collapsed on chairs below. (From Scott, G.A., Building Disasters and Failures, Construction Press Ltd., Lancaster, 1976. With permission.)
62 Failures in Concrete Structures
6.3 UNIVERSITY BUILDING ROOF
OnaJunemorning,followingaperiodofcoolnightsandveryhotsunnydays, aportionof a roof, including threeof theprestressedbeams, col-lapsedontothefloorbelow.Afourthbeamhadpulledoutofitsseatingbutremainedinplacejammedagainsttheedgebeam.
The roofwasconstructedofprestressedprecast concretebeamsmadewithHACspanning12.6m, supportingprecastconcrete slabsactingaspermanentformworkforaninsituconcretetopping(seeFigure 6.8).Theouterendsofthesebeamsrestedin50mmdeeppocketscastintotheverti-calinsidefacesoftheedgebeams.Inadditiontothese,theedgebeamshadrecessesjustbelowtheirseatings.Theseencroachmentsonthealreadynar-rowsectionledtotheadoptionofareinforcementarrangementthathadnomainsteelundertheseatingpocketsfortheprestressedbeams.
Thecauseof thiscollapsewasadisastrouscombination.ThecolumnsandedgebeamswereprecastasTunits(withwebsfacingoutward).Thearchitecthad,inordertoobtainadarkcolour,specifiedHACconcretetobemadewithaggregatethatwashighlyalkaline.Thespecificationhadbeenmistypedormisreadtocall foronepartHACtofourpartssandtotwoparts aggregate (thesemixproportionswere confirmedby analysis aftertheevent).CoresdrilledfromoneoftheTunitsafterthecollapserevealedstrengthsaslowas7MPaandsomecouldnotbeextractedinonepiece.
Figure 6.7 Collapsed roof lying on floor below. (From Scott, G.A., Building Disasters and Failures, Construction Press Ltd., Lancaster, 1976. With permission.)
Problems and Failures due to Poor Detailing 63
Thedepthof seating for thepretensioned roofbeams satisfied thecoderequirementforbearingpressure,butleftnomarginforerectiontolerancesortemperatureeffects.Therewasinadequatereinforcementdetailedunderthebeamseatings,andastheTunitswerecastwithexternalfacesdown,eventhatreinforcementwasdisplaceddownward,awayfromtheseatingledge.
Bearing stresseson the shallowseatingandvertical stressesunder thepocketswerewithinthecoderecommendations,andonlyaninvertedhatbarwasprovidedunderthebeamseating;thiswasdetailedinawaythatmade it difficult to construct in the correct position as the edgeprecastbeamswerecastwiththeoutsidesfacedowntoproduceasmoothfascia.
Linksprojectedfromtheprestressedbeamsintotheinsitutoppingandtheedgebeamshad9.5mmmildsteelbarsprojectingintotheinsitutop-ping,thusprovidingsometyingtogether,butnotattheleveloftheseating.
Theactualmechanismthat led to the failurewas found tobe thermalhoggingof theprestressedbeams.The tie steel projecting from the edgebeamintotheinsitutoppingactedasahinge,andtheendrotationaccom-panyingthethermalflexurebetweenthetwoextremescorrespondedtoahorizontalmovementattheleveloftheseatingofapproximately2mm.ThiscausedcrackingofthebearingnibasshowninFigure 6.9.Thetensionstressintheconcretesupportreduceditsshearresistancesufficientlytocausethefailure.Theprestressedbeamhadbeenmadewithhighaluminacementandalthoughitaggravatedthesituationitwasnotthecauseofthefailure.
Onanotherbuilding,anidenticaldetailwasfound,buttheedgebeamswereconstructedwithordinaryPortlandcementconcrete.Inthatcase,acracksimilartotheoneshowninFigure 6.9wasfound,butpresumablythereinforcementhadnotbeenbadlydisplaced,as fullcollapsehadnotoccurred.Thisdid,however,demonstratethatHACwasnottheprimarycauseofthecollapseofthisroof,butacceleratedthecrackinguntilcollapsehadbecomeinevitable.
Dry seat(no mortar bed)
Links projectinginto in situ topping
Prestressedbeam
150
50Bar under seat
Precast soffitslabs
Tie steel projecting into in situ topping
Figure 6.8 Prestressed beam and support.
64 Failures in Concrete Structures
Comment—Insummation,thecollapsewascausedby:
• Unfortunate selection of aggregate and ignorance of the material’sproperties
• Poordetailingoftheedgejoint• Insufficientbearingintheedgebeam• Insufficienthorizontalrestraintatthelevelofthebearingtoresistthe
relativemovementduetotemperatureeffects• Thereinforcementdetailedunderthebearingwasdisplaced
6.4 MINIMUM REINFORCEMENT AND CRACKING
Thedesignofpartsofabridgedeckslabdidnotrequiremorethanmini-mumreinforcement.Somemonthsafterconstructionlargecracksbegantoappear.Thesizesofsomecracksincreasedto2mmandthereasonwasnotimmediatelyclear.
Onerequisiteofacodeofpracticeforcrackcontrolistosatisfythemini-mum reinforcement percentage.Unless this is provided, the spacing andsizingofbarshavelittleeffectoncrackwidth.Thereasonforthisisbasedonthetensionstrengthoftheconcrete.Ifforanyreason(shrinkageetc.)theconcretecracks,itisessentialthatthereinforcementdoesnotyieldatthecrack.Ifitdoesyield,thecrackwillbecomelargeandthiswillpreventtheoccurrenceofsmallcracksatsmallspacings.Inordertoavoidsuchasituation,thetensionyieldstrengthofthereinforcementshouldbeatleastequaltothetensionstrengthoftheconcrete.Codesofpracticestipulateaminimumamountof reinforcementbasedon the tension strengthof theconcrete.
Thedesignforthisprojectspecifiedaconcretestrengthoffck=30MPaandtheamountofminimumreinforcementpercentagewascalculatedandprovided for this value. Unfortunately, the contractor provided concrete
Point ofrotation
�ermal gradient causesbeam to hog and rotate
Crack
Outwardmovement
Figure 6.9 Detail at support.
Problems and Failures due to Poor Detailing 65
withastrengthof50MPa.Becauseofitshighertensilestrength(4.1MPacompared with 2.9MPa), the reinforcement yielded when the concretecrackedandthecracksbecamelarge—upto2mm!
Comment — This is one of the few, but not insignificant, cases whereincreasingamaterial’sstrengthmakesthesituationworse.
6.5 PRECAST CONCRETE PANEL BUILDING
Intheearlyhoursof16May,1968,agasexplosioninabathroomonanupperfloorshookabuilding,resultingintheinstantaneouscollapseofpartofonewing(seeFigure 6.10).Fourpeoplewerekilled.Figure 6.11showsacloserviewoftheupperfloorsaftertheexplosion.
Figure 6.10 Progressive collapse showing point of explosion. (Ronan Point building col-lapse, May 16, 1968, London. Courtesy of Building Research Establishment.)
66 Failures in Concrete Structures
Thereasonsforthecollapsewere:
1.The possibility of unusual, and hence non-codified, loads was notconsidered.
2.Thestructurewasinadequatelytiedtogether.
Traditionalpre-WorldWarIItwo-storeyhousingwouldnothavehadanyengineeringinput;brickwallthicknessesandtimberfloorjoistsizeswereprescribedbytheLondonCityCouncilBuildingby-laws,andsimilarregu-lationsoutsideLondon.Therehadbeengasexplosionsbeforethisincidentinsimilar typesofdwellings,but thedamageandcasualtieshadusuallybeenlimitedtoonehousehold.Theriskwasacceptedasa‘factoflife.’
Therewerethereforenoprecedentsforprogressivecollapse,whensystembuildingwasintroduced.Forfour-storeywalk-upblocks,thematerialsandthicknessesofload-bearingwallswouldsimilarlybeprescribed.Anyfire-breakingfloorswouldbestraightforwardreinforcedconcreteslabs,designedforoccupationalloading.Theywerecastinsituontothewallsbelowandtherefore‘stucktothewalls.’Evenifthedesignspanwasparalleltothewallbelow,theslabwouldimposesomeloadonit.That,togetherwiththeloadfromthewallabove,wouldprovidesomuchpreloadthatalowerlevelwallwouldbeunlikelytobeblownoutduetoitsprescribedthickness.
Figure 6.11 Closer view of upper floors showing point of explosion. (Ronan Point building collapse, May 16, 1968, London. Courtesy of Building Research Establishment.)
Problems and Failures due to Poor Detailing 67
There was therefore a degree of tying together, albeit reliant on fric-tion,intraditionalconstruction.Inlarge-panelconstruction,mostofthisinherenttyingtogetherwaslostandnotreplaced(withsteelties)untiltherequirementforresistanceagainstprogressivecollapseenteredtheregula-tionsandcodes.
Drypackingofthepaneljoints,whetherwithmortarorfineconcrete,iscarriedoutasamenialtaskafterthe‘spectacular’eventofplacingandplumbing the panel. It is likely to attract less careful workmanship andsupervision.
Comment—ThiscollapsewasasignificanteventfortheindustryintheUKandmarkedthepartialdemiseoftheprecastindustry.Largeprecastpanelandframeconstructionbecamemuchlesspopularinthefollowingtwodecades.InformationgatheredfromtheincidentledtomajorchangestotheUK’sBuildingRegulations(1970)11andcodesofpractice(startingwiththeCP11612precastconcretecodein1970)withregardtoprogres-sivecollapseandrobustness.Morerecently,theEurocodeshaveincludedaccidentalloadandrobustnessclauses.
6.6 FOOTBRIDGE
Afootbridgewasbeingconstructedoveramotorway.ThebridgehadtwospanswithaninsituconcreteT-sectiondeck(seeFigure 6.12).Thespanlengthswere20and40m.Afterremovingtheformworkandprops,twolargetransversecracks(2and3mm)appearedinthesidesandsoffitofthelongerspan.Thecracksoccurredatthepositionsofthelapsofthemainbottomreinforcement.
Thereinforcementhadbeendetailedsothatallthemainbarsinthebot-tom,16T40s,werelappedatpositionsofhighstress.Itisgenerallyconsid-eredpoordetailingpracticetolapbarsinpositionsofhighstressandlapping
Reinforcement not shown
T12s @ 150 inpairs
16T40s lappedat one position
(no staggers)
Figure 6.12 Section through footbridge.
68 Failures in Concrete Structures
all thebars ina congested situation compounds the error.Thebarswerecrankedtoallowthemtobeascloseaspossible(seeFigures 6.13and6.14).Links,T12s@150inpairsenclosedallthemainbars.Noextralinkswereprovidedatthelapsorcranks.
Thelapjointsshouldhaveincludedverticallinkssurroundingeachpairof lapped bars. The presence of the vertical cranked bars increased theforcetoberesistedatthatendofthelapandadditionallinksshouldhavebeenprovidedatthisposition.Theforceduetothecrankedbarsactedasacrackinducer.
CoresweretakenandX-rayexaminationsshowedthatthe lapsofthemainreinforcementhadbeendetailedasshownonthedrawingsandthatconstruction had followed the drawings. It was decided that the bridgeshouldbedemolished.
Comment—Thisfailurewasaresultofpoordetailing.LappingofT40barscreateslargetransverseforcesthatmustberesistedwithlinks.Ifthecoderules(BS81101)hadbeenapplied,muchmoretransversereinforce-ment should have been specified. Good detailing practice would haveavoidedlappingthebarsatthepositionofmaximumstress.
13490
46
4T40s
T12s @ 150 in pairs40 cover
Figure 6.13 Layout of bars at laps.
3 mm crack
Figure 6.14 Side view of bottom reinforcement showing position of crack.
69
Chapter 7
Problems and Failures due to Inadequate Understanding of Materials’ Properties
7.1 CHANGES OVER TIME
Overtheyears, thechanges inmaterialsandreductions insafetyfactorsmake itmore important tounderstand thebehaviourof reinforced con-creteandprovidemorecare.Rulesofthumbandempiricalmethodsmayhave been developed for different conditions and may not be applicablefortoday’smaterials’propertiesanddesigncriterianeedtobecheckedtodeterminewhethertheyarestillapplicable.Anotherresultofthedevelop-mentofandchangestomaterialpropertiesisthattheultimatelimitstateisoftennolongercritical,andadesignnowoftendependsontheservice-abilitylimitstates,apartfrompunchingshear.
