Lecture 07.docx

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