ENCE 455 Design of Steel Structures：455钢结构设计经验

ENCE455

DesignofSteelStructuresVII.Fasteners/WeldingC.C.Fu,Ph.D.,P.E.CivilandEnvironmentalEngineeringDepartmentUniversityofMaryland2IntroductionFollowingsubjectsarecovered:IntroductionofFastenersFailuremodesofboltedshearconnectionsLRFD-FastenersLRFDofslip-criticalconnectionsHigh-strengthboltsintensionFastenersincombinedshearandtensionBasicsofweldingFilletweldLRFDofweldedconnectionsReading:Chapter7ofSeguiAISCSteelManualSpecifications,

ChapterJ3ImportanceofConnectionsBeamsandcolumnsrarelyfailManycatastrophicfailureresultedfrominadequateconnectionstrengthWhatcangowrong?HyattRegencyKansasCity,1981114Dead200+Injured4TheCulprit5ProblemandSolutionProblem-

LackofUnderstandingAISCAddresses“Typical”DetailsOnlyFailureModesmaybeneglectedSolution-DevelopConsistentMethodologySystematicallyIdentifyAllFailureModesIllustrateApplicableFailurePlanes6IntroductionofFastenersTypesofFasteners:rivets(obsolete)andbolts(high-strengthbolts:mostcommon)Propertiesofbolts7IntroductionofFastenersTwoconditionsofboltinstallationareusedwithhigh-strengthboltsSnug-tight(producingabearingconnection)FewimpactsofanimpactwrenchFulleffortofaworkerwithanordinaryspudwrench

Tensioned(producingaslip-criticalconnection)Turn-of-nutmethod:specifiednumberofrotationsofthenutfromsnugtight(nutrotationscorrelatedtoboltelongation)CalibratedwrenchtighteningAlternatedesignbolts:speciallydesignboltswhosetopstwistoffwhenthepropertensionhasbeenachievedDirecttensionindicators:compresswasher(underboltheadornut)withprotrusionstoagapthatiscorrelatedtobolttension

8IntroductionofFastenersWhenhigh-strengthboltsaretobetensioned,minimumlimitsaresetonthebolttension.SeeAISCTableJ3.1Tensionequalto70%oftheminimumtensilestrengthoftheboltPurposeoftensioningistoachievetheclampingforcebetweenconnectedparts.9LRFD-Fasteners generalwhere =resistancefactor(strengthreductionfactor) =nominalresistance(strength) =overloadfactors(LRFD-A4.1) =loads(suchasdeadload,liveload,windload,earthquakeload)ofloadeffects(suchasbendingmoment,shear,axialforce,andtorsionalmomentresultingfromthevariousloads)

fastenerswhere =resistancefactor,0.75forfractureintension,shearonhigh-strengthbolts,andbearingofboltagainstsideofhole =nominalstrengthofonefastener =factoredloadononefastener10FailureModeofBoltedShearConnectionsTwotypesofboltedconnectorfailureareconsideredinthissection

Failureoftheconnector

Failureoftheconnectedparts11FailureModeofBoltedShearConnections(cont.)ConnectorfailureSingleshearconnection–Singleshearplane.P=fvA,wherefvistheaverageshearingstressandAistheconnector’scross-sectionalarea.Doubleshearconnection–Doubleshearplane.P=2fvA

12FailureModeofBoltedShearConnections(cont.)Failureoftheconnectedparts,separatedintotwocategories.Failureresultingfromexcessivetension,shear,orbendinginthepartsbeingconnectedForatensionmembermustconsidertensiononthenetarea,tensiononthegrossarea,andblockshearForbeam-beamorbeam-columnconnections,mustconsiderblockshearGussetplatesandframinganglesmustbecheckedforP,M,andV13FailureModeofBoltedShearConnections(cont.)Failureoftheconnectedpartbecauseofbearingexertedbythefastener(averagebearingstressisfp=P/dt)Iftheholeisslightlylargerthanthefastenerandthefastenerisassumedtobeplacedlooselyinthehole(rarelythecase),contactbetweenthefastenerandtheconnectedpartwillexistoverapproximately50%ofthecircumferenceofthefastener.Thebearingproblemisaffectedbytheedgedistanceandboltspacing14LRFD–Fasteners(cont)DesignbearingstrengthUsualconditionsbasedonthedeformationlimitstate,accordingtoLRFD-Formula(J3-1a).Thisappliesforallholesexceptlong-slottedholesperpendiculartothelineofforce,whereenddistanceisatleast1.5d,thecenter-to-centerspacingsisatleast3d,andtherearetwoormoreboltsinthelineofforce. where =0.75d=nominaldiameterofboltatunthreadedareat=thicknessofpartagainstwhichboltbears =tensilestrengthofconnectedpartagainstwhichboltbears =distancealonglineofforcefromtheedgeoftheconnectedparttothecenterofastandardholeorthecenterofashort-andlong-slottedholeperpendiculartothelineofforce.15LRFD–Fasteners(cont)Designshearstrength–nothreadsinshearplanes

where =0.75,thestandardvalueforshear =tensilestrengthoftheboltmaterial(120ksiforA325bolts; 150ksiforA490bolts) =thenumberofshearplanesparticipating[usuallyone (singleshear)ortwo(doubleshear)] =grosscross-sectionalareaacrosstheunthreadedshankoftheboltDesignshearstrength–threadsinshearplanes

SequiExamples7.1&7.216LRFD–Fasteners(cont)Ref:AISCLRFDp.16.1-6117LRFD–Fasteners(cont)Minimumedgedistancerequirement(AISCJ3.4)Ref:AISCLRFDp.16.1-6318LRFD–Fasteners(cont)Maximumedgedistance–12t6”,wheretisthethicknessoftheconnectedpart.Maximumspacingofconnectors(a)Forpaintedmembersorunpaintedmembersnotsubjecttocorrosion,24t12”