Concrete — There has been a continuous increase in the strength ofconcreteoverthelasthundredyears;muchoftheincreasehasdevelopedsince1980(seeFigure 7.1).Aroundthattime,thevalueofblendedcementsand theuseofadmixtureswas realised.Modernconcreteshavebecomecomplexwithalmostinfinitevariationsavailabledependingontherequire-ments.Theunderstandingofhowtochangethepropertiesofconcreteandreinforcementisdevelopingrapidly.Itincludes:
• Theuseofadmixturesandblendedcements.Admixturesareessen-tialformodernconcrete.Self-compactingconcreteisoneimportantexample.Blendedcementsallow thecontrolof the rateof strengthgainandtheamountofheatcreated.
• Theuseofstainlesssteelwillincreaseforsituationswheredurabilityisparamount.
• Theuseof higher strength concretewill becomemorepopular forfloorslabs,particularlyflatslabs.Thiswillresultinthinnerandlon-gerspanslabs.
• Serviceability limit states have already become critical to flat slabdesignanditwillbecomemorecommontocheckvibrationoffloors.
• Theuseoffibreswillincrease;theuseofsteelfibreshasalreadybeenprovenforgroundfloorslabs.
70 Failures in Concrete Structures
Allthesedevelopmentsaddcomplexityandcosttoconcreteconstruction.Aboveacylinderstrengthof50MPa,thestress–strainpropertieschangewithincreasesinstrength.Figure 7.2showsthischangediagrammatically.Theconcrete itselfbecomesmorebrittleas the strength increases,but itshouldbenotedthatinflexuralmembers(beamsandslabs),theductilityandbrittlenessaredependentmostlyonthepropertiesofthereinforcement.
Theincreaseinconcretestrengthandreductioninoverallfactorofsafety(seeFigure 7.3)havemeantthat,formanystructuralelements,thedesignfortheserviceabilitylimitstateisbecomingmorecriticalthanthatfortheultimatelimitstate.
Up to C50/60
Stre
ss
Strain
C90/105
Strain at maximumstress increases
Ultimate strainreduces
Figure 7.2 Change in stress block for high strength concrete.
0
10
20
30
40
50
60
70
80
1915 1933 1938 1948 1957 1965 1969 1972 1980 1983 1985 2005
Concrete Cube Strength (MPa)
Figure 7.1 Increase of concrete strength during 20th century.
Problems and Failures due to Inadequate Understanding of Materials’ Properties 71
Reinforcement—Asimilarpatternofchangehasoccurredforreinforce-mentbothinstrengthandpartialsafetyfactors(seeFigures 7.4and7.5).
7.2 REBENDING OF REINFORCEMENT
In1964,theconstructionofa35mhighdustbunkerforacoal-firedpowerstationincludedanexternalconcretecantileverstaircasetobebuiltontothe faceof theoutsidewallof thebunker.The constructionof thewall
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
1915 1933 1938 1948 1957 1965 1969 1972 1980 1983 1985 2005
Overall Factor of Safety for Concrete
Figure 7.3 Reduction in concrete partial safety factor during 20th century.
0
100
200
300
400
500
600
1915 1933 1938 1948 1957 1965 1969 1972 1980 1983 1985 2005
Yield Stress of High Yield Reinforcement (MPa)
Figure 7.4 Increase in steel yield strength during 20th century.
72 Failures in Concrete Structures
meantthatthereinforcementrequiredforthestairwaywouldbecastflushwith thewalland thenbentoutafter removalof the formwork.At thattime,proprietaryreinforcementsystemsforsuchasituationdidnotexistandthebarswerebentbeforefixingwithintheshutter.Theradiusofbendwouldhavebeentoastandardofthreetimesthebardiameter.
Aftertheformworkhadbeenremoved,thesurfaceoftheconcretewasscabbled toexpose thesebarsandthescaffold tubeswere threadedoverthem.Thescaffoldtubeswerethenusedtoleverthebarsoutofthewallintotheirfinalpositions.About30%ofallthebarsbentoutsnappedoffduringtheoperation.Thereasonwasacombinationoffactors:
• Thebarsshouldhavebeenbentoutwithaspecialtoolthatensuredthattheradiusofbendwasatleastthreetimesthebardiameter.
• Theparticularbatchofreinforcementwasfoundtobemorebrittle(lessductile)thanspecified.
• Theworkwascarriedoutatatemperaturejustabovefreezing.
Theremedialactiontakenwastodrillholesintotheconcreteandgroutinreplacementbars.
Comment—Thebendingoutofreinforcementcast intowalls isacom-monprocedureand,alltoooftenisdonewithscaffoldtubesthatareread-ilyaccessibleonsite. It is regrettable thataproperrebendingtool isnotoften used which is a reflection of poor understanding of the material’sphysical and chemical properties. In the manufacture of reinforcement,specialproceduresare inplace tocheck the rebendingofbars toensurethat the reinforcement is sufficientlyductile. It is unfortunate that some
0.00
0.50
1.00
1.50
2.00
2.50
3.00
1915 1933 1938 1948 1957 1965 1969 1972 1980 1983 1985 2005
Reinforcement Overall Factor of Safety
Figure 7.5 Reduction in reinforcement overall safety factor during 20th century.
Problems and Failures due to Inadequate Understanding of Materials’ Properties 73
manufacturershavecontinuallytriedtoeliminatesuchtestsfromtherein-forcementstandard.
7.3 TACK WELDING OF REINFORCEMENT
The design of a building with large columns required 32 mm diameterstarterbarsprojectingfromthepilecaps.Thetemporaryworksforcon-struction included tack welding some small diameter bars to the starterbars.Whenthetimecametofixthecolumnreinforcementtothestarterbars,thecontractorattemptedtobendthestarterbarstoensurethattheywouldfitintothecolumnshutterwithsufficientcovertotheconcreteface.Alargesledgehammerwasusedtoeffectthis.Duringthisoperation,twoofthe32mmdiameterbarssnappedoff.
Thereasonwasthattackweldingthesmallbarsontothelargerdiameterstarterbarschangedthemolecularstructureofthelatter.Unlikestructuralwelding, tackweldingheats just the local spot,and theheat sinkof themainbarcoolsitveryrapidly.Theresultwasthatthestarterbarsbecamebrittle and requiredonlya sharpblow to fail. In thepast, tackweldingonsitewasforbidden.Todayitissometimespermittedifcarriedoutbyaskilledspecialist.Unfortunatelyoncepermitted,itisalltooeasyforanon-specialisttodothiswork,believingthatitwilldonoharm.
Comment—Toomanypeopleareunawarethattackweldingcanhavesig-nificantstructuraleffects.Thisisanothercasewhereamaterial’schemicalandphysicalbehaviourwasnotproperlyunderstood.
7.4 HIGH ALUMINA CEMENT
Highaluminacementconcretehasachievedacertainnotorietyfollowingthecollapseofseveralbuildings inthe1970s.Bytheendof1974,upto50,000buildingshadbeenreportedassuspectandamajoreffortwasmadetochecktheirsafety.Fortunately,manyoftheaffectedbeamsstoodindryconditions and the chemical deterioration had not reached an advancedstage.Theworstaffectedelementswerepositionedindampenvironments.
Description—Highaluminacementismanufacturedfromlimestoneorchalkandbauxite(theorefromwhichaluminiumisobtained).Thetwomaterialsarecrushedandfiredtogetherusingpulverisedcoalasafuel.Thematerials fusetogether,andaftercoolingarecrushedandgroundintoadarkgreypowder.
Thepredominantcompoundsarecalciumaluminates;calciumsilicatesaccountfornomorethanafewpercent.Thecalciumaluminatesreactwithwaterandtheprimaryproductiscalciumaluminatedecahydrate(CAH10).
74 Failures in Concrete Structures
Oneofitsmaincharacteristicsisthattheconcretemadewithitachievesitsfullstrengthafter24hourscomparedwith28daysforaconcretewithPortlandcement.However,itscrystalstructureisunstableandchangestotricalciumaluminatehexahydrate (C3AH6) spontaneously (albeit slowly).Thisprocessoccursatroomtemperatureandisacceleratedbyanincreaseintemperature.Thecrystalstructuretransformsitselftoamorecompactform,withtheresultthatthecementmatrixoftheconcretebecomesporousandweaker.Theextenttowhichthisconversion,asitisknown,occursislargelyafunctionofthe:
• Originalwater/cementratiooftheconcrete• Temperatureriseintheconcreteduringhardening• Temperatureandmoisturetowhichthehardenedconcreteissubse-
quentlyexposed
Degreeofconversion—ItwasfoundthatatthetimeofthecollapsesmostHACconcreteusedinbuildingswas90%ormoreconverted.Aconcretefrom a wet mix exposed subsequently to the sun was found to have itsstrengthreducedfrom40MPaat24hourstoanaverageofabout10MPaafter less than 10 years. In contrast, concrete from prestressed precastbeamswithalowwater-to-cementratioandhencea24hourstrengthof65MPafromthesamebuildingbut inadryenvironmentwas foundtohaveretainedastrengthofabout35MPa.
7.5 CALCIUM CHLORIDE
Manyreinforcedconcretestructureshavesufferedfromtoomuchchlorideintheconcretemix.Thiscausesthebreakdownofthehighlevelofalkalin-ity.Whenmoistureandoxygenarepresent,carbonationoccurs.Thisallowsthereinforcementtorustandleadstospallingoftheconcretesurface.
Before1980,calciumchloridewasusedextensivelyforinsituconcreteworks,frequentlywithoutadequatesupervision.Itwasusedprincipallyforfrostprotectionandtofacilitatetherapidstrippingofshutters.However,alltoooften,toomuchwasadded.Inthe1980s,thecodesofpracticeandconcretespecificationsweretightenedtoensurethattherustingandspall-ingshouldnothappenagain.Thefollowingthreeexamplesdescribewheretoomuchchlorideinconcretecausedstructuralfailures.
Example1—Aprimary school (built in1952)was shut in1973duetoextensivecorrosionof the reinforcementof factory-madeprecastcon-cretebeams.Thiswasduetothepresenceof toomuchcalciumchlorideaddedduringthemanufactureofthebeamstohastenthehardeningofthecement.The condensationunder thebeamsaccelerated the corrosionbycombiningwiththecalciumchloridetoproducehydrochloricacid.
Problems and Failures due to Inadequate Understanding of Materials’ Properties 75
Example2—In1974,theconcreteroofofaschoolcollapsed.Therea-sonwasfoundtobetoomuchcalciumchlorideintheconcrete,causingthereinforcementtodeteriorateandeventuallyfail.
Example3—Anindependentinvestigationofthecollapseofa100mlongpedestrianbridgefoundthecausetobehighlevelsofcalciumchlorideinthegroutusedintheductsfortheprestressedtendons.Thisledtocorro-sionandfailureoftheprestressingtendons.
7.6 ALKALI–SILICA REACTION
Thealkali–silicareaction(ASR)isaheterogeneouschemicalreactionthattakesplace inaggregateparticlesbetween thealkalinepore solutionofcement paste and silica in the aggregate particles. Hydroxyl ions pene-trate the surface regionsof theaggregate andbreak the silicon–oxygenbonds. Positive sodium,potassium, and calcium ions in thepore liquidfollowthehydroxyl ions so thatelectro-neutrality ismaintained.Waterisimbibedintothereactionsitesandeventuallyanalkali–calcium–silicagelisformed.
Thereactionproductsoccupymorespacethantheoriginalsilicasothesurfacereactionsitesareputunderpressure.Thesurfacepressureisbal-ancedbytensilestressesinthecentresoftheaggregateparticlesandintheambientcementpaste.Atacertainpoint,thetensilestressesmayexceedthetensilestrengthandbrittlecrackspropagate.Thecracksradiatefromtheinterioroftheaggregateoutintothesurroundingpaste.
