Portal Frames - Steelconstruction
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SteelConstruction.infoThefreeencyclopediaforUKsteelconstructioninformation
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PortalframesFromSteelconstruction.info
Portalframesaregenerallylowrisestructures,comprisingcolumnsandhorizontalorpitchedrafters,connectedbymomentresistingconnections.Resistancetolateralandverticalactionsisprovidedbytherigidityoftheconnectionsandthebendingstiffnessofthemembers,whichisincreasedbyasuitablehaunchordeepeningoftheraftersections.Thisformofcontinuousframestructureisstableinitsplaneandprovidesaclearspanthatisunobstructedbybracing.Portalframesareverycommon,infact50%ofconstructionalsteelusedintheUKisinportalframeconstruction.Theyareveryefficientforenclosinglargevolumes,thereforetheyareoftenusedforindustrial,storage,retailandcommercialapplicationsaswellasforagriculturalpurposes.Thisarticledescribestheanatomyandvarioustypesofportalframeandkeydesignconsiderations.
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Multibayportalframeduringconstruction
Contents
1Anatomyofatypicalportalframe2Typesofportalframes3Designconsiderations
3.1Choiceofmaterialandsection3.2Framedimensions
3.2.1Clearspanandheight3.2.2Mainframe3.2.3Haunchdimensions3.2.4Positionsofrestraints
4Actions4.1Permanentactions
4.1.1Serviceloads4.2Variableactions
4.2.1Imposedroofloads4.2.2Snowloads4.2.3Windactions4.2.4Craneactions4.2.5Accidentalactions4.2.6Robustness4.2.7Fire
4.3Combinationsofactions5FrameanalysisatULS
5.1Plasticanalysis5.2Elasticanalysis
6Inplaneframestability6.1Secondordereffects6.2Firstorderandsecondorderanalysis
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Principalcomponentsofaportalframedbuilding
6.3Calculationofcr6.4Sensitivitytoeffectsofthedeformedgeometry
7Design7.1Crosssectionresistance7.2Memberstability7.3Rafterdesignandstability
7.3.1Outofplanestability7.3.2Gravitycombinationofactions7.3.3Theupliftcondition7.3.4Inplanestability
7.4Columndesignandstability7.4.1Outofplanestability7.4.2Inplanestability
8Bracing8.1Verticalbracing
8.1.1Portalisedbays8.1.2Bracingtorestrainlongitudinalloadsfromcranes
8.2Planbracing8.2.1Restrainttoinnerflanges
9Connections9.1Columnbases
10References11Furtherreading12Resources13Seealso14Externallinks15CPD
Anatomyofatypicalportalframe
Aportalframebuildingcomprisesaseriesoftransverseframesbracedlongitudinally.Theprimarysteelworkconsistsofcolumnsandrafters,whichformportalframes,andbracing.Theendframe(gableframe)canbeeitheraportalframeorabracedarrangementofcolumnsandrafters.
Thelightgaugesecondarysteelworkconsistsofsiderailsforwallsandpurlinsfortheroof.Thesecondarysteelworksupportsthebuildingenvelope,butalsoplaysanimportantroleinrestrainingtheprimarysteelwork.
Theroofandwallcladdingseparatetheenclosedspacefromtheexternalenvironmentaswellasprovidingthermalandacousticinsulation.Thestructuralroleofthecladdingistotransferloadstosecondarysteelworkandalsotorestraintheflangeofthepurlinorrail
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Crosssectionshowingaportalframeanditsrestraints
towhichitisattached.
Portalframedstructuresoverview
Typesofportalframes
Manydifferentformsofportalframesmaybeconstructed.Frametypesdescribedbelowgiveanoverviewoftypesofportalconstructionwithtypicalfeaturesillustrated.Thisinformationonlyprovidestypicaldetailsandisnotmeanttodictateanylimitsontheuseofanyparticularstructuralform.
Pitchedroofsymmetricportalframe
GenerallyfabricatedfromUKBsectionswithasubstantialeaveshaunchsection,whichmaybecutfromarolledsectionorfabricatedfromplate.25to35marethemostefficient
PitchedroofsymmetricportalframeLancashireWasteDevelopment
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spans.
Portalframewithinternalmezzaninefloor
Officeaccommodationisoftenprovidedwithinaportalframestructureusingapartialwidthmezzaninefloor.TheassessmentofframestabilitymustincludetheeffectofthemezzanineguidanceisgiveninSCIP292.
