Lecture 07.docx
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Lecture 07.docx
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Lecture07
StructuralSteelworkEurocodesDevelopmentof
ATrans-nationalApproach
Course:
Eurocode3
Module3:
Framedesignapproaches
Lecture7:
Traditionalandmoderndesignapproaches
Summary:
∙Thetraditionalapproachestothedesignofframesareconciselydescribed:
-continuousframingwithrigidjointsand/orsimpleframingwithpinnedjoints,
-thesocalled“windmoment”method
-the“partialstrength”approach
-rigid-plasticdesign.
∙Themodernapproachtoframedesign,i.e.semi-continuousframingusingsemi-rigidjoints,isthenoutlined;howitistobedistinguishedfromthetraditionalapproachesisexplainedandthepotentialbenefits(scientificandeconomic)foritsuseareraised.
∙Aconsistentdesignprocessisdescribedinwhichjointbehaviourisaccountedforintheglobalanalysisfromtheoutset.Itisshownhowtomodifythesimpleframingorcontinuousframingapproachestobemoreinlinewithaconsistentdesignprocess.Itisexplainedthataconsistentdesignprocesscantakedifferentformswhichdependontheassumptionsaboutjointbehaviourintheglobalanalysis,whoisresponsibleforjointdesignand/orthedegreeofcollaborationbetweentheparties(designerandfabricator).
∙Designpracticesareidentifiedwhichshowhowdesignresponsibilityis,orcanbe,shared.Itisexplainedthatanunderstandingofthesharingofdesignresponsibilityisessentialinordertomodifycurrentpracticesoastoallowaconsistentdesignprocesstobeused.
Pre-requisites:
∙Aknowledgeofthefundamentalsofthetheoryofresistanceofmaterials(forbeamsandtensionmembers)andofstructuralanalysis.
∙Aknowledgeoftheelasticandplasticdesignofsimplemembers.
∙Module1forloadinganddefinitionsoflimitstates.
∙Lecture4“Frameidealisationandanalysis”.
∙Lecture5“Frameclassificationandjointrepresentation”.
NotesforTutors:
Thismaterialcomprisesone45minutelecture.
Objectives:
Thestudentshould:
∙Understandthedifferentapproachestoframedesign,bothtraditionalandmodern.
∙Haveanappreciationofthepotentialbenefitsofusingaconsistentdesignprocesswhichbestaccountsforjointbehaviour.
∙Understandtheconsequencesthatthesharingofdesignresponsibilitymayhaveonthesubsequentdesign.
∙Knowhowtoputaconsistentdesignprocessintopractice.
References:
[1]Anderson,D.,Reading,S.J.,TheWind-momentdesignforunbracedframes,SCIpublicationP-082,1991.
[2]AndersonD.,ColsonA.,JaspartJ.-P.,Connectionandframedesignforeconomy,ECCS/TC10publicationN°,1991(alsopublishedinanumberofnationaljournals).
[3]Maquoi,R.,ChabrolinB.,Framedesignincludingjointbehaviour,ReportEUR18563EN,ECSC/EuropeanCommission,1998.
[4]ENV1993-1-1:
Designofsteelstructures:
Part1-1:
Generalrulesandrulesforbuildings.
1Traditionalapproachestodesign
1.1Pinned-Rigidjointapproach
Uptonow,thetypicallyusedprocessofdesigningbuildingstructuresinvolvedthefollowingsuccessivesteps:
∙framemodellingincludingthechoiceofrigidorpinnedjoints
∙initialsizingofbeamsandcolumns
∙then,foreachultimatelimitstate(ULS)andserviceabilitylimitstate(SLS)loadcombination:
-evaluationofinternalforces(loadeffects)
-checkofULSandSLScriteria
∙iteration,ifneeded,onmembersizesuntilallchecksaresatisfactory.
∙atthefinalsatisfactorystageafterthepreviousiterations:
-designofthejointstoresisttherelevantmembersendforcesinaccordancewiththeinitialassumptions(framemodelling)aboutthejointstiffness.
TheapproachisgiveninflowchartforminFigure1.Sincethejointsareconsideredaseitherpinned(simpleframing-nomomentassumedtobetransferred)orrigid(continuousframing-rigidmoment-carrying),theirdesignbecomesaseparatetaskfromthatofthedesignofthemembers.Itisoftenperformedatalaterstageintheoveralldesignprocessbyotherpersonnel(usuallythefabricator).
5.2.2.2
5.2.2.3
Thisapproachissuitedforframedstructureswhichareclassifiedasbracednon-swaywheremostofthebeamtocolumnjointsarenotrequiredtotransmitmoment.Otherjointsmayformpartofabracingsystemwheretheywouldberequiredtoberigidandhavehighresistance.
Theuseofmoment-carryingjointsinframesofanyheightornumberofstoreysmayoftenprovideaneconomicalternativetothecasewhenbracingalonemustprovidelateralstability.Becausefloordepthsarereduced,theoverallvolumeofthebuildingforagivenfloorspaceisless.Eliminatingbracinggivesanincreasedfreedomforuseandresultsinaestheticallypleasingdesigns.Formediumtohighmulti-storeybuildings,itisoftennecessarytohavemomentcarryingbeamtocolumnjointssoastoprovideobstructionfreeofficefloorspacewhichcanberapidlyre-organisedatleastcost.However,assumingsuchjointsasrigidmaynotbethemosteconomicsolution,andconsiderationshouldbegiventothesemi-rigidjointalternative.