Thecracksareempty(notgel-filled)whenformed.Smallorlargeamountsofgelmaysubsequentlyexudeintothecracks.Smallparticlesmayundergocompletereactionwithoutcracking.Formationofthealkali–silicageldoesnot cause expansion of the aggregate. Observation of gel in concrete isthereforeno indicationthat theaggregateorconcretewillcrack.ASRisdiagnosedprimarilybyfourmainfeatures
• Presenceofalkali–silicareactiveaggregates• Crackpattern(oftenappearingasthree-pointedstarcracks)• Presenceofalkali–silicagelincracksand/orvoids• Ca(OH)2depletedpaste
In mainly unidirectional reinforced members, the cracks become linearandparalleltothereinforcement.Thedegreeofcrackingdependsontheamountofconfiningreinforcement,i.e.,links,etc.OnemajorconcernwasthatASRcausedcrackingthatledbitsofconcretetofalloffstructuralele-mentsandhitpeoplebelow.Thisledtodemolitionofthestructuresinsomecases.ExamplesofASReffectsaregiveninFigure 7.6.
76 Failures in Concrete Structures
Figure 7.6 Examples of alkali–silica reaction. (Top: From the US Department of Transportation Highway Administration; middle: From Dr. Ideker, http://web.engr. oregonstate.edu/~idekerj/; bottom: From the US Department of Transportation Highway Administration.)
Problems and Failures due to Inadequate Understanding of Materials’ Properties 77
7.7 LIGHTWEIGHT AGGREGATE CONCRETE
Duringthe1960s,amedium-sizecivilengineeringcontractorwantedtojointhehousingdrive,thenatitspeak.Atthetime,anAustrianconstructionfirmusedcrushedbrickrubbleasaggregateinun-reinforcedconcretewallsforsix-andseven-storeyblocksofflats.Inspiredbythis,itwasdecidedtotrytodevelopasimilarformofload-bearingwallwithadequatethermalinsulation,madeoflean-mixplainconcretewithlightexpandedclayaggre-gate(LECA).A12-storeyblockwasconstructedasapilotproject.
Thestrengthofthewallconcretewasreducedinstages—about2000psi(14MPa)at28daysforthefourbottomstoreys,1600psi(15MPa)forthenextfour,and1200psi(8MPa)forthetopstoreys.Thefloorslabswereoftraditionalreinforcedconcrete,buttheroofslabwasreinforcedLECAconcretewithastrengthof3000psi(21MPa).
Therewasnosignificantadversefeedbackfromthetenantsnorthebuild-ingauthority.Theblockremainedstandingandinuseforover40years.Encouragedbytheapparentsuccess,thecontractorstartedpromotingthe‘system.’Aboutthesametime,lightweightaggregateconcretewasincludedinthecodeofpracticeandaminimumstrengthof3000psi(21MPa)wasstipulated.Thisrequiredarichermixthanthatusedforthewallsoftheearlierblock.The resultingeffectsof thison the thermal insulationandshrinkagepropertiesoftheLECAconcreteappeartohavebeenoverlookedbythedesignteam.
AfewblockswerebuiltforlocalauthoritiesoutsidetheLondonCountyCouncil area. These were higher than the first block utilising the higherconcretestrengthrequiredbythecodeinthewalls.Manyoftheflatswereallocatedtotenantsinpoorfinancialcircumstances,whocouldnotaffordthe charges for the underfloor heating and used paraffin heaters instead.This,combinedwiththereducedthermalinsulationoftheexternalwalls,ledtoseverecondensation.
Structurally more important, however, were the diagonal cracks thatdevelopedonthetopfloorofoneoftheblockswithinashorttimeafterhand-over.Fromtheirgeometry,theyappearedtobeduetolowershrinkageandgreaterthermalexpansionoftheroofslabrelativetothewallconcrete.
Definitelyalarmingwastheoccurrenceofhorizontalcracksinoneofthe200mmthickinternalcrosswallsconnectedtothe300mmthickexternalwallatrightangles.Oneofthesecracksonthe13thfloorofa16-storeyblock opened suddenly with a noise like a gun shot. The wall was 200mmthickand,accordingtothedesignassumptions,carriedthefloorslabsthatspannedabout3.5moneitherside.Thismeantthatthebuildingcon-tainedthreestoreysofunreinforcedconcretecrosswall,withtheloadfromapproximately3.5mwidthoffloorsplustheroofhangingorcantileveringofftheexternalwall!
78 Failures in Concrete Structures
Structurally,theonlyexplanationforthesecracksseemedtobethatthe internal wall was drying out, and therefore shrinking and short-ening, while the external wall with very little load to carry (at leastinitially)andexposedtotheBritishweatherwasnotshorteningatthesamerate.
Discussion—TheporousLECApelletsweresoakedjustbeforethemix-ingoftheconcrete,topreventthemfromabsorbingwaterfromthefreshmixandthusmakingittoostiff.Theythereforeconstitutedareservoirofwater,overandabovethatrequiredforthehydrationofthecement.ThisextrawatermeantthattheLECAconcreteneededmoretimetodryoutandthe300mmexternalwallswouldhaveaslowerrateofdryingoutthanthe250mminternalwallseveniftheyhadthesameenvironmentonbothfaces.
Comment—Thesecrackswereduetothechangedpropertiesofthewallconcrete.Aproper studyof thepropertiesof thematerialsalongwithareviewofthedesignwouldhaveshownthatthetwo-stageextrapolationfrommediumrise tohigh riseand from leanmix,brick rubble,unrein-forcedconcretetodense,albeitlightweightaggregate,concretecouldnotbesustained.
79
Chapter 8
Problems and Failures due to Poor Construction
8.1 FLAT SLAB CONSTRUCTION FOR HOTEL
Forashorttimeintheearly1970s,thegovernmentprovidedloansfortheconstructionofhotels.Inorderforaprojecttobeeligible,theconstructionperiodhadtobeverytight.Theworkmanshipofsomeofthehotelsbuiltthenwasshoddy.Foronesuchhotel,theshoddyconstructionwasnotdis-covereduntil20yearslaterwhenamajorrefurbishmentwastakingplace.
Figure 8.1 shows the structural layoutofa typicalflat slabfloor.Thedepthoftheslabwas250mm.Thespansalongthebuildingwere7.2mandacrossthebuildingwere6.1and7.4m.Thetopsurfaceoftheslabwasveryunevenanddidnotappeartohavebeenlevelled(byhandorpowerfloat).Insomeplaces,bootmarkshadbeenleft.Cracks(generallynotlargerthan0.3mmwidth)occurredontheuppersurfaceradiatingfromthecornersofthecolumnswithoneortwosmallcracksrunningtangentially.Largecracks(upto1mmwidth)appearedatsomeoftheconstructionjoints.Thedeflectionofoneoftheslabbaysofanupperfloorwaslarge—over75mm.
Severalorganisationsbecameinvolvedinassessingthesituation.Itwasunfortunatethatoneofthemconcludedthatthestructurewasunsafeandthatoneofthefloorswasindangerofimminentcollapse.Onebayofanupperfloorinquestionwasthensetasideforinstrumentation(bothcompli-catedandcostly).Afternearlyayearofmonitoringmovement,theresultsshowednoperceptibleincreaseofdeflection.However,duringthatperiod,additionalsteelbracketshadbeendesignedtopreventtheslabsfailingbypunchingshear,andtheyhadalreadybeenfittedtothecolumn–slabjunc-tionsofseveralfloors.Afurtherindependentcheckwasinstigatedandthefollowingwereitsfindings:
Excessivedeflection—Inordertounderstandhowtheexcessivedeflec-tionhadcomeabout,itwasnecessarytochecknotonlythedesign,buttheinsituconcretestrength,reinforcementdetails,andcovertothereinforce-mentasbuilt.Thischeckfoundthattheonlyfactornotasspecifiedwasthetopcovertothereinforcementnearthecolumnsupports.Thiswasfoundtobeonaverage30mmmorethanspecified.Thisreducedthemomentof
80 Failures in Concrete Structures
resistanceatthesupport.However,afterreasonablemomentredistributionwasincludedinthecalculations,therewassufficientoverallmomentcapac-ityintheslabwithoutrequiringanyreductiontothedesignsafetyfactors.
This conclusion raised the question ofwhy such large deflections hadoccurred.Oncloseexamination,itwasnoticedthattheedgedeflectionoftheslabalonggridlineBwasalsoverylarge.Inoneofthebays,theskirtingboardbetweentwoedgecolumnshadbeenmadeintwoequallengthssplitinthemiddle(seeFigure 8.2).Deflectionsof15to20mmoccurredbeloweachhalfoftheskirtingboard.Thisrepresentedanedgedeflectionupto50mm.Sincetheskirtingboardwasattachedtothewall,itwaslikelythatitwasfittedthiswayandthatmuchofthedeflectionhadtakenplacebeforeconstructionofthewall.Thiswasconfirmedbyfindingthatthebottomcoursesoftheexternalwallhadbeenlaidonthesaggingshapeoftheslabandthefollowingcoursesadjustedsothattheywerelevelatthewindowsillsabovethefloor.
The conclusion was that much of the deflection occurred during con-struction,someofwhichwasduetothesaggingoftheformworkandsomeduetoearlyremovaloftheformwork.
Edge column
Skirting board
Floor surface15–20 mm
Edge column
Figure 8.2 Skirting board deflection.
1 2 3 4 5 6 7
7200 (all bays)
Down
Down
Down
7400
6100
DC
B
Column size900 × 300 mm
Figure 8.1 Structural layout of hotel floor.
Problems and Failures due to Poor Construction 81
Punchingshear—Punchingshearfailureisdifficulttopredict.Failurecanoccurwithlittlewarning.Thepresenceofcracksintheslab,tangentialtoacirclearoundthecolumninthetopsurfaceoftheslab,indicatedthatthepossibilityofapunchingshearfailureshouldbeconsidered.Thecovertothereinforcementwaslargerthanspecifiedinthisareawhichmeantthatthecracksizewasmagnified.However,thefactthatthesurfacecrackswerelargedidnotnecessarilymeanimpendingshearfailure.
Areliablemethodofpredictingpunchingshear failure isbyusing thecodedesignexpression(BS81101orBSEN1992-1-12)forshearwiththecharacteristicvaluesfortheconcretestrengthinsteadofthefactoreddesignvalues and the as-built information concerning the reinforcement (i.e.,size,spacingandcovertothebars).Anassessmentofsafetycanbemadebycomparingthe ‘worstcredible’ loadswiththeresistanceascalculatedabove.Thecalculationsforthiscaseshowedthattheworstcredibleloadscouldbecarriedwithasufficientsafetyfactor.
Comment—Itwasunfortunatethatthereasonfortheexcessivedeflectionwas not found earlier and that calculations were not made to check thepunchingshearcapacity.Muchcostlyremedialworkcouldhavebeensaved.
8.2 STEEL PILES SUPPORTING BLOCK OF FLATS
Thepilecapsupportingthestructureforamulti-storeyblockofflatsincor-poratedsteelHpiles.Theseshouldhavebeencutoffclosetothebottomofthepilecapbutweretakenuptowithin100mmofthetop.Thepileswerecoatedwithbitumenthathadnotbeeneffectivelyremovedfromtheprotrud-inglengthsthathadtotransfertheloadfromthepilecapsthroughbond.Theresultwasthatasthebuildingworkcontinued,thepilecapsstartedtosettlewiththepilesactingaspistons.Theconcreteatthetopofthepilecapsfailedinpunching.Theremedialwork includeddiggingunder thepilecapsandweldinglargecollarsaroundthepilestopreventfurthermovement.
8.3 SHEAR CRACKS IN PRECAST T UNITS
RemedialworktothebearingofaprecastcolumnrequiredlocaljackingofprecastTfloorunitsforseveralfloorsabovetoreleasetheirload.Thejacksandpropsweretakenrightdownthestructuretotheground.Atonefloor,thepropsaboveandbelowdidnotlineup.TheeccentricloadcrackedtheTunitatthatfloor.Figure 8.3showstheopeningupof2mmshearcracks.
The subcontractor proposed to repair the damaged Tunit bydrillingangledholesdownthecentreofthewebandgroutinginstraightdeformedreinforcingbars.Hewas sufficientlyconfident toagree to then test load
82 Failures in Concrete Structures
thatunitwiththefullultimatedesignloadtocheckthatitwasserviceable.Figure 8.4showshowthebarswerearranged.Therepairworkrequiredcarefuldrillingtoensurethattheprestressingtendons(showndashed)werenotdamaged.Theunitpassedtheextremetest loadandpermissionwasgivenforittoremainaspartofthestructure.
Comment—Itwassurprisingthatstraightbarswerecapableofgeneratingenoughbondandprovidingsufficientshearresistance.