PortalframewithinternalmezzaninefloorWatersMeetingHealthCentre,Bolton(ImagecourtesyBDStructuresLtd.andASDWestokLtd.)
Craneportalframewithcolumnbrackets
Whereatravellingcraneofrelativelylowcapacity(uptosay20tonnes)isrequired,bracketscanbefixedtothecolumnstosupportthecranerails.Useofatiememberorrigidcolumnbasesmaybenecessarytoreducetheeavesdeflection.Thespreadoftheframeatcraneraillevelmaybeofcriticalimportancetothefunctioningofthecranerequirementsshouldbeagreedwiththeclientandwiththecranemanufacturer.
Tiedportalframe
Inatiedportalframethehorizontalmovementofthe
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eavesandthebendingmomentsinthecolumnsandraftersarereduced.Atiemaybeusefultolimitspreadinacranesupportingstructure.Thehighaxialforcesintroducedintheframewhenatieisusednecessitatetheuseofsecondordersoftwarewhenanalysingthisformofframe.
Monopitchportalframe
Amonopitchportalframeisusuallychosenforsmallspansorbecauseofitsproximitytootherbuildings.Itisasimplevariationofthepitchedroofportalframe,andtendstobeusedforsmallerbuildings(upto15mspan).
Proppedportalframe
Wherethespanofaportalframeislargeandthereisnorequirementtoprovideaclearspan,aproppedportalframecanbeusedtoreducetheraftersizeandalsothehorizontalshearatthefoundations.
ProppedportalframeRebottlingPlant,Hemswell(ImagecourtesyofMetsecplc)
Mansardportalframe
Amansardportal
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framemaybeusedwherealargeclearheightatmidspanisrequiredbuttheeavesheightofthebuildinghastobeminimised.
Curvedrafterportalframe
Portalframesmaybeconstructedusingcurvedrafters,mainlyforarchitecturalreasons.Becauseoftransportlimitationsrafterslongerthan20mmayrequiresplices,whichshouldbecarefullydetailedforarchitecturalreasons.Thecurvedmemberisoftenmodelledforanalysisasaseriesofstraightelements.GuidanceonthestabilityofcurvedraftersinportalframesisgiveninSCIP281.Alternatively,theraftercanbefabricatedasaseriesofstraightelements.Itwillbenecessarytoprovidepurlincleatsofvaryingheighttoachievethecurvedexternalprofile.
Cellularbeamportalframe
Raftersmaybefabricatedfromcellularbeamsforaestheticreasonsorwhenprovidinglongspans.Where
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transportlimitationsimposerequirementforsplices,theyshouldbecarefullydetailed,topreservethearchitecturalfeatures.Thesectionsusedcannotdevelopplastichingesatacrosssection,soonlyelasticdesignisused.
CellularbeamportalframeHayesgardencentre(ImagecourtesyofASDWestokLtd.)
Designconsiderations
Inthedesignandconstructionofanystructure,alargenumberofinterrelateddesignrequirementsshouldbeconsideredateachstageinthedesignprocess.Thefollowingdiscussionofthedesignprocessanditsconstituentpartsisintendedtogivethedesigneranunderstandingoftheinterrelationshipofthevariouselementsofthestructurewithitsfinalconstruction,sothatthedecisionsrequiredateachstagecanbemadewithanunderstandingoftheirimplications.
Choiceofmaterialandsection
SteelsectionsusedinportalframestructuresareusuallyspecifiedingradeS275orS355steel.
Inplasticallydesignedportalframes,Class1plasticsectionsmustbeusedathingepositionsthatrotate,Class2compactsectionscanbeusedelsewhere.
Framedimensions
Acriticaldecisionattheconceptualdesignstageistheoverallheightandwidthoftheframe,togiveadequateclearinternaldimensionsandadequateclearancefortheinternalfunctionsofthebuilding.
Clearspanandheight
Theclearspanandheightrequiredbytheclientarekeytodeterminingthedimensionstobeusedinthedesign,andshouldbeestablishedearlyinthedesignprocess.Theclientrequirementislikelytobethecleardistancebetweentheflangesofthetwocolumnsthespanwillthereforebelarger,bythesectiondepth.Anyrequirementforbrickworkorblockworkaroundthecolumnsshouldbeestablishedasthismayaffectthedesignspan.
