A basic understanding of open web steel joist framing systems, including their history, their unique features, and certain challenges associated with their evaluation and modification.

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An Overview of Open Web Steel JoistsTheownerormanagerofabuildingthatfeaturesopenwebsteeljoistframingsystemsforthefloorsand/orroofmayoccasionallywonderiftheyarestrongenoughtosupportneworadditionalloads,and,ifnot,whethertheycanbemodifiedtodoso.ThisWJEPrimerisintendedtoprovidebuildingownersandmanagerswithabasicunderstandingofthesecommonstructuralmembers,includingtheirhistory,theiruniquefeatures,andcertainchallengesassociatedwiththeirevaluationandmodification.

A Brief History

Thenameopenwebsteeljoistsmayconcealthefactthatthese“joists”arereally“trusses.”Trussesmadeofwoodhavebeenutilizedthroughouttheworldforcenturies,buttheuseoftrussesbecamefarmoreprevalentinthelate19thandearly20thcenturiesfollowingimprovementsintheproductionofstructuralmetals,suchascastironandlatersteel.Today,openwebsteeljoistsareoneofthemostcommonlyusedframingsystemsforfloorsandroofsinbuildings.

SomeofthefirststeeljoistsusedintheUnitedStatesinthemidtolate1800sweremadeofsheetsteelwebsandflangesrivetedtogether.Useofthesememberscontinuedintotheearly1900s,whenquantityproductionofpressedsteeljoistsmadeoflightgaugesteelsheetsbegantoreplacetherivetedversions.Thesejoistsweremadebypressingthesheetsintotheformofchannelsandweldingthemback-to-back,givingtheappearanceoflightI-shapedbeams.Oneoftheprimaryobjectionstothesejoistswasthattheirsolidsteelwebsprovidednopractical

meansofinstallingconduitsandpipeswithinthedepthofthefloorandroofmembers.Thisandotherconcernsledtothedevelopmentofwhatcametobecalledopen web steel joists.

ThefirstopenwebsteeljoistsintheUnitedStatesweremanufacturedinthe1920s.Theseearlysteeljoistsfeaturedtopandbottomchordsmadeofroundbars,steelangles, orproprietaryshapes,withwebscommonlyformedfromsinglecontinuousbentbars. Asthesteeljoistindustrybegantogrow,engineersandarchitectsfoundthatthelackofuniformdesignandfabricationstandardsamongthevariousmanufacturersmadeitdifficulttoachievethemostcost-effectivestructures.Consequently,membersoftheindustrybeganworkingtostandardizethemanufactureofthesecomponents.TheseeffortsresultedintheformationoftheSteelJoistInstitute(SJI),whichpublisheditsfirst setofStandardSpecificationsforOpenWebSteelJoistsin1928,followedbythefirstloadtablesin1929.Today,theSJI,anonprofitorganizationofactivejoistmanufacturers,

By Ben COOk aND Jerry mAlyWiss, Janney, Elstner Associates, Inc.

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andotherorganizationsandcompaniesconnectedtotheindustry,continuestodevelopandmaintainstandardsforthedesign,manufacture,anderectionofsteeljoistconstruction.

ThefirstSJISpecificationpermittedthe useofrolledshapesorstripsofsheetsteel,roundbars,anglesorspeciallyrolledbarsrivetedorweldedtogether.Modern-dayopenwebsteeljoistsaresimilartothoseproducedinthe1920sand1930s,andallfeaturetopandbottomchords,withdiagonal,andsometimesvertical,webmembersbetweenthechords.Today’ssteeljoistsareoftenfabricatedusingdoubleanglechordsandwebsmade fromanglesorroundrods.However,the SJIStandardSpecificationprovidesperformancerequirements,andeachmembercompanyispermittedtousevariousproductsandgeometriesintheirjoists,therebyprovidingproprietarydesigns,solongasthespecificationissatisfied.Thisincludestheuseofbothhot-andcold-rolledsteelshapes,bars,sheet,andstrip.Photographsofacirca1930openwebsteeljoistandamoderneraK-seriesopenwebsteeljoistareshowninFigures1and2,respectively.

Typical and Atypical Applications in new Buildings

Sincetheywerefirstpublished,eachSJIStandardSpecificationhasincludedloadtables,whichprovideallowableuniformloadsforvariousspanlengthsandjoistdepths,makingitpossibleforengineersandarchitectstospecifyratherthandesignthesetypesofstructuralcomponents.Manyjoistmanufacturersprovidecustomdesignsofopenwebsteeljoists.Thisisadvantageousforapplicationswheretheengineerofrecordspecifiesspecificloadingsonthestructuraldrawingsas

opposedtothestandardSJIjoistdesignations.Somemanufacturersalsoallowothervariablesinthedesignandfabricationoftheiropenwebsteeljoists,includingslopedorcurvedtopchords(ratherthanthetraditionalparallelchordjoists),variationsinthedepthsofjoistbearingseats,slopedbearingseats,greaterjoistdepths,andtheabilitytoincorporatesteeljoistsintothelateralforceresistingsystems.