8.4 CANTILEVER BALCONIES TO BLOCK OF FLATS
Fiveyearsafterconstructionoftheflats,cracksappearedonthetopsur-facesofthecantileverbalconies.Beforecarryingoutrepairs,investigationrevealedthatthereinforcementrequirednearthetopsurfacewasplacedhalf way down the section. Although the amount of cracking varied ateachfloorlevel,itwasdecidedtorebuildthebalconiesateveryfloorlevel.
Shear crack
Figure 8.3 Cracked T unit.
(a) Elevation showing the extra shear reinforcement (b) Section through repair
6 No. 20 mm reinforcingbars grouted into web
Figure 8.4 Repair of T unit.
Problems and Failures due to Poor Construction 83
Theengineerwascuriousbecausethefifthfloorlevelshowednosignsofcrackingatall.Whenthebalconyatthatlevelwasdemolished,itwasdis-coveredthatthereinforcementhadbeenleftoutaltogether!
Theconcretehadsurvivedsowellbecauseitstensionstrengthwassuf-ficient to resist all the loads that the slab sustained. The reason for thecrackingoftheotherbalconieswasthatthereinforcementrestrainedtheshrinkageoftheconcreteandthuscreatedtensionstressesintheconcrete.Normallythereinforcementshouldbedetailedwithbarsatsmallspacing.Ifthebalconieshadbeenconstructedwiththespecifiedconcretecover,anycrackingwouldhavehadasmallwidthandbeenatasmallpitch.
Comment—Althoughtheupperbalconyshowednosignsoffailureitwasmuch more dangerous than those for the floors below. If subjected to alargeimpactload,itwouldhavecollapsedsuddenly.
8.5 PRECAST CONCRETE TANK
Aliquidstoragetankwasconstructedwithprecastwallpanels.Thediam-eterandheightofthetankwere12.3and7m,respectively(seeFigure 8.5).The vertical panelswere held inplace byunbondedprestressed tendonsthreadedthroughhorizontalPVCductsembeddedintheconcreteandfullyencirclingthetankatsetlevelsthroughouttheheight.Thetankcollapsedwithoutwarningwithin2yearsofconstruction.Failurewascausedbyanumberofseparatemistakes.
Watertightness—Inorderforthetanktobewatertight,eachprecastverticalpanelhadtobuttuptotheadjacentpanelevenlythroughoutthe7mheight.Arubberstripwasinsertedwithinthejointbetweeneachsetofadjacentpanelsand incorporatedholes throughwhich theprestress-ingtendonswerethreaded.Theprestressingwasintendedtocreateuni-formcompressionthroughouttheheightofthetank.However,toachieve
12.2mAnchor unit
Figure 8.5 Precast elements of tank.
84 Failures in Concrete Structures
watertightconstruction,theedgesofthewallunitshadtobebuiltwithverysmalltolerances.Thedesignoftheseprecastpanels(seeFigure 8.6)allowedforseveraldifferentdiameter tanks.Thismeant that theangleof the edge formwork for the panel was adjustable and had to be setwith extreme care for each diameter of tank. The water tests showedleaks. Several attempts were made to seal them before water tightnesswasachieved.
Prestressing ducts — During the water tests and possibly afterward,waterpenetratedthePVCprestressingducts.Thedesignassumedthatevenifwaterhadpenetratedtheducts,therewouldbesufficientprotectionoftheprestressingtendonsfromtheenclosingsheathandgrease.
Sheathingandgrease—Thesheathandgreaseprovidedacontinuouscoveringofthetendonsuptotheanchoragezone.Inordertoattachtheprestressingjacksandinsertthewedges,thetendonswerestrippedofthesheathingforsomedistance(seeFigure 8.7).
The amount of sheathing required to be cut back depended on theextentoftendonstretchduringprestressing.Inevitablyaftertheoperationsof stressing and locking off, there remained a length of bare tendon. Itwouldhavebeenoverlyoptimistictoassumethatthegreasecovertothestrippedstrandswouldbeintactafterthreadingthemthroughtheanchor-age.Althoughpossible, itwouldhavebeenadifficulttasktoreplacethegreasearoundthebaretendonafterstressingandunlikelytobecompletedsuccessfully.Anywaterthatpenetratedtheprestressingductswouldhavereachedthispartofthetendon.
23 mm PVC ductInterface withadjacent unit
Figure 8.6 Section through wall panel.
7-wire greased tendonPVC duct castinto concrete
Anchorage castinto concrete
Screw in capfilled with grease
Sheath over tendoncut back from end
Figure 8.7 Detail of anchorage zone.
Problems and Failures due to Poor Construction 85
Grease—Thegreaseusedinthisparticulartypeofunbondedtendon(12.5mmdiameterTyesaseven-wirestrandmanufactured inSpain)wasfoundtoemulsifywhenincontactwithwater.Thisallowedanywaterthatpenetrated theanchorzone tonotonlycome intocontactwith thebarepartsofthetendonsbutalsotopenetratethesheathing.
Stress-corrosioncracking—Thealloysteeloftheprestressingtendonsused in this structure had a microstructure susceptible to stress–corro-sioncracking,andthestressinthetendonswasgreaterthan50%oftheyieldstrength.Moistureincontactwiththetendonsprovidedacorrosiveenvironment.Onexaminationafterthecollapse,itwasfoundthatstress–corrosioncrackinghadtakenplaceinmanypartsoftheunbondedtendons.
Comment—IfsimilarstrandmanufacturedintheUKhadbeenused,thegreasewouldnothaveemulsified.Nevertheless,itwouldstillhavebeenadifficulttasktoensurethatthebaredendsofstrandsneartheanchorageswereadequatelyprotected.Althoughitwaslesslikelythatstress–corrosioncrackingwouldtakeplace,therewasstillasignificantrisk.Thereisnorea-sontosupposethatothertanksofthistypedidnotleakwhenwatertestedasitwasanexactingtasktoensurethateachverticalfaceofeverypanelmatchedupexactlywithitspartner.Henceitislikelythattheprestressingductsformanyofsuchtankswouldbefloodedatsomestageintheirlives.Intheopinionoftheauthor,thevulnerabilityofsuchconstructioncastsdoubtontheviabilityandsafetyofsuchsystems.
8.6 CAR PARK
InMarch1997,a120tonnesectionoftheroofofacarparkcollapsedontothefloorbelow(seeFigure 8.8).Thisoccurredat3a.m.when,fortunately,nopeoplewereinthestructure.Itwasimmediatelyclearfromthedebristhatapunchingshearfailurehadtakenplace.
Figure 8.8 Collapse of roof of car park. (From Jonathan Wood, http://www.hse.gov.uk/research/misc/pipersrowpt1.pdf.)
86 Failures in Concrete Structures
Thecarparkwasconstructedusingtheliftslabmethod.Thisinvolvedcasting the slabsoneon topof anotheron theground.Precast columnswerepositionedandthentheslabswerejackedupthecolumnstothecor-rectlevel.Finalconnectionbetweentheslabandthecolumnwasmadeviaasteelcollarintheslabandasteelinsertinthecolumnintowhichwedgeswerefixed.ThesteelcollarsupportedtheslabonanglesthateitherformedasquareoranHinplan.Figure 8.9showsthetypicalHconfigurationusedfor internalcolumns.Thecolumnconnection isverydifferent fromthatfoundinatypicalflatslab.
The230mmthick slabwasconstructedwithconcreteofhighlyvari-ablequality.Areasoflowqualityconcretedeteriorated,probablythroughfreeze–thawaction.Insomeplaces,thisdeteriorationoccurredtoadepthof100mmandhadbeenrepaired.Therepairwaspoorlybondedtotheparentmaterial.This leftaslabthatwaseffectivelysplit into two layerswith the only connection being the longitudinal steel passing throughtherepairintotheoriginalconcrete.Furtherdeteriorationoftheoriginal
Top reinforcementonly shown
Shear headcollar
Punching shearperimeter (1.5d)
Wedge
19
16
130
130
241241H
H22
Figure 8.9 Plan layout at column.
Problems and Failures due to Poor Construction 87
concrete,andinparticularitsbondstrengthtothetopsteel,reducedwhatcompositeactionexisteduntilfailureoccurred.
Comment—ThisformofconstructionhadbeenusedinmanyplacesintheUKduringthe1970sand1980sandhasbeenacommonformofconstruc-tionintheU.S.Ithasprovidedreasonablyrobuststructures.Theverynatureoftheconstructionmethodfocusesattentiononthecolumn–slabjoint.Insomesituations,thestructurehasreliedonthemomentresistanceofthesejoints,i.e.,unbracedframes.Inothersituations,separateinsitucorestruc-tureswerebuilttohandletheswayforcestowhichthecompletestructuremayhavebeensubjected.Insomeofthelaterexamples,Ubarswereweldedtothesteelcollarsandembeddedintothesurroundingconcrete.
8.7 CRACKING OF OFFSHORE PLATFORM DURING CONSTRUCTION
Thesubstructuretothisplatformconsistedofthreeconcretetowerssetonacellularconcretestructurethatwouldsitontheseabedinthefinalloca-tion(seeFigure 8.10).Itwasconstructedonshoreinadrydock.
Duringtheoperationoffloatingtheplatformouttotheoilfieldandsink-ingitintothecorrectposition,itwasnecessarytofillsomeofthecellswithwater(seeFigure 8.11).Thisensuredthattheplatformfloatedattherightlevelasitwastowedtotheoilfield.Thecellsandshaftswerethenfilledinacontrolledmannertoallowtheplatformtosettleontheseabedintothecorrectposition.Thecellswerecompletelyfilledwithwaterduringopera-tionandnonewasusedforoilstorage.
Whilstthemovingoperationwastakingplace,thepartiallyfilledcellscreateddifferentialpressuresamongthecells.Itwasimportanttoensurethat the cells remained watertight during this operation. Arrangements
Figure 8.10 Elevation of offshore platform.
88 Failures in Concrete Structures
weremadeduringconstructiontotestthewatertightnessofthecells.Theintentionwastofilleachoneofthecellstoitstoponly.Eachcellwascon-structedwithaplastictubethatledtothetopoftheshaftstoallowairtoescape from thecellwhen itwasfilledduring thefinal installation.Themainwater supplyused tofill cellswas capableof providing apressuremuchmorethanthatrequired,andthewaterlevelwasallowedtoriseuptheplastictube(seeFigure 8.12).
Water surface
Water ballast
Figure 8.11 Water ballast pumped into some of the cells during sinking operation.
Plastic tube to let air outof cell to sea surface
Excessive pressure causedwater to rise up plastic tube
To test water tightness of cellswater let into cell from the mains
Figure 8.12 Water tightness test during construction.
Problems and Failures due to Poor Construction 89
Onlyaverytinyamountofwaterwasneededtofilltheventpipe,butthisvastlyincreasedthehydrostaticpressure.Thegreatlyincreasedwaterpres-sureinthecellsexceededthedesignpressureandcausedoneofthewallstocrack(Figures 8.13and8.14).TherepairworkincludedcastingreinforcedconcretebuttresseseithersideofthecrackedwallandprestressingthemtothewallwithMacalloybars(seeFigure 8.15).
Comment—Thisfailurecouldhavebeenavoidedifithadbeenrealisedthatthetestingarrangementcouldpermittoohighapressuretobeappliedtothestructure.Thefailurelayintheinadequatemonitoringandcontrolof thewaterpressure in the cell,not in thedesignof the structure.Theoverloadwasaboutthreetimesthedesignpressure.
To test water tightness of cellswater pumped into cell.
Shear cracksoccurred
Figure 8.13 Shear crack in wall.
Shear cracksoccurred
Water pressure
Figure 8.14 Enlarged view of shear crack.
90 Failures in Concrete Structures
8.8 SPALLING OF LOAD BEARING MULLIONS
The facadesof a tallofficeblockweremadeofprecast concrete sculp-tured panels (see Figure 8.16) and incorporated load bearing mullions.Thehorizontalpanelsandmullionjointswereabout600mmabovethefloorlevel.Ithadbeenspecifiedthatthepanelsbelevelledbymeansofsteelshimsandthenmortarbedsbelaidtoaleveljustproudoftheshims.Thepanelswiththemullionsattachedwouldthenbelowereddownontothebedswith the intention that the loadwouldbe sharedbetween themortarandshims.
