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HSS SEAM WELDS

by Jeffrey A. Packer1

1Bahen/Tanenbaum Professor of Civil Engineering, University of Toronto, Ontario, Canada _____________________________________________________________________________________Therearetwoprincipalmanufacturingmethodsinternationallyforcold-formingsquareandrectangularHSS:direct-formingandcontinuous-forming.Thedirect-formingprocessincludes:(1)roll-formingacoilstripdirectly intoanopensectionwiththedesiredrectangularshape;and(2) joiningtheedgesoftheopensectionbyweldingtoformaclosedrectangularshape.Thecold-workinginthiscaseisconcentratedat the four corners. The continuous-forming process includes: (1) roll-forming a coil strip first into acircularopentube;(2)joiningtheedgesoftheopentubebyweldingtoformaclosedcircularshape;and(3)flatteningthecirculartubetoformthedesiredrectangularshape.BothproductionmethodsareusedinNorthAmerica,althoughthelatter(continuous-forming)ismuchmorecommon.Thestaticpropertiesof these two HSS products have been compared by Sun and Packer (2014). Both methods of HSSmanufacture involve forming a closed section by continuous “welding”, using an induction heatingprocess,withouttheuseofanyadditionalweldconsumable.WeldFlashThe “flash” (protruding weld metal) of the longitudinal seam weld is always removed, during themanufacturingprocess,ontheoutsideofHSS,thusmakingtheexterioroftheHSSsurfacesmoothandflush.However,weldedtubingisnormallyfurnishedwithoutremovaloftheinsideflash,asnotedbybothA500(ASTM,2013a)andA1085(ASTM,2013b).SomemanufacturersareabletoprovideHSSwithinsideflashremoval,butthisneedstoberequestedandconfirmedpriortoproduction.InsideflashremovalismostcommonlystipulatedforroundHSSwhereonetubemustfitsnuglyinsideanother(e.g.telescopingapplications).Sometimestheprotrudinginsideweldflashmaycomplicatetheexecutionofaparticularboltedconnection,suchasaspliceconnectionusinginteriorspliceplateswheretheintentionistoplacetheboltsindoubleshear(seeFigure1).Thispotentialproblem(whereaspliceplatecannotlieflatontheinsideHSSsurfaceduetoaraisedweld)canbeovercomebythefabricatorpayingattentiontotheseamweldlocationandavoidingboltingtothatwall,ifnotallfourfacesaretobeconnected.IfallHSSfacesaretobeconnectedthenshims,eithersideoftheseamweld,couldbeemployed.

Figure1:BoltedspliceplateconnectiononlargerectangularHSS

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WeldinCornerRegionsIthasbeengenerallyacceptedbyHSSmanufacturersthatlongitudinalseamweldsshouldnotlieinthecornerregionsofsquareandrectangularHSS.Thereisessentiallyalackofevidenceandtestingtosupportcorner seamwelds, in general structural applications. Although corner seamswere notmentioned inASTMA500-10,inASTMA500-13Clause6.3wasaddedtospecificallystipulatethat…“Theweldshallnotbe locatedwithintheradiusof thecornersofanyshapedtubeunlessspecifiedbythepurchaser”. InASTMA1085-13,however,thisprohibitionoftheseamweldinthecornerisnotincluded–yet.Thus,aweldseamlocatedbeyondthetangentpointofanHSS“flat”iscauseforrejectionofASTMA500material,butnotASTMA1085material.Through-ThicknessWeldingHSS manufacturers also generally concur that the longitudinal weld should fuse the HSS across thecompletewall thickness.ASTMA500-13,however,only requires that…“The longitudinalbutt jointofweldedtubingshallbeweldedacrossitsthicknessinsuchamannerthatthestructuraldesignstrengthofthetubingsectionisassured”.Sincethisspecificationnotesthat…“Forsquareandrectangulartubing,thewallthicknessrequirementsshallapplyonlytothecentersoftheflats”(whichwouldnotbeconstruedasincludingaweldlocation),theamountofrequiredthrough-thicknessweldfusionisnotspecified.Aslightconcavityisnotuncommonthough,asshowninFigure2,andtherearenoacceptanceorrejectioncriteriawithASTMA500forthedegreeoffusion,ormis-alignmentofedgeseithersideoftheweld.

Figure2:LackofcompleteweldfusionthroughtheHSSthickness

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Somecasesofpoorweldfusionhavebeenfoundinthepast,asillustratedinFigure3.Extremecasesareevidentvisually,butpartiallack-of-fusioncangenerallybedetectedbyUT.

Figure3:DefectiveseamweldswithimportedHSS(DivisionoftheStateArchitect,California,2007)

SinceASTMA500stipulatesthattheHSS…“shallbeweldedacrossitsthicknessinsuchamannerthatthestructuraldesignstrengthofthetubingsectionisassured”,thedegreeofthrough-thicknessconnectionthat isrequireddependsontheHSS loading. If loaded inpurecompression,asanaxiallycompressedcolumn,only intermittentpartial connection is required–sufficient for themember toachieveglobalflexural buckling without suffering from local buckling of the open cross-section between points ofconnection.ThishasbeenillustratedexperimentallybycolumntestsinwhichtheHSSwasslitopenalongitslength,tosimulatezerothrough-thicknessconnectionbya“seamweld”,andthenloadedtofailureinaxial compression (Figure 4). Providing the open section was joined intermittently, across the free-standingedges,atreasonable(calculated)distances,globaloverallbucklingcouldbeattained.

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(a) (b)

Figure4:Large-scaleHSScolumncompressiontests(UniversityofToronto)onslit-opensectionstoillustrate:(a)globalflexuralfailure,withintermittentpatching;(b)possiblelocalbucklingfailure

betweenpointsofdistantinter-connection

While even an extreme case of zero through-thickness weld fusion can be repaired, by intermittentpatches,toserveasanaxiallyloadedHSScolumn,itisclearlydangeroustoassumethatsuchamemberwouldbeadequateundermanyotherloadingconditions,andespeciallyatconnections.ASTMA1085-13,ontheotherhand,statesthat…“Thelongitudinalbuttjointofweldedtubingshallbeweldedacrossitsthickness”.Thus,withthisnewhigherperformancespecification,anythrough-thicknessweldfusionlessthan100%iscauseforrejection.

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ReferencesASTM. 2010. “Standard Specification for Cold-FormedWelded and Seamless Carbon Steel StructuralTubinginRoundsandShapes”,ASTMA500/A500M–10,ASTMInternational,WestConshohocken,PA.ASTM. 2013a. “Standard Specification for Cold-FormedWelded and Seamless Carbon Steel StructuralTubinginRoundsandShapes”,ASTMA500/A500M–13,ASTMInternational,WestConshohocken,PA.ASTM.2013b.“StandardSpecificationforCold-FormedWeldedCarbonSteelHollowStructuralSections(HSS)”,ASTMA1085–13,ASTMInternational,WestConshohocken,PA.Sun,M.andPacker,J.A.,2014.“Direct-FormedandContinuous-FormedRectangularHollowSections–ComparisonofStaticProperties”,JournalofConstructionalSteelResearch,Vol.92,pp.67-78.April2014