SEISMIC APPLICATIONS OF HOLLOW STRUCTURAL SECTIONS (HSS) IN MOMENT RESISTING FRAMES

By Emily Guglielmo, SE, Martin/Martin, STI Technical Committee Member UseofHollowStructuralSection(HSS)membersinmoment-framestoresistseismicforceshasnumerouspotentialadvantages.HSSshapesprovideeffectivebi-axialcolumnstrength,whilehavingminimalimpactonarchitecturallayout.Furthermore,HSSshapesareparticularlyefficientandeconomicalundercompressionloads.Finally,HSScolumnshapesofferinternalfireproofingoptions,attributesunavailablewithcomparablewideflangeshapes.Asmentionedabove,HSSsectionsarecommonlydesiredinsteelframesinwhichmembersexperienceloadinginmultipledirections.SquareandcircularHSShaveveryefficientshapesformulti-axisloadingduetotheiruniformgeometryalongeachcross-sectionalaxis,resultinginuniformstrengthcharacteristics.Inadditiontohavingsuperiormulti-axisproperties,HSSsectionsareinherentlymoreefficientcompressionmembersthanotherstructuralsteelsections,includingwideflanges.Thereasonforthisefficiencyistheirgeometricshape.Theradiusofgyration(rx,ry),particularlyabouttheminoraxis,issignificantlygreaterthanthatofanopensectionofasimilarsize,area,andweight.Thisrobustradiusofgyrationresultsinamuchlowerslendernessratioforthesameeffectivelength,resultinginagreatercompressioncapacity.

PerAISC341ChapterE,aseismicforce-resistingsystemutilizingmoment-resistingframeswithHSScolumnsandeitherwideflangeorHSSbeams,canbedesignedasanOrdinaryMomentFrame(OMF).ClassificationasanOMFrequiressimplecalculationsandminimalsupplementalattentiontotheconnectiondetails,reinforcement,andsizing.However,ifamomentframeistobeclassifiedasanIntermediateMomentFrame(IMF)oraSpecialMomentFrame(SMF),therearetwooptionscurrentlyavailabletodemonstrateacceptablebeam-columnconnections.Thedesignermusteitheremploya“pre-qualifiedconnection”,asapprovedbyAISC358-10,ormeeta“conformancedemonstration”throughspecificlaboratorytesting.

Whilenumerouspublicdomainconnectionsformoment-resistingframesunderhighseismicloadingareavailableasprequalifieddesignsinAISC358,nonedirectlypertaintoHSSmembers.ThisabsenceofAISC-endorsed,genericseismicmomentconnectionsisasubstantialbarriertousingHSScolumnsinsuchframesinNorthAmerica.

AcomparisonofHSSuseintheUnitedStatestoourseismicallyactiveJapanesecounterpartsrevealslimiteduseintheUnitedStates,butsignificant(greaterthan90%ofsteelbuildings)usageinJapan(Kurobane2002).Researchisongoingtowardthepre-qualificationofHSSmomentframesintodailypracticeintheUnitedStates.Oneparticularstudy“SeismicApplicationsofHollowStructuralSectionsinMomentResistingFrames”isledbyDr.JasonMcCormickattheUniversityofMichiganandfundedbyAISC.ThisresearchfocusesondevelopinginnovativeconnectionmethodsforHSSmembersinhighseismicstructures.Specifically,thestudyinvolvesadetailedstudyofthebehaviorofHSSmembers,developmentofconnectiondetailsforvariousHSSmomentconnections,andtestingofthemostpromisingconnectionconfigurationsforuseinbothIMFandSMFsystems.SeephotosoftestingofHSSmomentframes.

However,untilthisresearchandpre-qualificationprocessiscomplete,connectionsnotprequalifiedbyAISC358mustbetestedinaccordancewithAppendixSofAISC341.InthecaseofHSScolumn(orbeam)momentframeapplications,thebeam-columnjointshouldbetestedperguidelines,mimickingtheactualproposedconfiguration.

Worthnoting,AISC358doespermitaprequalifiedproprietaryproductwhichallowsfortheutilizationofHSSshapesasmomentframecolumns.TheConXtechConXLmomentconnectionforSpecialMomentFrames(SMFs)andIntermediateMomentFrames(IMFs)ispermittedwithspecificbeamclearspan-to-depthratiosand16”squarecolumns.ThisproprietaryconnectionutilizescollarassembliesaroundasquareHSScolumnbyfasteningwithhigh-strengthpre-tensionedbolts.Aspracticingengineersareaware,currentpracticegenerallyminimizesthenumberofmoment-resistingframesinabuildingtoreducecost.OnemajorbenefitoftheConXtechframingsystemisthataredundant,well-distributedmoment-resistingspaceframeisachievedasnearlyallbeam-columnconnectionsaremoment-resisting.Thisincreasednumberofframesresultsingreaterrobustness,amorewell-distributedseismic-resistingsystem,andincreasedresistancetoprogressivecollapse.

Insummary,numerousreasonsexistwhyHSSshapesmaybebeneficialforuseinmoment-frameapplications,rangingfromefficiencytoaesthetics.However,currentcodesprecludecommonusageoftheseshapes.Whilethereareseveralwork-aroundoptions,includingtestingandproprietaryconnections,greaterusageofHSSshapesinmomentframeapplicationsrequirescarefulconsiderationandfurtherinvestigation.

References:Kurobane,Y.(2002).Connectionsintubularstructures.ProgressinStructuralEngineering.4,35-45.McCormick,Jason.SeismicApplicationsofHollowStructuralSectionsinMomentResistingFrames,2010–2014

ImagescourtesyofDr.JasonMcCormick