Sometimeafterconstruction,someofthemullionsbegantospallaroundthebearings.Itwasdiscoveredthatsomeofthemortarbedsweremissing.Inplaceof these, the contractorhadundertaken todrypack the joints,butthishadnotbeencarriedoutthoroughly,probablybecauseitwasanimpracticaltask.Themortarusedwasmuchsofterthanthesteelshimsanddidnotcarryitsshareoftheloadthatwaslargelytransferredthroughtheshims.Theconcentrationoftheloadonsuchasmallareaofconcreteinthemullionscausedseverespalling(seeFigures 8.17and8.18).
Therepairworkincludedinsertingasetofsteelcolumnsateveryfloor(seeFigure 8.19).Toensurethecorrectloadwastakenbyeachcolumn,flatjackswereinsertedateachfloorlevel.Eachjackwasinflatedwithresinthatwasallowedtohardenoncethecorrectpressurewasattained.
Comment—Thisjointdetailmighthaveworkedhadhardwoodplywoodbeenusedfortheshimsinsteadofsteel.TheEvaluewouldhavebeenclosetothatofthesetmortarand,ifpre-soaked,theshimswouldhaveshrunkastheydried,thustransferringtheloadtothemortar.
Prestressed withMacalloy bars
Concrete buttresses
Figure 8.15 Repair work.
Problems and Failures due to Poor Construction 91
Figure 8.16 Office block with sculptured panel facades. (Courtesy of Poul Beckmann.)
Steel shim
Figure 8.17 Spalling of mullions. (Courtesy of Poul Beckmann.)
92 Failures in Concrete Structures
Althoughthisincidenthasbeenincludedinthechapterbasedonpoorconstruction,thedesigndetailforthejointbetweenmullionswasdifficult,ifnotimpossible,toconstructasintended.Itwascriticallyimportantforthestructuralintegrityofthefacadeandmorethoughtshouldhavebeengiventoproducingamorepracticaldetail.
8.9 TWO-WAY SPANNING SLAB
Atwo-wayspanningslabwasdesignedinaccordancewiththedesignrules.Onesidewassignificantly longer thantheotherandthismeantthat the
Concrete profileafter spalling
Steel shims
Figure 8.18 Plan section through mullion after spalling.
Additional steel columns
Figure 8.19 Layout of additional columns. (Courtesy of Alan Steele.)
Problems and Failures due to Poor Construction 93
reinforcementprovided for the short spanwasconsiderablygreater thanthatforthelongspan.
Whenthedrawingarrivedonsite,theengineerdecidedthatthedrawingcontainedamistakeandthatthereinforcementinthelongspanshouldbethegreater.Heinstructedthereinforcementlayouttobeswappedaroundandtheslabwasbuiltthatway.Later,whenthefullloadwasapplied,theslabcollapsed.
Comment—Thisfailurecouldhaveeasilybeenavoidedifthesiteengineerhadcheckedwiththedesignerbeforemakinghisdecision.
8.10 CHIMNEY FLUE FOR COAL-FIRED POWER STATION
Thewindshieldofthischimneywas270mhighand20mindiameter.Atlowlevel,three‘portals’forthehorizontalboilerfluesweretoconnectwiththethreeverticalflues.Theflueswereabout8mindiameterandlinedwithfirebricks.Theirwall thicknesswasreducedabove theportalsbyataperon theoutside surface.The reinforced concrete shaftsof thewindshieldandtheflueswereconstructedsimultaneously,usingslip forming.Theconcretewasmadewithahighproportionofblast-furnaceslagcementreplacement;thePortlandcementandtheslagwerenotpremixed,butsup-pliedseparatelyandstoredinseparatesilosatthebatchingplantthathadasinglemixer.
TheroofslabandoneortwointernalfloorslabswereseparatedfromthefluewallswithFlexcelorasimilarjointmaterialtoallowunrestrainedtem-peratureexpansion.Thefirebrickliningstothefluesweresupportedoncor-bels,andthecastingandthelayingofthebrickworkfollowedtheslipforming.
Abovetheroofslabatthetop,theflueswerealsocladexternallywithbrickwork.Thebricklayershadjustfinishedcappingthelastfluewhenafracturelowerdowncausedthetophalfofoneofthefluestoslidedown,disintegrate,andfillthebottomofthewindshieldwithdebristhatspilledoutoftheportalforthatflue.Oneworkerwaskilled(seeFigure 8.20).
Itwasstillpossibletoenterthechimneyspacethroughoneoftheotherportals, and the inside of one of the intact flues could be examined. Asubstantialhorizontalcrackwasfoundonpartofthewallsurface;insert-ingaknifebladeintothecrackindicatedthatthefracturesurfaceslopedupward.Subsequently,morecracksofasimilarnaturewerefoundontheinsideofthewindshield.
The fracture surfaces on the concrete fragments in the debris seemedmore square and sharper than expected. This was later explained as afeature of sudden disintegration of high-strength concrete by impact, asopposedtotheslowfailuresobservedinlaboratorytests.Coresweretaken
94 Failures in Concrete Structures
atvariouslocations.Anumbershowedpatchesofbluecolouronthefreshlydrilledsurfaces,somedistancefromthewallfaces.Hydratedslagisblueuntil ithasbeenoxidizedbyexposuretotheatmosphere.Thisthereforeindicated that the cementand slaghadnotbeenproperly intermixed.Anumberofcores,someofwhichwereabout0.75mlong,weresawnintotestablelengthsandallshowedadequatestrengths.Othercoresweretakenacrossthehorizontalcracksandshowedclearseparation.
Anexperiencedslipformingexpertwascalledintogiveasecondopin-ion.Heexplainedthatthehorizontalcrackswere liftingcracksthatcanoccurwhenliftingisresumedafterconcretinghasbeeninterrupted.Ifsomeofthehalf-setconcretethenstickstotheform,itisliftedwithit,untiltheweightofthefreshconcretedepositedontopforcesittodropback.Astheconcretemaynotalldropneatlyinonepiece,cavitiesmayresult,obviouslyweakeningthewall.Liftingcrackscanbecausedoraggravatedbyuneven-nessofformworksurfaces;theyareusuallyhiddenbytheslurryrub-downthatiscustomarilycarriedoutfromafinishingplatformsuspendedsome2mbelowaworkingplatform.Recordsandanecdotalevidenceindicatedanumberofoccurrences:
• Ashortdistanceabovethefoundation,awholebandhadbeencastwith practically pure slag. This was discovered early, cut out, andre-concreted.
• Where the wall thickness was reduced above the flue portals, thewholeoftheformworkhadtobeliftedclearoftheconcretetoallowadjustmentoftheoutsideformworktothesmallerdiameter.Thishadtobedonetwice—atthebeginningandattheendofthetaper.Therewassomecongestionofreinforcementinthiszone,someofitduetodesign,someofitduetoineptsteelfixing.
See detail
Detail: Close up of debris
Figure 8.20 Debris from collapsed flue. (Courtesy of Alan Steele.)
Problems and Failures due to Poor Construction 95
• Deliveries of materials were sometimes late, causing concreting tohalt.
• Themixerbrokedown,causingconcretingtohalt.• Supervisionandinspectionwereinadequate.
Theconclusionofhowthecollapseoccurredwasasfollows:
• Onoccasionwhenconcretingstopped,asevereliftingcrackformedintheflue,extendingaboutthreequartersofthecircumference.Becauseofitslocationsomewayuptheshaft,theintactquadrantcouldcarrytheweightoftheshaftabove,albeitwithasubstantiallyreducedfac-torofsafety.
• Solarheatingcaused thewind shield tobend.Theodolitemeasure-mentsshowedthatthetopofthewindshieldonsunnydaysdescribedanirregularhorizontalorbitwithadiameterintheorderof0.75m.
• Theflueshadtofollowthismovement,whichatcertaintimesofthedayincreasedthecriticalstressontheintactquadrantofthecrackedflue. Italsocausedthe liftingcracktoopenandcloseonadiurnalcycle.Thiscausedtheconcretetogrindawayjustbeyondtheendsofthecrack,graduallyreducingtheareaofloadcarryingconcreteuntilfailureoccurred.
Comment—Morethoroughsupervisionwouldhavepreventedthisfailurefromoccurring.The slip formingprocess requires carefulmonitoringatalltimes.
97
Chapter 9
Problems and Failures due to Poor Management
Manyofthecasestudiesintheotherchaptersmayhavebeenavoidedwithbetter management. The three cases reported in this chapter emphasisehowimportantanengineeringbackgroundistothemanagementofstruc-turalengineeringprojects.
9.1 COLUMN–SLAB JOINT
Attimeswhensomuchworkmakesaprojectteamover-committed,itiscommon to assignpackagesofwork toother teams. For this particularjob,aprojectteamdesignedtheslabsandhandedoverthedesignofthecolumnstoanotherteam.Thereinforcementdetailingwascarriedoutbyanothergroup.Allthedesignworkshouldhavebeencheckedbytheprojectteambutinthiscasetheseparatepartsofthedetaildesignwerepassedtothedetailinggroupwithoutanoverallcheck.Itwascommontodetailtheslabsandcolumnsonseparatedrawings.Figure 9.1showsthearrangementoftheedgecolumn–slabjoint.
The reinforcement arrangement intended for this joint is shown inFigure 9.2.Construction reached the secondfloorwhena younggradu-ate visited the building to gain some site experience. He reported backhisconcernaboutthereinforcementlayoutalongtheedgeoftheslabthatwasabouttobeconcreted.Figure 9.3showswhathesaw.Therewasnomechanical linkbetween the slab and the column reinforcement! Itwasimmediatelyrealisedthatnotonlyhadthisoccurredatalledgesoftheslabonthesecondfloor,butalsoonthefirstfloorthatwascompletedseveralweeksearlier.
Immediateremedialworkwasputintoaction.Temporarysupportswereputinplaceforbothfloorsandthenholesweredrilledthrougheachjointandlongboltsgroutedinplace(seeFigure 9.4).
98 Failures in Concrete Structures
Slab
100
Edge column800 × 400
Figure 9.1 Plan layout of edge column–slab joint.
T16s
T12s
T10s
Figure 9.2 Intended layout of reinforcement.
Problems and Failures due to Poor Management 99
Comment—Oneofthereasonsthissituationoccurredwasthatnocheckhadbeenmadeinthedesignofficetoensurethatamechanicallinkcouldbemade. In fact,when thecoverand theactual sizeofbarswere takenintoaccount,itwasclearthatthereinforcementcouldnotfitasintended.Thefabricatorofthereinforcementfittedthebarsasclosetothecorrectpositionaspossiblewithoutthinkingthatamechanicallinkwasessential.
Nevertheless,theunderlyingcauseofthismistakewasinfailuretoman-agethedesign.
T12s
T16s
T10s
No mechanicalconnection
Figure 9.3 Actual layout of reinforcement.
T16s
T12s
T10s
Figure 9.4 Remedial work.
100 Failures in Concrete Structures
9.2 PLACING OF PRECAST UNITS
Aspinebeamcarryingprecastplankslostitsbearingbecausealabourertrying to jack one of the final planks into position actually levered out
Section
Plan View
spine beam
Wall supportingspine beam
Precast slab jackedinto position
Wall shifted outwards causingspine beam to fall off its bearing
Precast slab jackedinto position
Lacer bars not inplace at time of jacking
Precastslabs
Figure 9.5 Adjusting positions of precast planks.
Problems and Failures due to Poor Management 101
thewallpanelsupportingtheendofthespinebeam(seeFigure 9.5).Thiscaused the spinebeam to lose its bearing and led to the collapseof thefloor.TheconnectionbetweenthespinebeamandthewallpanelhadbeendesignedtohavetwooverlappingUbarslockedtogetherwithlacingbarsthreadedthroughthespacebetweenthem.Theselacerbarshadnotbeeninsertedatthetimeoferectingandlayingofthefloorelements.Iftheyhadbeeninplace,theywouldhavepreventedthewallpanelfrommovingawayfromthespinebeam.
Comment—This isanexample inwhichmanagementshouldhavehadmorecontrolonhowtheerectionandplacingofprecastunitstookplaceand,moreimportantly,ensuredthatthelacerbarsattheendsofthespinebeamwereinplacebeforetheerectionoffloorunitstookplace.
