Lecture 08.docx

Lecture 08.docx

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Lecture08

StructuralSteelworkEurocodesDevelopmentof

ATrans-nationalApproach

Course:

Eurocode3

Module3:

Framedesignapproaches

Lecture8:

Practicalapplicationofmoderndesignapproaches

Summary:

∙Designstrategiesareintroducedtopermitaneasyadaptationofthetraditionaldesignapproachesandallowconsiderationofthesemi-continuityofjoints.

∙Elasticdesignandrigid-plasticdesignmethodscanbeused.

∙Thedesignapproachisaimedatavoidingcomplex（costly）jointdesignswhiletakingfullaccountofjointbehaviourfromtheoutsetofthedesignprocess.

∙Tablesareprovidedwhichpermitobtaininggoodestimatesofjointstiffness（forelasticdesign）andjointresistance（forpartialstrengthrigid-plasticdesign）fortheglobalanalysis.

∙Inoneofthesedesignstategies,useismadeofthefactthatwithsemi-rigidjoints,gooddesigncanbeobtainedwithapproximatejointstiffnessvalueswithinafairlyconsiderablerangeoftheactualvalue.

Pre-requisites:

∙Aknowledgeofthefundamentalsofthetheoryofresistanceofmaterials（forbeamsandtensionmembers）andofstructuralanalysis.

∙Aknowledgethebasictheoryofcolumnbucklingandotherformsofinstability.

∙Aknowledgeoftheelasticandplasticdesignofsimplemembers.

∙Module2“Frameanalysisanddesign”.

∙Lecture7“Traditionalandmoderndesignapproaches”.

NotesforTutors:

Thismaterialcomprisesone75minutelecture.

Objectives:

Thestudentshould:

∙Understandtheinadequaciesoftraditionaldesignapproachestoaccountforjointbehaviour.

∙UnderstandthattheEurocodePart1-1allowsnewapproachestothedesignofframes.

∙Beabletousethepracticaltoolsgivenforthedesignofframes.

∙Beawarethatthetraditionalapproachesmaynolongerbethemostefficienttoproduceeconomicbuildingdesigns.andthatnewapproachesareeasytointegrateintocurrentpractice.

References:

[1]SteenhuisM.,GresnigtN.,Predesignofsemi-rigidjointsinsteelframes,Workshop1994–COSTC1,ed.,WaldF.,Proceedingsofthestateoftheart,Prague,pp131-140,1995.

[2].MaquoiR.andChabrolin,B.,Framedesignincludingjointbehaviour,

ReportN°EUR18563EN,ECSCContractNo7210-SA/212/320,EuropeanCommission,1998.

[3]6]ENV1993-1-1:

Eurocode3:

Designofsteelstructures:

Part1-1:

Generalrulesandrulesforbuildings.

1Introduction:

designmethodologyanddesignstrategy

Thedesignstrategyfollowedbythedesignerisofvitalimportanceforanefficientdesignprocess.Agoodstrategyleadsnormallytoeconomicalsolutionsforbothmembersandjoints.

Inlecture7“ Traditionalandmoderndesignapproaches ”,severalapproachesforthedesignprocesshavebeenoutlined.Thetraditionaldesignapproachreflectscurrentpracticewherejointsaremodelledaseithersimpleorcontinuousand,furthermore,wherejointdesigniscarriedoutseparately（subsequenttotheglobalanalysisandmemberdesign）.Intheconsistentdesignapproach,globalframeanalysisiscarriedoutwithdueaccountbeingtakenofthejointresponsefromtheoutset.

Whenthejointsaresemi-continuous,thetraditionaldesignapproachmaybeused,butinaniterativewayonly,inordertointegratethejointresponseinthefinalglobalanalysis.Obviouslyitisbestthatasinglepartybeinchargeofthewholedesignprocess.Howeveronemustovercomethefollowingdifficultywiththetraditionalapproach.Theactualjointpropertiesaredependentonthejointsizinganddetailing,ataskusuallycarriedoutbythefabricator.Theengineerdoingtheanalysisandmemberdesignmustobtaintheneededinformationonthejointsbeforeperformingthefinalglobalframeanalysis.Suchauseofthetraditionaldesignapproachisneitherveryefficientnordoesitfiteasilywithcommonpractice.

Forthesereasonsthefocushereistogivestrategiessuitedforthedesignofframeswithsemi-continuousjointsbutwhichcanbeeasilyintegratedintoamodifiedtraditionalapproach.Threestrategiesareputforwardwhichstillallowforatwotaskprocessi.e.thedesignoftheframemembersisseparatedfromthefinaldesignofthejoints,asinthetraditionaldesignapproach.Theprincipalaimisoneofefficiencywhilstincorporatinganallowanceforsemi-continuitywhencarryingouttheglobalanalysis.

Thefollowingthreestrategiesareproposed:

∙Theuseofagoodguessforjointstiffnessesforuseinanelasticglobalframeanalysis.

∙Theuseofafixityfactorinthetraditionaldesignapproach.

∙Thedesignofbracedframesusingrigid-plasticanalysis.

Thefirsttwostrategiesareespeciallyapplicablewhenelasticframeanalysisisused,althoughitmayalsobepossibletousethemwhenelastic-plasticanalysisisadopted.Theymainlyaddressunbracedframesbutbracedframescanbecoveredalso.