(b)Forunpaintedmembersofweatheringsteelsubjecttoatmosphericcorrosion,14t7”19LRFDSlip-criticalConnectionsAconnectionwithhigh-strengthboltsisclassifiedaseitherabearingorslip-criticalconnection.Bearingconnections-theboltisbroughttoasnug-tightconditionsothatthesurfacesoftheconnectedpartsareinfirmcontact.SlippageisacceptableShearandbearingontheconnectorSlip-criticalconnections-noslippageispermittedandthefrictionforcedescribedearliermustnotbeexceeded.Slippageisnotacceptable(Properinstallationandtensioningiskey)Musthavesufficientshearandbearingstrengthintheeventofoverloadthatcausesslip.AISCJ3.8fordetails.OverviewofTheoryforDesign2021LRFDSlip-criticalConnections(cont)

WhereRstr=nominalslipresistanceperboltatfactoredloads

m=numberofslip(shear)planes

Ti=minimumfastenerinitialtensiongiveninAISCTableJ3.1 =meanslipcoefficient,asapplicable,orasestablishedbytests =0.35forClassAsurfacecondition =0.50forClassBsurfacecondition =0.40forClassCsurfacecondition =1.0forstandardholes =0.85foroversizeandshort-slottedholes =0.70forlong-slottedholestransversetoload =0.60forlong-slottedholesparalleltoloadSequiExample7.422LRFD–Fasteners(cont)Designtensilestrength

where =0.75,avalueforthetensilefracturemode =tensilestrengthoftheboltmaterial(120ksiforA325bolts;150siforA490bolts) =grosscross-sectionalareaacrosstheunthreadedshankoftheboltHigh-StrengthBoltsinTensionFigures7-24&7-2523PryingAction24Figure7.27BolttensionB0BPryingforceQThecorrespondingboltforce,includingtheeffectsofprying,isBcPryingAction25Figure7.28PryingAction26PryingAction27SequiExample7.8LRFDSolutionForEvaluation:Forbackchecking:28CombinedShearandTensionBearing-typeconnectionsSlip-criticalconnectionsSequiExample7.929BasicofweldingStructuralweldingisaprocesswherebythepartstobeconnectedareheatedandfusedwithamoltenfillermetal.Uponcooling,thestructuralsteel(parentmetal)andweldorfillermetalwillactasonecontinuouspart.Thefillermetalisdepositedfromaspecialelectrode.Anumberofweldingprocessesareused,dependingontheapplication

FieldweldsShopwelds30Basicofwelding(cont)Basicprocess:ShieldedMetalArcWelding(SMAW):NormallydonemanuallyandiswidelyusedforfieldweldingCurrentarcsacrossthegapbetweentheelectrodeandthebasemetalConnectedpartsareheatedandpartofthefillermetalisdepositedintothemoltenbasemetalCoatingontheelectrodevaporizesandformsaprotectivegaseousshield,preventingthemoltenmetalfromoxidizingbeforeitsolidifiesTheelectrodeismovedacrossthejointandaweldbeadisdeposited.SizeoftheweldbeaddependsontherateoftravelAstheweldcools,impuritiesrisetothesurfaceandformacoatingcalledslag.Slagmustberemovedbeforethenextpassortheweldispainted.31Basicofwelding(cont)Basicprocess(cont.usedforshopwelding):SubmergedArcWelding(SAW)GasMetalArcWelding(GMAW)Endoftheelectrodeandthearcaresubmergedinagranularfluxthatmeltsandformsagaseousshield.FluxCoredArcWelding(FCAW)ElectroGasWelding(FGW)ElectroslagWelding(ESW)32Basicofwelding(cont)33Basicofwelding(cont)Minimumweldsize,maximumweldsize,andminimumlength:Theminimumsizeofafilletweldisafunctionofthethicknessofthethickerconnectedpart.SeeAISCTableJ2.4fordetails.Themaximumsizeofafilletweldisasfollows:Alongtheedgeofaconnectedpartlessthan¼-inchthick,themaximumfilletweldsize(w)equalstheplatethicknessForothervaluesofplatethickness,t,themaximumweldsizeist-1/16in.34Basicofwelding(cont)Theminimumpermissiblelengthofafilletweldis4timesitssize.Ifonlyashorterlengthisavailable,w=L/4.Fortheweldsintheconnectionshownbelow,LWtoaddressshearlaginsuchconnections.Whenaweldextendstothecornerofamember,itmustbecontinuedaroundthecorner(anendreturn)PreventstressconcentrationsatthecorneroftheweldMinimumlengthofreturnis2w35Basicofwelding(cont)CommontypesofweldsareFilletwelds-WeldsplacedinacornerformedbytwopartsincontactGroovewelds-WeldsdepositedinagapbetweentwopartsPlugwelds-Circularorslottedholethatisfilledwithweldmetal.Usedsometimeswhenmoreweldlengthisneededthanisavailable

36FilletWeldThedesignandanalysisoffilletweldsisbasedontheassumptionthatthegeometryoftheweldisa45-degreerighttriangle

Standardweldsizesareexpressedinsixteenthsofaninch.Failureoffilletweldsisassumedtooccurinshearonthethroat.37FilletWeld(cont)Thestrengthofafilletwelddependsonthestrengthofthefillerorelectrodemetalused.Thestrengthofanelectrodeisgivenintermsofitstensilestrengthinksi.Strengthsof60,70,80,90,100,110,