9.3 WEAK AGGREGATE CONCRETE IN CHIMNEY
A tall chimney with a single flue shaft for a coal-to-petrol synthesizingplantwasbeingslip-formed.Half-wayup,the3-daycylindertestsresultssuddenly showed a serious dip in the strengths. The samples had beentakenfromconcreteplacedbetweenmidnightandthefollowingmorning.Subsequenttestresultsweresatisfactory.Aftersomeprobingandquestion-ing,thefollowingscenarioemerged.
Thereweretwobatchingplantsonthesite,eachwithitsownday-workstockpile.Onewassuppliedwithlocallyavailable,butsomewhatinferior,aggregateforuseinlow-gradeconcreteforplantfoundationblocks.Theotherhadtoobtain‘good’aggregateforthechimneyfromsomedistanceaway.Thebatchingplantswereatoppositecornersofthesite.
Onthenight inquestion,themidnightaggregate lorrydidnotappearon time.The foreman inchargeof slip-forming foundhimself facinganunplanned stop to the sliding thatwouldhave causeda calamity.Apartfromtheresultingdelay,itcouldeasilyhavecausedaproblemwhenrestart-ingtheslip-forming,forwhichhecouldhavebeenblamed.Notawareofthedifferentqualitiesoftheaggregates,hissolutionwastogetacoupleofdumpertruckstofetchaggregatefromtheotherbatchingplantthatwasshutdownforthenight.Theslidingcouldthencontinue.
However, this left the other batching plant short of aggregate. Whenthelorrywiththe‘good’aggregatearrivedinthesmallhours,theslidingforemandirectedittounloadatthe low-gradebatchingplanttoavoidadaybreakdisputewiththatforeman.Normalservicewasthenresumed.
Bythetimeallthiscametolight,asubstantialheightofgoodconcretehadbeenplacedontopofthelow-gradeaggregatebelt.Toavoiddemolish-ingsome20mofshaft,itwasproposedtoconstructa“collar”or“sleeve”ofgoodconcreteorgunitearoundtheweakzone.Compositeactioncouldbe provided by drilled-in anchor bolts acting as shear connectors. Thiswouldnecessitateaplannedinterruptionoftheslidingtoenableaworking
102 Failures in Concrete Structures
platformtobesuspendedfromtheslipform,sotheremedialworkcouldbecarriedout,butthiswasacceptedasanecessarydelay.
Comment—Carefulmanagementisessentialfortheslipformingprocess.Thereshouldhavebeenproceduresinplacethatensuredconsistentsuppliesofthecorrectaggregate.Thestockpileshouldnothavebeenallowedtofalltosuchalevelthattheslipformingproductionlinewaswaitingforalorrytoarrive.
103
Chapter 10
Problems and Failures due to Poor Construction Planning
Planningofanyconstructionworkrequirestheinputofengineeringthoughtandthisincludesthetimeneededtoconsider‘whatifs?’.Handsketchescanbeveryusefultoexplainwhatshouldorcouldhappen.
10.1 POWER STATION ON RIVER THAMES
ApowerstationwasconstructedonthenorthbankoftheThamesintheearly1960s.Originallyitwastobecoalfiredtoproduce1500MW.Thefoundationsofthepowerstationsaton20,000reinforcedconcretepilesandaspecialcastingyardwassetuponsitetoproducethem(seeFigure 10.1).
Thepileswere430mmsquareand18mlong.Severalpilerigsweresetupwithdiesel-drivenhammers(seeFigure 10.2).Apilewashoistedintopositionandthengivenatapbythehammertogetthepointofthepilethroughthetopcrustofthemarshland.Thenunderitsownweightthepiledropped15mthroughthemud.Eachpilewasthendrivenintothegraveluntilaspecifiedsethadbeenreached.
Pilingcommencedfromtheedgeofthesiteclosesttotheriverandcontin-uedinlandforadistanceofover250m.Pileswereplacedat1.5mcentres(onaverage).Theexactpositionsofpilesshownonthedrawingsrelatedtothetypeoffoundation(turbine,boiler,culvert,ormiscellaneous).Thetimeperiodforthispartoftheprojectwasabout18months.Excavationforthefoundations,alsostartingfromtheriverendofthesite,commenced6monthsafterthestartofpiling.Thedepthofexcavationvariedfrom1to3m.Thisexposedthepilesthatwerethencutdowntotheleveloftheconcreteblinding.Concretingofthefoundationsthencommenced,startingfromthesameendofthesiteasthepilingandexcavation.
A year after the start of piling, when concreting of the foundationsprogressedaboutathirdofthewayalongthesite,itwasdiscoveredthatthetopsofthepilesthatwerestillexposedweremoving.Measurements showed that the movement was up to 1.5m!
104 Failures in Concrete Structures
Figure 10.2 Pile driver.
Figure 10.1 Pile casting yard.
Problems and Failures due to Poor Construction Planning 105
Thefirstmajorconcernwastocheckwhetherthepileswerestillintact.Onepilewasextracted.Itcameoutstraightandappearedundamaged.Onclose examination,fine cracks couldbe seen throughout its length.Thisindicatedthatithadbentevenlyoveritsfulllength.
Ittooksomeweekstofullyunderstandwhathadhappened.Figure 10.3showsaplanof the site andattempts todemonstrate the situation.Thesteady addition of piles into the ground built up ground pressure. Thispressurewasappliedincreasinglyfromonedirection.Atthesametime,theexcavationforthefoundationsofthestructureclosesttotheriverreleasedanygroundpressure thatbuilt up.The combinationof these twomajoractionscausedthemovement.
Theresultingremedialworkincluded
(1)addinganadditional600verticalpilestocompensatethereductioninverticalcapacityoftheexistingpiles
(2)adding200morerakedpilestocompensateforthehorizontalforcecomponentcausedbythebentpiles.
Large amounts of remedial and extra work were required because ofthemovementofallthesepiles.Forexample,theexistingpilesnolongerfollowedtheplanlayoutfortheeightinletandoutletculvertsthatwound
Turbinefoundations
Suction
Boilerfoundations
Chimneyfoundations
Chimneypiles
River �ames
Pressure
Figure 10.3 Diagram showing cause of movement.
106 Failures in Concrete Structures
theirway through the site tobring coolingwater to the condensers andreturn it to theRiverThames.On-sitedecisions tomakechanges to thedesign had to be made each day. It became very tricky trying to fit theextrapiles,especiallyforthe170mchimneypilecaps.Theoriginaldesignincludedacarefularrangementof rakedpileswhichhadbeendrivenallpointingtowardsthecentreofthecap.Theyhadalltippedover,makingacompletetangle.Astheextrapilesweredrivendowntheykepthittingexistingpiles.Thismeantthattheyhadtoberepositionedandangledtoobtainaclearroute.
Comment—Theprogrammeforthecontractsonthisprojectdidnotfore-seetheproblemscausedastheworkprogressedfromoneendofthesitetotheother. Inprevioussimilarprojects,asignificantdelaybetweenpilingandthestartofexcavationallowedenoughtimeformuchofthesoilpres-suretodissipate.Theneedtoreducetimeandcostsonthisprojectwasnotbalancedbycarefulconsiderationoftheconsequence.
Wherepilinglayoutdrawingsaremadeforlargesites,eachpileisindi-catedbyacrossonthedrawing,sothevolumetriceffectofeachpileandtheentiregroupisnotimmediatelyapparent.Tokeepatightprogramme,onepossiblesolutionmighthavebeentostartthepilingfrombothendsofthesite.
10.2 TOWER BLOCK
An underground car park was to be added to one of several residentialtowerblocks.Excavationcommencedonthesouthsideoftheblockandthe excavated soil was piled on the north side to a height of 10 m (seeFigure 10.4).Aheavyrainfalloccurredsometimeaftertheexcavationfin-ishedanditincreasedthelateralgroundpressureonthepiles.Thebuildingbegan tomove sideways toward theexcavation.This caused thepiles tofailastheirshearresistancewasexceeded(seeFigure 10.5).Thebuildingbecameunstableasitmovedtowardtheexcavationandeventuallyfellover(seeFigure 10.6).
Itwasveryfortunatethattheothertowerblockswerenotcloseenoughtocauseadominoeffect.Figures 10.7and10.8showhowclosethistowerblockwasfromitsneighbours.Figure 10.8showsaclose-upviewofthepilesthatfailedinshear.Thepilesweremadeofcircularhollowunrein-forcedconcrete.Theywereconstructedwithshortlengthsatthetopwithsolidreinforcedsections(seeFigure 10.9).
Comment—Thereappearstohavebeennoengineeringplanningtopre-ventthecombinationofexcavationandpilingupofthesoilonopposite
Problems and Failures due to Poor Construction Planning 107
sidesofthebuilding.Somehandsketchesshowingtheintentionsoftheplanwouldhavehelpedidentifythelikelyproblem.
Itwasprobablyreasonableforthepilestobedesignedforcompressiononlysincetheloadingtowhichthefoundationsweresubjectwasbeyondthescopeofthedesign.
Figure 10.4 Diagrammatic view after excavation.
Figure 10.5 Diagrammatic view of pile shear failure.
108 Failures in Concrete Structures
Figure 10.6 Diagrammatic view of collapse.
Figure 10.7 View of underside of collapsed building. (Courtesy of Gillian Whittle; redrawn from Reuters, http://www.telegraph.co.uk/news/worldnews/ asia/china/ 5685963/ Nine-held-over-Shanghai-building-collapse.html.)
Problems and Failures due to Poor Construction Planning 109
Figure 10.8 View of piles that failed in shear. (Courtesy of Gillian Whittle; redrawn from Reuters, http://www.telegraph.co.uk/news/worldnews/asia/china/5685963/Nine- held-over-Shanghai-building-collapse.html.)
Figure 10.9 Unreinforced piles with short reinforced length at tops. (Courtesy of Gillian Whittle; redrawn from Reuters, http://www.telegraph.co.uk/news/world-news/asia/china/5685963/Nine-held-over-Shanghai-building-collapse.html.)
111
Chapter 11
Problems and Failures due to Deliberate Malpractice
11.1 FLOOR WITH EXCESSIVE DEFLECTION
Thebuildinginquestionwasatelephoneexchangebuiltinthemid-1970s,10yearsearlier.Figure 11.1showsaplanandsectionofatypicalendbay.Theslabhadbeendesignedassinglewayspanningbetweentwoshallowhaunchedbeams.Thedesignincludedaspanof9mwithaslabonly250mmthickwhichmanyengineerswouldconsidertoothin.
Ten years after the building had been completed, the operators com-plainedthatthedeflectionwasstillincreasingandhadcausedsomeoftheswitchgeartobecomefaulty.Thedesignersaskedforasecondopiniononthedesignoftheslab.Thecalculationsanddrawingswerecheckedandnomajorflawswerefound.Itwasconceivablethatcreepandshrinkageeffectswerestillincreasing.
The designers also asked the local university to run an independentcheck using its finite element modelling package.The subsequent reportconcluded that there was some major overstressing that could lead to ashear failure. The finite element analysis assumed elastic behaviour thatledtohighstressesatthesupports.Thiswasemphasisedbytheshallowsupporting beams that allowed the slab to behave more like a flat slab.However,ifareasonableamountofmomentredistributionwasassumed,althoughsomecrackingoftheconcretecouldbeexpected,thedesignwassafe.Neverthelessitwasdifficulttoconvincethedesignerthatthiswassoandavisittothesitewasarranged.
Thevisittositeincludedtheinspectionoftheslabclosetoacolumn.Thescreedhadbeenremovedtoexposethetopsurfaceofthestructuralslab.On close inspection, the reinforcement appeared tobebadly rusted andbreakingthroughthetopsurface.Asacrudecheckofthehardnessoftheconcretesurface,itwasscratchedwithapenknife.Quiteunexpectedly,thebladeoftheknifepenetratedintotheconcretesurfacerightuptothehilt!Afurthercheckofthesoffitoftheslabgaveasimilarresult.