New or Additional Loads on Open Web Steel Joists in Existing Buildings

Situationsroutinelyarisewheretheownersofexistingbuildingswanttomakecertainalterations,suchasinstallingnewandheavierroofingsystems,replacingexistingrooftopmechanicalunitswithupgradedequipment,orinstallingrooftopmechanicalunitswherenonepreviouslyexisted.Proposedalterationscanalsoincludetheinstallationofheavierflooringmaterialsorhighdensityfilingsystems,whilechangesinoccupancymayresultinincreasedfloorliveloadrequirementsormodificationstotheroofframingtoaccommodatesnowdriftloadsthatwerenotnecessarywhenthebuildingwasoriginallydesignedandconstructed.Whensituationslikethesearise,thequestionisoftenasked:Can the roof or floor joists in my building support these new loads, and if not, can they be strengthened so that they can?

Similartomostexistingstructuralcomponentsorsystems,theloadcarryingcapacitiesofexistingopenwebsteeljoistscanbeestimated;however,thereliabilityofanysuchassessmentcanbeaffectedbyavarietyoffactors.Thecapacityofexistingopenwebsteeljoistscanbedeterminedwiththegreatestconfidencewhentheoriginaldesign,construction,orshopdrawingsareavailable.Whenthese

Open Web Steel Joists (COnTInUEd)

Figure 1

Photograph of an open web steel joist circa 1930

Figure 2

Photograph of a modern open web steel joist

Figure 3

Typical joist tag

documentscannotbeobtained,thenextbestoptionistoseeifjoisttagsarestill inplaceontheaffectedjoists.Ajoisttag isametallabelwiredtoeachjoistduringfabricationthatprovidesinformationsuchasthemanufacturer’sname,jobnumber,marknumber,plantlocation,anddate; anexampleisshowninFigure3.When joisttagscanberetrieved,itislikelythatthemanufacturercanbeidentifiedwho

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maybeabletoprovideinformationonthedesignandfabricationofthejoistsinquestion.Ifthemanufacturerisnolongerinbusiness,theSJImaybeabletoprovideusefulinformation.Alternatively,whendrawingsandjoisttagsarenotavailable,oreveniftheyare,buildingowners,engineers,andcontractorstryingtoidentifyexistingjoistscanfilloutandsubmittheJoist Investi gati on Form availableontheSJIwebsite,which,weunderstand,hasproventobeaneffectiveandvaluableresource.

Finally,ifdrawingsand/orjoisttagsarenotavailable,anduseoftheSJIJoist Investi gati on Formhasbeenfruitless,thecapacityofsteeljoistscanalwaysbeestimatedbyanexperiencedstructuralengineer,usingfieldmeasurementsoftheoverallgeometryandmeasurementsofthejoistchordsandwebs,togetherwithassessmentsofthevariouschord-to-webmemberconnections.Evaluationssuchasthiswillalsorequireassumptionsregardingtheyieldstrengthofthesteel,whichcanusuallybedeterminedwithreasonableaccuracyusingtheSJIStandardSpecificationthatwasinuseatthetimethebuildingwasdesignedandconstructed.However,wheregreaterreliabilityisneeded,samplescanberetrievedfromthejoistsinquestionandthematerialpropertiesdeterminedbytests.

increasin g the load Carrying Capacity of Existi ng Open Web Steel Joist Framing Systems

Oncethecapacitiesoftheexistingjoistsareknownwithinareasonabledegreeofengineeringcertainty,theycanbecomparedtotheproposeddemandstodetermineifthenewloadscanbecarriedwithanadequatefactorofsafety.Whentheanswertothisquestionisno,thereare

Figure 4

Original top chord strengthened with new welded steel rod

Figure 5

Original top chord strengthened with a new screw-fastened steel angle

Figure 6

Original web members supplemented with new steel angles

Figure 7

Original web member strengthened with new steel rod and angles.

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severalapproachesthatcanbetaken sothattheexistingframingcanbemadestrongand/orstiffenoughtosupport theproposedloaddemands.Theseapproachesinclude:

1.Reinforcingorstrengtheningthe existingjoists

2. Addingnewjoistsorsteelbeamsbetweentheexistingjoists

3. distributingtheloadstoseveral adjacentmembers.

Approach 1

Reinforcingorstrengtheningtheexistingjoistscanbeaccomplishedinavarietyofways,dependingtosomedegreeonthemagnitudeandextentofthenewloads. Ifthenewloadswillbeappliedatdiscretelocations(e.g.anewrooftopmechanicalunit),itispossiblethatonlyarelativelysmallnumberofthejoistcomponents willrequirereinforcement.Inthiscase, theaffectedchordsandwebsmaybereinforcedwithnewsteelrodsoranglesasshowninFigures4,5,6and7.However,ifthenewloadingisappliedoveralargearea(e.g.fromanewhighdensityfilingsystemorincreasedfloorliveload),allofthejoistchordsandwebsmayneedtobereinforcedinasimilarmanner.