Whereaclearinternalheightisspecified,thiswillusuallybemeasuredfromthefinishedfloorleveltotheundersideofthehaunchorsuspendedceilingifpresent.
Mainframe
Themain(portal)framesaregenerallyfabricatedfromUKBsectionswithasubstantialeaveshaunchsection,
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Dimensionsusedforanalysisandclearinternaldimensions
Typicalhaunchwithrestraints
whichmaybecutfromarolledsectionorfabricatedfromplate.Atypicalframeischaracterisedby:
Aspanbetween15and50mAnclearheight(fromthetopofthefloortotheundersideofthehaunch)between5and12mAroofpitchbetween5and10(6iscommonlyadopted)Aframespacingbetween6and8mHaunchesintheraftersattheeavesandapexAstiffnessratiobetweenthecolumnandraftersectionofapproximately1.5LightgaugepurlinsandsiderailsLightgaugediagonaltiesfromsomepurlinsandsiderailstorestraintheinsideflangeoftheframeatcertainlocations.
Haunchdimensions
Theuseofahaunchattheeavesreducestherequireddepthofrafterbyincreasingthemomentresistanceofthememberwheretheappliedmomentsarehighest.Thehaunchalsoaddsstiffnesstotheframe,reducingdeflections,andfacilitatesanefficientboltedmomentconnection.
Theeaveshaunchistypicallycutfromthesamesizerolledsectionastherafter,oroneslightlylarger,andisweldedtotheundersideoftherafter.Thelengthoftheeaveshaunchisgenerally10%oftheframespan.Thehaunchlengthgenerallymeansthatthehoggingmomentattheendofthehaunchisapproximatelyequaltothelargestsaggingmomentclosetotheapex.Thedepthfromtherafteraxistotheundersideofthehaunchisapproximately2%ofthespan.
Theapexhaunchmaybecutfromarolledsectionoftenfromthesamesizeastherafter,orfabricatedfromplate.Theapexhaunchis
notusuallymodelledintheframeanalysisandisonlyusedtofacilitateaboltedconnection.
Positionsofrestraints
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Generalarrangementofrestraintstotheinsideflange
Duringinitialdesigntheraftermembersarenormallyselectedaccordingtotheircrosssectionalresistancetobendingmomentandaxialforce.Inlaterdesignstagesstabilityagainstbucklingneedstobeverifiedandrestraintspositionedjudiciously.
Thebucklingresistanceislikelytobemoresignificantintheselectionofacolumnsize,asthereisusuallylessfreedomtopositionrailstosuitthedesignrequirementsrailpositionmaybedictatedbydoorsorwindowsintheelevation.
Ifintroducingintermediatelateralrestraintstothecolumnisnotpossible,thebucklingresistancewilldeterminetheinitialsectionsizeselection.Itisthereforeessentialtorecogniseatthisearlystageifthesiderailsmaybeusedtoproviderestrainttothecolumns.Onlycontinuoussiderailsareeffectiveinprovidingrestraint.Siderailsinterruptedby(forexample)rollershutterdoors,cannotbereliedonasprovidingadequaterestraint.
Wherethecompressionflangeoftherafterorcolumnisnotrestrainedbypurlinsandsiderails,restraintcanbeprovidedatspecifiedlocationsbycolumnandrafterstays.
Actions
AdviceonactionscanbefoundinBSEN1991[1],andonthecombinationsofactionsinBSEN1990[2].ItisimportanttorefertotheUKNationalAnnexfortherelevantEurocodepartforthestructurestobeconstructedintheUK.
Permanentactions
Permanentactionsaretheselfweightofthestructure,secondarysteelworkandcladding.Wherepossible,unitweightsofmaterialsshouldbeobtainedfrommanufacturersdata.Whereinformationisnotavailable,thesemaybedeterminedfromthedatainBSEN199111[3].
Serviceloads
Serviceloadswillvarygreatlydependingontheuseofthebuilding.Inportalframesheavypointloadsmayoccurfromsuspendedwalkways,airhandlingunitsetc.Itisnecessarytoconsidercarefullywhereadditionalprovisionisneeded,asparticularitemsofplantmustbetreatedindividually.
Dependingontheuseofthebuildingandwhethersprinklersarerequired,itisnormaltoassumeaserviceloadingof0.10.25kN/m2onplanoverthewholeroofarea.