Thethirdstrategyfocusesononetypeofplasticframedesign;itsusemayberecommendedforbracedframesonly.

2Useofagoodguessforjointstiffness

Thisstrategyreferstothetraditionaldesignapproach（seelecture7（Figure1））.However,twochangesareneededtomakeitsuitableforsemi-continuousdesign（Figure1）.

1）Accountforjointstiffnessintheelasticglobalframeanalysis:

InStep3,useismadeofagoodguessfortheinitialjointstiffness.Incommonpractice,aroughguessistoassumethatthejointisrigid.Thereforeatableisgiventomakeabettergoodguessoftheactualinitialjointstiffnessratherthanassumingittoberigid.Thisgoodguessisbasedonthebeamandcolumnpropertiesandonthetypeofthejoints.Thestiffnessofthejointthusobtainedshouldbeuseddirectlyfortheelasticglobalframeanalysis（Step4）whenthedesignerintendstoperformelasticjointandmemberdesignchecks.Itshouldbedividedbyanappropriatefactorwhenplasticdesignchecksaretobecarriedout（seemodule5“Joints”）.

AnnexJ

2）Verificationofstiffnessinthedesignofjoints:

InStep8ofFigure1,onemustverifythattheactualstiffnessofanyofthejointsisinreasonableagreementwiththerelevantapproximatestiffnessintroducedinitiallyintheelasticglobalframeanalysis.Thisreplacestheverificationsforrigidand/orpinnedjointclassificationsinthetraditionaldesign.Somerulesaimedatenablingthisverificationaregivenbelow;theirphilosophyisquitesimilartotheclassificationdiagramsforjointsofEurocode3,Part1-1.Theserulescanbeappliedeasilyincombinationwithaglobalanalysissoftware.

AnnexJ

3Simplepredictionoftheinitialjointstiffness

Atthepreliminaryframedesignphase,oneneedstomakeapreliminaryassessmentofthestiffnessesforthejoints（yettobedesigned）.Forthispurpose,somesimplifiedformulaehavebeenderivedbasedonEurocode3-（revised）AnnexJ.Bymeansoftheseformulae,thedesignercanmakereasonableestimatesofthejointstiffnessbasedonaconsiderationofthejointconfigurationonly.

AnnexJ

Ofcoursetheseformulaearebasedonsomechoicesconcerningtheconnectiondetailingparametersfortypicaljointsusedinframes.Thechoiceoftheseparameters,whicharerelatedtoplatethickness,boltdiameter,boltpositioningetc,andnotgivenhere[1].

TheapproximatevalueSj,appoftheinitialjointstiffnessisexpressedas:

wherethevaluesofthefactorCaregiveninTable1fordifferentjointconfigurationsandloadings.Thesevaluesofthejointstiffnessinvolvetwoparametersonly:

zandtfc;zisthedistancebetweenthecompressionandtensileforceresultantsinthejointandtfcisthethicknessofthecolumnflange.Inanextendedend-plateconnectionwithtwoboltrows,thedistancezisapproximatelyequaltothebeamdepth.Forthesamejointwithahaunch,zisequaltothesumofthebeamdepthandthehaunchdepth.AnnexJnowgivesatableofrecommendedapproximatevaluesfortheleverarmvalue.

Figure1Designstrategyallowingforsemi-continuousjoints

（elasticglobalanalysis）

4Requiredjointstiffness

Eurocode3providesthedesignerwithtwodiagramswhichenabletheclassificationofjointstobemadeaccordingtotheirstiffness（pinned,semi-rigid,rigid）:

oneforbracedandoneforunbracedframes.

Figure6.9.8

Accordingly,forbracedframes,ajointmayberegardedasrigidiftheactualinitialjointstiffnessfulfils :

6.9.6.2

Figure6.9.8

Thatconditionensuresthattherealflexibilityofthejointsdoesnotresultinareductionofmorethan5%intheultimateresistanceoftheframebasedontherigidjointassumption.Theacceptanceofareductionofthismagnitudepermitstheacceptanceoftheglobalframeanalysiswithafinite（ratherthaninfinite）valueofthejointstiffness.Here,thewordingactualinitialstiffnessmeansthebestvalueadesignercanobtainfortheinitialstiffnessofaparticularjoint.Thisis,forexample,thevalueobtainedfromexperiments,fromnumericalsimulationsorfromcomputationsbasedonEurocode3-（revised）AnnexJ.

6.4.2.2

（2）

AnnexJ

Inordertocheckwhetherajointisrigid,athreestepprocedureisrequired（seeFigure2） :

∙Stepa:

aframeanalysisconductedwiththeassumptionofrigidjoints（Step3ofFigure1）.

∙Stepb:

checktherangeinwhichtheactualinitialstiffnessshouldbe（inStep8ofFigure1）.

∙Stepc:

checkwhethertheactualinitialstiffnessisinthisrange（inStep8ofFigure1）.

Jointconfiguration

C

Sj

Extendedend-plate,

singlesided,

unstiffened

（=1）

13

Extendedend-plates,

doublesided,

unstiffened,

symmetrical

（=0）

7,5

Extendedend-plate,

singlesided,

stiffenedintensionandcompression

（=1）