Alternatively,insomecircumstances,itmaybepossibletostrengthentheexistingjoistsbyreducingtheirspanswiththeinstallationofanewbeamorbeamsperpendiculartothespansofthejoists.Inmostcases,thiswouldrequirestrengtheningatleastsomeofthewebmembersinthevicinityofthenewbeamorbeams.However,thismaybeamorecost-effectiveapproachthan

reinforcingallormostofthejoists’chordsandwebmembersovertheentirelengthoftheaffectedjoists.

Approach 2

Theadditionofnewframing,suchassteelbeamsoradditionaljoistsbetweenandparalleltotheexistingjoists,issometimesanoptionworthyofconsideration.Ifthenewloadswillbeappliedatdiscretelocations,newjoistsorbeamsmaybedesignedandinstalledtosupportthe newloadsintheirentirety,andrelativelyindependentoftheexistingloads.However,ifthenewloadsaretobeappliedoverlargeareas(e.g.fromanewhighdensityfilingsystemorincreasedfloorliveloads),thenewframingwilllikelyhavetoworkinunisonwiththeexistingframingtosharealltheloads.Inthiscase,thestiffnessofthenewframingwillneedtoapproximatelymatchthatoftheexistingmembers.

Theadditionofnewjoistsorbeamsbetweenandparalleltotheexistingjoistsmaybemorechallengingthanitinitiallyseems.Forexample,moststeeljoistsaretopchordbearing,whichmeansthatthetopchordsextendseveralinchesoverthetopofthesupportingbeam,girder,orwallateachend;averyefficientandcosteffectivebearingdetailfornewconstruction.However,inanexistingbuilding,itcanbeverydifficult,ifnotpracticallyimpossible,toinstallanewjoistwiththissametopchordbearingdetailthatwasusedfortheoriginaljoistateachend.Onesolutionistoinstallthenewjoistswithfield-boltedsplices.Ontheotherhand,theuseofsupplementalsteeljoistsorbeamssupportedatthefaceoftheexisting

supportsatoneendmayeliminatethisproblem.Otherchallengesincludethepossibilitythataccesstotheareabetweenexistingjoists,forinstallationofnewframingmembers,maybelimitedbythepresenceofmechanicalducts,plumbingpipes,and/orelectricalconduits.Otherdifficultiesmaybeencounteredbecausetheexistingframingwillhavedeflectedbetweensupportsduetothedeadloadsalreadyonthestructure,andthiswillneedtobeconsideredinthedesignandinstallationofthenewmembers.

Approach 3

Whenthenewloadsareisolatedtoarelativelysmalllocationofafloororroof(e.g.whenamechanicalunitistobesuspendedfromtherooforceiling),itmaybepossibletodistributethenewloadsoveranumberofadjacentjoistsbytheinstallationofloaddistributionorspreaderbeams.Thissolutionisverystraightforwardwhenthenewloadsaresmallenoughthattheycanbecarriedbytwoadjacentjoists.However,thisapproachbecomesslightlymorecomplexwhenthenewloadmustbedistributedoverseveraljoistssothatnoneareloadedbeyondtheirallowablecapacitywhenthenewloadsareconsideredtogetherwiththeexistingdeadloadsandothercode-requireddesignloads.Inthiscase,thespreaderbeamsmustberelativelystiffcomparedtothestiffnessofthejoists.Asanalternativetoinstallationofastiffbeambelowthejoists,itmaybepossibletoconstructatrussperpendiculartoandwithinthedepthofthejoiststotransfertheloadappropriatelytotheexistingjoists.

Open Web Steel Joists (COnTInUEd)

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references

�� TheBergerManufacturingCo.,Handbook Describing Berloy Building Materials, First Edition,1921,Canton,OH

�� TrusconSteelCompany,Truscon Steel Joist Data Book,1924,Youngstown,OH.

�� nucorCorporation,Designing with Vulcraft Steel Joists, Joist Girders and Steel Deck,SecondEdition,2002,JamesM.Fisher,Ph.d.,P.E.,MichaelA.West,P.E.,AIA,JuliusP.VandePas,P.E.

�� SteelJoistInstitute,75 - Year Steel Joist Manual,2003,MyrtleBeach,SC.

�� SteelJoistInstitute,Technical Digest 12, Evaluation and Modification of Open-Web Steel Joists and Joist Girders, 2007, JamesM.Fisher,Ph.d.,P.E.,MyrtleBeach, SC.

�� SteelJoistInstitute,Standard Specifications,43rdEdition,Florence,SC.

Contact:T: 303.914.4300 E: bcook@wje.com E: jmaly@wje.comW: www.wje.com

Location:DENVER OFFICE 3609 South Wadsworth Boulevard Suite 400 Lakewood, CO 80235

Open Web Steel Joists (COnTInUEd)