Themostcommonexampleofasteelframeinwhichrigidmoment-carryingbeamtocolumnjointsareusedistheonestoreypithed-portalframeindustrialbuilding.Whilestillkeepingthejointinthe“rigidjoint”classification,designtoEurocode3Part1-1permitslesscostlyjointdesignsthanthoseusuallyadopted.Moreeconomicdesigns,ascomparedtocommonpractice,shouldbepossible.
AnnexJ
.
Figure1Traditionaldesignprocessofasteelframeforrigidand/orpinnedjoints.
1.2Windmomentmethod
Inthewindmomentmethod[1],thebeamtocolumnjointsareconsideredtotransmitnomomentforverticalloadingcasesbuttotransmitmomentwhenwindloadingisconsidered.Ithastheadvantageofallowingtheuseofrathersimplebeamtocolumnflangejoints.Thedesignprocessfitswellintotheflowchartforthetraditionalpinned-rigidapproach
Whatisinterestingaboutthisapproachisthatitisaway,albeititrathersimple,ofaccountingfortheactualstiffnessandfortheinherentmomentcarryingcapacity,althoughlow,ofthejoint.However,itisnotcleartowhatextentthewindmomentmethodcanbeadoptedwithinthecontextoftheapplicationofEC3.ThejointsdetailshavetobecarefullychosenfortheassumptionsontheirbehaviourtobeactuallyvalidasregardstheEurcodePart1-1rquirements.
1.3Partial-strengthbeamtocolumnjoints
Althoughitisnotaverytraditionalapproach,likethewindmomentmethod,ithasfoundsomeapplicationalreadyinindustry[2].ThisapproachfitsintothetraditionalbreakdownindesigntasksasshowninFigure1.
Inthiscase,oneavoidsanyriskofitbeingdifficulttodesignajointwiththerequiredmomentresistanceandstiffness.Thelatterisaproblemwhicharisesregularlybetweenthefabricatorandthedesignerforstructuresdesignedassumingrigidjoints.
Usuallyusedforthedesignoffloorsofbracednon-swayframes,themethodinvolvesadoptingpartialstrengthductilebeamtocolumnjoints.Thebeamisdesignedonthebasisofasimplemechanismwithtwojointhingesandathirdhingeatthemid-spanofthebeam.Figure2showsanexampleofthebeammechanism.
Figure2Applicationofthepartial-strengthapproach
Economicfloorbeamsofreduceddepth,comparedtothebeamobtainedbytheusualassumptionofpinnedjoints,canbeproposed.Sincethejointdesignmomentsareusuallychosenatlessthan40%ofthatoftheplasticmomentresistanceoftheattachedbeam,economicjointtypes,whichareusuallyoftheboltedflushendplatetype,canbeused.
Thedesignmethodisverysimpleandrapidtouse.
The“ partialstrength approach”,likethewindmomentmethod,maybealsobeconsideredasaparticularapplicationofthesemi-rigidapproachwheretheaccentisnotonflexuralstiffnessbutonchoosingjointswhichhavemoderateorlowbendingresistancesandareductile.
1.4Rigid-Plasticdesign
Asthismethodisasyetlittleusedinmanycountries,itcanbecalledtraditionaltoaproportionofengineersonly.Nevertheless,ithasbeenusedforsometimenowforthedesignofthemostcommonsteelstructure,theone-storeyportalframe.
5.2.1.4
Thisapproachhasbeendescribedinmoredetailinthemodule2“FrameAnalysisandDesign”.The“ partialstrengthjoint ”methodforrectangularframestructuresisasimplevariantofit.
Themethodcanbeappliedtocertaintypesofswayframes,althoughmostwouldlimititsusetoswayframesofonlyonestoreyhigh.
5.2.6.3
ItismuchusedintheUKforthedesignofportalframeindustrialbuildings.Intheseframes,hingesdonotformintherafterattheeavesjointssincehaunchesareusedandtheloadingissuchthatthehingeintherafterisnotattheapex.
Itsuseforothertypesofbuildingseemstoberare,althoughthe“partialstrength”approachhasbeenusedforthedesignofmulti-storeyframes.Therequirementthatmembercross-sectionsbeofclass1orofclass2isprobablythemajorhindrancetoitswiderapplication.
2.Themodernapproachtoframedesign
Itisnowwellrecognisedthatassumingjointstoberigidorpinnedmayneitherbeaccuratenorresultbeeconomical.Simplybecauseajointhassufficientstrengthdoesnotmeanithassufficientstiffnessforittobereasonabletomodelitasrigid.Manyjoints,oftenassumedtoberigidexhibitanintermediatebehaviourbetweenthe"rigid"and"pinned"states.Eurocode3Part1-1hastakenthisfactintoaccountandindoingsoopenedthewaytowhatisnowknownas"thesemi-rigidapproach"(seeFigure3).
5.2.2.4.
Inthesemi-rigidapproach,th
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