Anadditionalinterestingfeatureofthesoffitwasthepresenceofanum-ber of shallow disc shaped (‘flying saucer’) pieces of concrete (150 mm
112 Failures in Concrete Structures
diameter)thatwereseparatingfromthesurface.Onesuchpiececameawayasitwasbeingexamined.Althoughtheslabhadbeendesignedtospanoneway, thesupportingbeamwassufficientlyflexible for theslabtobehavemorelikeaflatslab.The‘flyingsaucers’appearedinthecompressionareasofthesoffitandwereconsideredtobetheeffectsofspalling.Itwasclearthattheslabinquestionrequiredimmediateadditionalsupportandtherestofthebuildingrequiredcoretesting.
Aftercoreshadbeentakenthroughoutthebuilding, itwasdiscoveredthattheconcretecubestrengththatshouldhavebeen25MPawasonaver-age only 5MPa. The subcontractor had deliberately reduced the cementcontentinthespecifiedmix.Majorremedialworkfollowed.
Comment — It is very surprising that the building structure remainedintactforsolonganddemonstratesthat,ifthereisapossibleforcepathtoholdastructuretogether,thestructurewillfindit.Theclientwasrelievedtolearntherealcauseoftheproblems.Itisremarkablethatthetelephoneexchangeremainedinoperationthroughouttheperiodofremedialwork.
250 thick slab
Excessive deflection(still increasing after 10 years)
9 mA
AA - A
600
300
Figure 11.1 Plan and section of typical end bay.
Problems and Failures due to Deliberate Malpractice 113
11.2 PILES FOR LARGE STRUCTURE
Overthirtyreinforcedconcretepiles(3mdiameter)wererequiredtosup-portacomplexstructure includinga3mthicktransferslabandseveraltowerblocksabove.Whentheconstructionofatowerblockabovereachedseveral storeys, it was discovered that the piles were less than half therequiredlength.
Manyideastoresolvethesituationwereconsidered,butintheendthechosensolutionwastoprovideasetofninesteelHsectionpilesaroundtheexistingpilesanddrivethemtotherequireddepth.TheHpileswerethenconnectedtotheexistingstructurethroughnewpilecaps(oneperoriginalpile).Tocarryout the remedialwork,oneof the intermediatebasementfloorshadtoberemovedtoallowthepilingrigstooperatewithsufficientheadroom.Access intothebasementwasgainedbycuttinga largeholethroughtheexistingperimeterretainingwall.
Comment—Thepileshorteningwasadeliberateactofthesubcontractorthatresultedinalongdelaytothecompletionoftheprojectatenormousextracost.
11.3 IN SITU COLUMNS SUPPORTING PRECAST BUILDING
Thisbuildingwasconstructedwithprecastelementsaboveground.Belowground,thefoundations,columns,andbeamswereconstructedinsitu(seeFigure 11.2).Constructionhad reached an advanced stagewhen cracksappearedintheinsitucolumnsjustbelowtheconnectionswiththepre-castcolumns.
TheconstructionoftheinsitucolumnsshouldhaveproceededasshowninFigure 11.3.Thecentralcircularhollowsection(CHS)dowelwascastintothetopoftheinsitucolumnreadytoreceivetheprecastcolumn.
Inordertocheckthattheconcreteinthebox-outwasinplaceasintended,holesweredrilledthrougheachface.Itwasdiscoveredthatthepolystyrenehadbeenleftinandonlyathinlayerofconcretehadbeenplacedatthetop(seeFigure 11.4).
Theloadintheprecastcolumnfromsevenfloorsabovetransferredtotheouterrimoftheinsitucolumn.Thisloadwasintendedtobetakenbythewholesectionoftheinsitucolumns.However,thethinlayerofconcreteatthetopofthebox-outandthepolystyrenebelowwerequiteincapableoftakinganysignificantload.Theoutershellofinsituconcretethathadtotaketheloadbecameoverstressedandconsequentlycracked,providingthefirstsignofimminentfailure(seeFigure 11.5).
Inordertorepairthetopsoftheinsitucolumns,theloadfromthepre-castbuildinghadtoberemoved.Thiswasachievedbyprovidingpropsand
114 Failures in Concrete Structures
jacksclosetotheexistingprecastcolumnsateachfloorlevelandcreatinganewloadpathtotheground.Thisreleasedtheloadontheinsitucolumnsbelowandallowedtherequiredremedialwork—reconstructionofthetopsoftheinsitucolumns—totakeplace.
Street level Transfer beams
Precast beams, columns and slabs
Existingretaining
wall
In situ beams andcolumns
See detail of column connection
Figure 11.2 Section through lower part of building.
Column reinforcedas normal
Column cast with largepolystyrene box-out
Polystyrene totallyremoved; CHS 114 diadowel cast in with freshconcrete filling box-out
Figure 11.3 Intended construction of top of in situ column.
Problems and Failures due to Deliberate Malpractice 115
Comment—Althoughtheactthatcausedtheproblemwasnotintendedtocausecollapseofthebuilding,itwas,intheauthor’sopinion,adeliberateacttoleavemostofthepolystyrenebox-out.Thiswasdonetosavetimeandeffortforthecontractor.Itwasextremelyfortunatethatthefaultwasfoundbeforecollapseofthebuildingoccurred.
Only thin layer ofconcrete cast in topof column
CHS dowel pushedinto polystyrene
Existing insitucolumn
Only top layer ofpolystyrene removed
Figure 11.4 Actual construction of top of in situ column.
Load from 7floors above
Precast column
Grouting tube
Load from precast unitsupported on thin outershell of insitu column
Insitu column
Severe cracking ofinsitu column wall
Figure 11.5 Cracking of outer rim of in situ column.
117
Chapter 12
Problems Arising from the Procurement Process
12.1 EFFECTS OF DIFFERENT FORMS OF CONTRACTS
Typicallytheformofprocurementliesbetweenthetwoextremes:
• Traditionalarchitect-ledcontracts(e.g.,oneoff)• Design–buildcontractor-ledcontracts(e.g.,systembuild)
Theformerprovidestheclientandarchitectwiththegreatestfreedomoflayoutandform.Thelattercanleadtothemosteconomicandfastestcom-pletiontime.Bothrelyontheintegrityandcareofthewholeteam.
Therearesignificantdifferencesinsomeaspectsofthedesignanddetail-ing between these two forms of contracts. The reasons are not difficulttounderstandbuttheyhaveamarkedeffectonthewaytheinformationisproduced.Whereaproject isarchitect led, thecontractor thatwillbeawardedtheworkisnotknownatthetimeofthedesign.Thismeansthatthedesignanddetailing shouldbecarriedoutwithout involvingpropri-etarysystemsunlesstheyarerequiredbythedesign.Thisisbecauseeachcontractorislikelytofavouraparticularproprietarysystemthatisnotpre-ferredbyothers.Thedetailingshouldincludeonlyreinforcementrequiredbythedesignandnotbecontrolledbypossiblesiterequirements(suchashealthandsafety).Thiscanbebestexplainedthroughexamples.
Example1—Onemethodthatcontractorsusetoresolvethedangerthatpeoplewilltripoverslabreinforcementbeforeandduringplacingofcon-creteistoplacetopreinforcingbarsatsmallcentres(say,200mmorless).Thisisoftennotadesignrequirement.Infact,veryoftenthereisnodesignneedfortopreinforcementinthemiddleareaofaslab.
Hence, if the drawings show only the reinforcement required for thedesign,thecontractorwillhavetodecidewhethertoproposeextrarein-forcement or provide some other method of ensuring safe passage forpeople(suchasprovidingtemporaryplanksandwalkways).Theclientisinterestedinthecheapestmethod,notnecessarilythemostconvenientone
118 Failures in Concrete Structures
for thecontractor.All toooften, theextrareinforcementalongwith theextra cost is assumed tobe a requirementofdesignwhen theremaybecheaperwaysofachievingtherequiredsafety.
Example2—Anothercommonexampleconcernstheprotectionofcol-umnreinforcementstarterbars.Healthandsafetyregulationshaverequiredtheendsofthestarterbarstobebentintohooks.Thisiscostlytoachieveandacheaperalternativewouldbetoplaceanopenplasticboxoverthetopofeachgroupofcolumnbars.Theboxescouldbereusedforeachfloor.
Example3—Asignificantdifferencebetweenthetwoformsofcontractsconcernsthepositionsofconstructionjoints.Forbuildingstructures,goodpracticeensuresthatreinforcementislappedatpositionsoflowstress.Thelapandanchoragelengthsusedbydetailersareoftensetbyasimple“ruleof thumb.” Often40×bar diameter is used.This is consideredbymostengineerstobeasafeapproachandnormallynofurtherchecksaremade.
However,somecontractorsrealisedthatthisisapointwheresavingscanbemadeandfordesign–buildcontracts,theyinstructthedetailertouse35×bardiameter.Thiswouldprobablybeacceptable fora traditional typeofcontractwherethespecificationdemandsthatconstructionjointsbeposi-tionedawayfromhighlystressedareas.However,fordesign–buildcontracts,acontractoroftendecidestopositionconstructionjointsatpositionscon-venientforconstruction—wherereinforcementmaybehighlystressed.Therequiredlaplengthinsuchapositioncouldbeashighas60×bardiameter.
Thiscausesadilemmaforadetailer.Shouldheorsheuse60×bardiam-eteranchorageandlaplengthsfordesign–buildcontractstoensuresafetyinallsituations?
12.2 WORKMANSHIP
Thequalityofworkmanshipislikelytoremainthegreatestissuewithintheindustryfortheforeseeablefutureandwillrelyontheintentandenthusi-asmofthemanagerstoimprovestandards.Itisnotjustaquestionoftick-ingboxesonaform.Itisaboutensuringthattheengineeringofeachpartofajobisunderstoodandcarriedoutcorrectly.Itrequiresindividualstotakeontheownershipandresponsibilityfortheexecutionofgoodqualitywork.Workmanshipqualityisstronglyaffectedby:
• Anintelligentworkforceandrequirementforchecking• Achievingindividualsatisfactionandprideinwork• Utilizingsensibleprocedurestoavoidmistakes
Onecriticismlevelledagainstthetraditionalformofcontractisthatmanydesignsare tooconservativeandasa result concreteconstruction is lesseconomicalthanconstructionusingothermaterials.Itiscertainlytruethat
Problems Arising from the Procurement Process 119
manydesignerserrontheconservativesidewhensizingelements.Thishascomeabout frompast experiencewhen itbecamecommon forarchitectclientstomakechangeslateoninthejob,leadingtoincreasedloadingorchangesinstructure(movingcolumnpositions,addinglargeholes,etc.).
Duringtheconstructionphase,acontractormayseewaystoreducehiscostsandoftenthisispossiblebecauseoftheconservatismofthedesign.Forastandardformofstructure(e.g.,officeorresidentialblock),thiscon-servatismcanbeconsideredinefficientandwasteful.
Attheotherextreme,forsomedesign–buildcontracts,theworkmanshipisconsideredtobeoflowquality.Occasionallymethodsadoptedforapar-ticulartypeofconstruction(e.g.,hybridcarparks)areextrapolatedforlon-gerspansandareillthoughtout,sometimesproducinganunsafestructure.
It isnotuncommonforadesign–buildcontracttousehybridconcreteconstruction(insituandprecastconcrete).Thismaywellleadtothedesignoftheindividualelementsbydesignersworkingfordifferentcompanies.In such situations it is essential that there should be a single responsibility of one engineer for the stability of the structure, and the compatibility of the design and details of the parts and components, even where some or all of the design, including the details of those parts and components, are not carried out by this engineer.
The lackof systematicor thirdpartycheckinghas led topoorand insomecasesunsafeconstruction(e.g.,ungroutedprestressingducts).Thirdparty checking shouldbe carriedoutwhere considerednecessaryby theengineer and, in the author’s opinion, should be incorporated more fre-quentlyinallformsofcontract.
12.3 CHECKING CONSTRUCTION
Ononemajorproject, in situpost-tensioningwas required for thefloorslabs.Anindependentresidentengineerwasnot includedinthecontractanditwasassumedthatthecontractorwouldprovidesufficientsupervisionofthework.
Afterconstruction,waterwasdiscovereddrippingfromthesoffitoftheslab. Investigationrevealedthat theprestressingduct justabove the leakhadnotbeengrouted.Afurther investigationrequiredthatall theductsinthebuildinghadtobecheckedandthisrevealedthatmanyotherductshad not been grouted. The possibility of stress corrosion and failure ofprestressingtendonscanbegreatlyincreasedbythepresenceofwateraswasthecasewiththeseungroutedducts.Theremedialworktoresolvethismistakewascostlyandtimeconsuming.