Variableactions
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ImposedloadsonroofsRoofslope, qk(kN/m)
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Gantrygirderscarryinganoverheadtravellingcrane
Collapsemechanismofaportalwithaleantounderfire,boundaryconditionongridlines2and3.
Driftedsnow,determinedusingAnnexBofBSEN199113[5]TheopeningofadominantopeningwhichwasassumedtobeshutatULS
Eachprojectshouldbeindividuallyassessedwhetheranyotheraccidentalactionsarelikelytoactonthestructure.
Robustness
Robustnessrequirementsaredesignedtoensurethatanystructuralcollapseisnotdisproportionatetothecause.BSEN1990[2]setstherequirementtodesignandconstructrobustbuildingsinordertoavoiddisproportionatecollapseunderaccidentaldesignsituations.BSEN199117[9]givesdetailsofhowthisrequirementshouldbemet.
FormanyportalframestructuresnospecialprovisionsareneededtosatisfyrobustnessrequirementssetbytheEurocode.
FormoreinformationonrobustnessrefertoSCIP391.
Fire
IntheUnitedKingdom,structuralsteelinsinglestoreybuildingsdoesnotnormallyrequirefireresistance.Themostcommonsituationinwhichitisrequiredtofireprotectthestructuralsteelworkiswherepreventionoffirespreadtoadjacentbuildings,aboundarycondition,isrequired.Thereareasmallnumberofother,rare,instances,forexamplewhendemandedbyaninsuranceprovider,wherestructuralfireprotectionmayberequired.
Whenaportalframeisclosetotheboundary,thereareseveralrequirementsaimedatstoppingfirespreadbykeepingtheboundaryintact:
TheuseoffireresistantcladdingApplicationoffireprotectionofthesteeluptotheundersideofthehaunchTheprovisionofamomentresistingbase(asitisassumedthatinthefireconditionraftersgointocatenary)
ComprehensiveadviceisavailableinSCIP313.
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Bendingmomentdiagramresultingfromtheplasticanalysisofasymmetricalportalframeundersymmetricalloading
Combinationsofactions
BSEN1990[2]givesrulesforestablishingcombinationsofactions,withthevaluesofrelevantfactorsgivenintheUKNationalAnnex[10].BSEN1990[2]coversbothultimatelimitstate(ULS)andserviceabilitylimitstate(SLS),althoughfortheSLS,onwardreferenceismadetothematerialcodes(forexampleBSEN199311[11]forsteelwork)toidentifywhichexpressionshouldbeusedandwhatSLSlimitsshouldbeobserved.
Allcombinationsofactionsthatcanoccurtogethershouldbeconsidered,howeverifcertainactionscannotbeappliedsimultaneously,theyshouldnotbecombined.
GuidanceontheapplicationofEurocoderulesoncombinationsofactionscanbefoundinSCIP362and,specificallyforportalframes,inSCIP400.
FrameanalysisatULS
Attheultimatelimitstate(ULS),themethodsofframeanalysisfallbroadlyintotwotypes:elasticanalysisandplasticanalysis.
Plasticanalysis
Thetermplasticanalysisisusedtocoverbothrigidplasticandelasticplasticanalysis.Plasticanalysiscommonlyresultsinamoreeconomicalframebecauseitallowsrelativelylargeredistributionofbendingmomentsthroughouttheframe,duetoplastichingerotations.TheseplastichingerotationsoccuratsectionswherethebendingmomentreachestheplasticmomentorresistanceofthecrosssectionatloadsbelowthefullULSloading.
Therotationsarenormallyconsideredtobelocalisedatplastichingesandallowthecapacityofunderutilisedpartsoftheframetobemobilised.ForthisreasonmemberswhereplastichingesmayoccurneedtobeClass1sections,whicharecapableofaccommodatingrotations.
Thefigureshowstypicalpositionswhereplastichingesforminaportalframe.Twohingesleadtoacollapse,butintheillustratedexample,duetosymmetry,designersneedtoconsiderallpossiblehingelocations.
Elasticanalysis
Atypicalbendingmomentdiagramresultingfromanelasticanalysisofaframewithpinnedbasesisshownthefigurebelow.Inthiscase,themaximummoment(attheeaves)ishigherthanthatcalculatedfromaplasticanalysis.Boththecolumnandhaunchhavetobedesignedfortheselargebendingmoments.