Comment—Inaprestressedfloor,theamountofreinforcementisnegli-giblecomparedtoafloorwithreinforcementonly.Thespacingoftendons
120 Failures in Concrete Structures
canbeupto2or3m.Ifonetendonweretofail,upto6mwidthofslabwouldremainunsupported.Theriskofcollapseoftheslabandprogressivecollapseof thewhole structurewouldbe significant.There isagrowingconcernamongstmanyengineersthatthequalityofdesignandconstruc-tionhasdeclinedasaresultoftheincreasingpressuretocutthetimeandcostofprojects.
IthasbeenreportedtoCARES13thatasurveyhasshownthatanumberofducts,approaching1%onaverage,havebeeneitherpartiallyorcom-pletelyungrouted,therebypotentiallyaffectingtheintegrityofanumberofbuildingsinthelongterm.Thisisanaveragefigure,anditislikelythattheaveragehasbeenexceededgreatlyinanumberofstructures.
Inmany situations, the checkingof constructionworkhas reduced tounacceptablelevels.Theremustbeabalancebetweenrigorousproceduresandtheamountofcheckingrequired.Thepresenttrendistoincreasetherequirementswithin the specifications.Theauthor isnot convinced thatthis issufficientorthebestwayto improvethequalityofwork.Havingathirdpartycheckforcriticalpartsofastructurewouldappeartobeasensiblesolution.
121
Chapter 13
Contributions of Research and Development toward Avoidance of Failures
13.1 LINKS BETWEEN PRACTICE AND RESEARCH
Theneedforresearchanddevelopmentcontinuestoincreaseasmaterials,designs, and constructionmethods are refined. It is very important thatgood communication links aremaintainedbetween engineeringpracticeandresearchinstitutions.Thisisoftenbestachievedthroughpersonalcon-tacts. It isunfortunatethatsuch linksarebecoming lesscommonintheUK,probablyduetoreductionsinavailablefunds.Thefollowingexamplesdescribehowtheauthorhasbeeninvolvedinsuchlinks.
13.2 FLAT SLAB BEHAVIOUR
Inthelate1970s,flatslabconstructionwasbecomingpopularasitresultedinthinnerslabs.Italsoallowedsimpleandquickmeansofconstruction.However,designerswerelookingformoretoolstohelpthemanalysesuchstructuresanddemandingmoreinformationabouttheirstrengthandper-formance. The author became involved in a research project to test flatslabsatthePolytechnicofCentralLondon.Thisresultedfromquestionsabout thebehaviourand strengthof this formof construction.Dr.PaulRegansetupthetests(seeFigure 13.1)andproducedagroundbreakingresearchreport forCIRIA(Report89:Behaviourof reinforcedconcreteflatslabs14).
Oneimportantareaofdoubthadbeenpunchingshearbehaviour.Regan’stestsprovidedvaluableinformationonthissubject,takingintoaccounttheeffectofmomenttransferbetweenslabandcolumn.ThisinformationhassincebeendevelopedandincludedinboththeUKandEuropeancodesofpractice.
122 Failures in Concrete Structures
13.3 SPAN AND EFFECTIVE DEPTH RATIOS FOR SLABS
BS 81101 and previous UK codes of practice provided clauses on spanand effective depth. Modification factors were provided for tension andcompressionreinforcement.In1994,duringthedraftingoftheConcreteSociety’sTechnicalReport49:Designguideforhighstrengthconcrete,15itwasrealisedthatsomeconcretepropertiesimprovedwithstrength.Theseincludedanincreasedmodulusofelasticity,increaseofcrackingmoment,andreductionsinshrinkageandcreep.
Professor Beeby, who provided the supporting information for theexistingcodeclauses,wasaskedwhether itwasappropriate tomakeanadjustmenttothespanand/oreffectivevalueforthestrengthofconcrete.Afterfurtherresearch,heprovidedanexpressiontoincludethisvariable.Figure 13.2showsaplotoftheexpressionthatwasadoptedinthetechnicalreportandlater,in2004,wasalsoincludedwithinEurocode2.2
13.4 BEAM AND COLUMN JOINTS
TestswerecarriedoutatDurhamUniversitytodeterminethestressesinboththeconcreteandthereinforcementastheloadwasincreasedtofail-ure.Someof the reinforcingbarshadspecial treatment to inserta largenumberofstraingaugesintoslotsthroughthecentresofthebars.Eachbarconsistedoftwoseparatebarsmilledlongitudinallysothatcross-sections
Figure 13.1 Flat slab tests carried out at Polytechnic of Central London. (Courtesy of Jonathan Wood.)
Contributions of Research and Development toward Avoidance of Failures 123
werehalved.Aslotwascutdownthecentreofeachofbarandthestraingaugesplaced alongonebar at varyingpitches (as little as50mm)andgluedintoposition.Thetwohalveswerethenjoinedtosimulateasinglecompletebar.Allthewiresfromthestraingaugeswereledoutoftheendsofthebarstoacontrolbox.Figure 13.3showsoneofthetestsatthepointwherethejointfailed.
Differentarrangementsofreinforcementweredetailedandtested.Oneimportant result showed that if thebeam topbarswerebentupward inthe column, with gravity loading on the beam, the joint resistance wasdrasticallyreducedcomparedwiththebarsbentdown.ThereasonisthatthehighcompressionforcecreatedwithinthejointwasnotresistedbytheenclosingbarasshowninFigure 13.3a.
13.5 TENSION STIFFENING OF CONCRETE
The importance of time effects of tension stiffening of concrete becameapparentaftertestingwascarriedoutatLeedsandDurhamUniversities.ThisworkwasundertakenasaresultofsomeunansweredquestionsfromaBriteEuramprojectonhighstrengthconcretecarriedoutin1995.Aspartof thatprojectTaywoodEngineeringbuilt a full-scale thinflat slab (seeFigures 13.4and13.5).
10
15
20
25
30
35
40
45
50
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Practical limit
C30/37 C50/60 C70/85 C90/105C20/25 C40/50 C60/75 C80/95
l/d
100 As,rqd/bd (mean value for span)
BS8110
Figure 13.2 Effect of concrete strength on span and effective depth ratios.
124 Failures in Concrete Structures
Thebehaviouroftheslabwascarefullymonitoredduringitsearlylifeandtheauthorcomparedthemeasurementswiththeresultsfromafiniteelementmodel.Theresultscorrespondedverycloselyuptoandjustafterthefulltestloadhadbeenapplied.Itcameasasurprisetodiscoverthat2weeksafterunloadingtheslab,itdeflectedmorethanthemodelhadpre-dicted.Dr.RobertVollumofImperialCollegesetupanindependentmodelbuttheresultsdidnotexplainthedifferences.
(b) Failure(a) Severe cracking
Figure 13.3 Loading column joint to failure. (Courtesy of Richard Scott.)
3 m
3 m
9 m 250 thick solid slab
300 × 300 columns
Figure 13.4 Layout of Taywood test flat slab.
Contributions of Research and Development toward Avoidance of Failures 125
Thesolutiontothisproblemcameaboutasaresultofachancemeet-ingwithProf.AndrewBeebytwoyearslater.HesuggestedthatheandDr.RichardScottcarryout testsatLeedsandDurhamUniversities toexaminetheconcretetensionstiffeningeffectsinmoredetail.Theproj-ectwasanunmitigated successand resolved theproblemwith respectto theTaywoodEngineering slab. It revealed thatwhathadbeencon-sidered short term effects of tension stiffening lasted much less timethanassumedandthatthelongtermeffectstookoverafterafewdays.Figure 13.6showshowtensionstiffeningaffectsthebehaviourofacon-creteelement.
Theknock-oneffectofthatprojecthasbeenasignificantchangeintheapplicationoftheBritishandEuropeancodes.Eachofthesecodesstatedthattensionstiffeningishalvedinthelongterm.Untilrecently,longterm
Figure 13.5 Construction of test flat slab. (Courtesy of Richard Scott.)
Calculated assumingconcrete has notensile strength
Calculated assumingno cracking
Actual behaviour
Deflection
Load
Figure 13.6 Effect of concrete tension stiffening on deflection of slabs.
126 Failures in Concrete Structures
was assumed to be several years after loading instead of the actual fewdays. This means that for design purposes the long term value shouldalmostalwaysbeused.
This research has provided an advance in the understanding of thebehaviourofreinforcedconcretewhenitcracks.ThereportontheprojecthasbeenconvertedtoConcreteSocietyTechnicalReport59:Influenceoftensionstiffeningondeflectionofreinforcedconcretestructures.16
127
References
1. BritishStandardsInstitution.BS8110-1Thestructuraluseofconcrete.Part1:Codeofpracticefordesignandconstruction,London,1997.
2. BritishStandardsInstitution.BSEN1992-1-1:Eurocode2,Designofconcretestructures.Generalrulesandrulesforbuildings,London,2004.
3. ConstructionIndustryResearchandInformationAssociation.Report91:Early-agethermalcrackcontrolinconcrete,1992.ReplacedbypublicationC660in2007.
4. WhittleR.andTaylorH.Designofhybridconcretebuildings,ConcreteCentre.2009.
5. ConcreteSociety.TechnicalReport67:Movement, restraintandcracking inconcretestructures,2008.
6. ConcreteSociety.TechnicalReport64:Guidetothedesignofreinforcedcon-creteflatslabs,2007.
7. BritishStandardsInstitution.BSEN1994-1-1:Eurocode4,Designofcompos-itesteelandconcretestructures.Generalrulesandrulesforbuildings,London,2004.
8. BritishStandardsInstitution.CP114:Thestructuraluseofreinforcedconcreteinbuildings,London,1957.
9. RoweR.E.,CranstonW.B., andBestB.C.Newconcepts in thedesignofstructuralconcrete.StructuralEngineer,43,399–403,1965.
10. BritishStandardsInstitution.CP110:Thestructuraluseofconcrete,London,1972.
11. Department for Communities and Local Government. Building regulations,5thamendment,HMSO,London,1970.
12. BritishStandardsInstitution.CP116:Thestructuraluseofprecastconcrete,London,1965(revised1970).
13. UnitedKingdomCertificationAuthorityforReinforcingSteels(CARES). 14. ConstructionIndustryResearchandInformationAssociation(CIRIA).Report
89:Behaviourofreinforcedconcreteflatslabs,1981. 15. ConcreteSociety.TechnicalReport49:Designguideforhighstrengthconcrete,
1998. 16. ConcreteSociety.TechnicalReport59:Influenceoftensionstiffeningondeflec-
tionofreinforcedconcretestructures,2004.
Robin Whittle
Failures in ConCrete struCturesCase Studies in Reinforced and Prestressed Concrete
A S P O N B O O K
ISBN: 978-0-415-56701-5
9 780415 567015
9 0 0 0 0
Y106141
Cover image: © Dr Jonathan G M Wood
STRUCTURAL ENGINEERING
“The book provides a fascinating insight into what can go wrong. It illus-trates the importance of approaching problems from first principles and identifying the key load paths, loads, etc. This type of material rarely sees the light of day for reasons of litigation and commercial confidentiality.” —Robert Vollum, Imperial College, London
“This book is essential reading for any experienced engineer and of-fers insight into how designers and builders accidentally create struc-tures that fail. This book both explains how things fall down and what was done or could have been done to overcome these errors. Learning from errors is an essential part of any engineer’s knowledge and expertise.” —David M. Scott, Laing O’Rourke, Connecticut
Failures in Concrete Structures: Case Studies in Reinforced and Prestressed Concrete presents a selection of the author’s firsthand experience with incidents related to reinforced and prestressed concrete structures. He includes mistakes discovered at the design stage, ones that led to failures, and some that involved partial structure collapse—all of which required remedial action to ensure safety. The book focuses on specific incidents that occurred at various points in the construction process, including mistakes related to structural misunderstanding, extrapolation of codes of practice, poor detailing, poor construction, and deliberate malpractice. The author provides this information for readers to gain an understanding of errors that can occur in order to avoid making them. The book also examines issues during the procurement process, as well as the importance of research and development to gain a deeper knowledge of materials and help prevent future failures.
Failures in ConCrete struCturesCase Studies in Reinforced and Prestressed Concrete
Y106141_Cover_mech.indd 1 8/24/12 2:11 PM