Wheredeflections(SLS)governdesign,theremaybenoadvantageinusingplasticanalysisfortheULS.Ifstiffersectionsareselectedinordertocontroldeflections,itisquitepossiblethatnoplastichingesformandthe
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frameremainselasticatULS.
Bendingmomentdiagramresultingfromtheelasticanalysisofasymmetricalportalframeundersymmetricalloading
Portalframeanalysissoftware(FastrakmodelcourtesyofCSC)
Inplaneframestability
Whenanyframeisloaded,itdeflectsanditsshapeunderloadisdifferentfromtheundeformedshape.Thedeflectionhasanumberofeffects:
Theverticalloadsareeccentrictothebases,whichleadstofurtherdeflectionTheapexdrops,reducingthearchingactionAppliedmomentscurvemembersAxialcompressionincurvedmemberscausesincreasedcurvature(whichmaybeperceivedasareducedstiffness.)
Takentogether,theseeffectsmeanthataframeislessstable(nearercollapse)thanafirstorderanalysissuggests.Theobjectiveofassessingframestabilityistodetermineifthedifferenceissignificant.
Secondordereffects
Thegeometricaleffectsdescribedabovearesecondordereffectsandshouldnotbeconfusedwithnonlinearbehaviourofmaterials.Asshowninthefiguretherearetwocategoriesofsecondordereffects:
Effectsofdisplacementsoftheintersectionsofmembers,usuallycalledPeffects.BSEN199311[11]describesthisastheeffectofdeformedgeometry.Effectsofdeflectionswithinthelengthofmembers,usuallycalledPeffects.
Secondorderanalysisisthetermusedtodescribeanalysismethodsinwhichtheeffectsofincreasingdeflectionunderincreasingloadisconsideredexplicitlyinthesolution,sothattheresultsincludethePandPeffects.
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PandPeffectsinaportalframe
Firstorderandsecondorderanalysis
Foreitherplasticanalysisofframes,orelasticanalysisofframes,thechoiceoffirstorderanalysisorsecondorderanalysisdependsontheinplaneflexibilityoftheframe,characterisedbythecalculationofthecrfactor.
Calculationofcr
Theeffectsofthedeformedgeometry(Peffects)areassessedinBSEN199311[11]bycalculatingthefactorcr,definedas:
where:
Fcristheelasticcriticalbucklingloadforglobalinstabilitymode,basedoninitialelasticstiffnesses
FEdisthedesignloadonthestructure.
crmaybefoundusingsoftwareorusinganapproximation(expression5.2fromBSEN199311[11])aslongastheframemeetscertaingeometriclimitsandtheaxialforceintherafterisnotsignificant.RulesaregivenintheEurocodetoidentifywhentheaxialforceissignificant.Whentheframefallsoutsidethespecifiedlimits,asisthecaseforverymanyorthodoxframes,thesimplifiedexpressioncannotbeused.Inthesecircumstances,analternativeexpressionmaybeusedtocalculateanapproximatevalueofcr,referredtoascr,est.FurtherdetailsaregiveninSCIP397.
Sensitivitytoeffectsofthedeformedgeometry
ThelimitationstotheuseoffirstorderanalysisaredefinedinBSEN199311[11],Section5.2.1(3)andtheUK
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NationalAnnex[12]SectionNA.2.9as:
Forelasticanalysis:cr10
Forplasticanalysis:
cr5forcombinationswithgravityloadingwithframeimperfections,
providedthat:a)thespan,L,doesnotexceed5timesthemeanheightofthecolumns
b)hrsatisfiesthecriterion:(hr/sa)2+(hr/sb)20.5inwhichsaandsbarethehorizontaldistancesfromtheapextothecolumns.Forasymmetricalframethisexpressionsimplifiestohr0.25L.
cr10forcombinationswithgravityloadingwithframeimperfectionsforcladstructuresprovidedthatthestiffeningeffectsofmasonryinfillwallpanelsordiaphragmsofprofiledsteelsheetingarenottakenintoaccount
Design
Oncetheanalysishasbeencompleted,allowingforsecondordereffectsifnecessary,theframemembersmustbeverified.
Boththecrosssectionalresistanceandthebucklingresistanceofthemembersmustbeverified.Inplanebucklingofmembers(usingexpression6.61ofBSEN199311[11])neednotbeverifiedastheglobalanalysisisconsideredtoaccountforallsignificantinplaneeffects.SCIP400identifiesthelikelycriticalzonesformemberverification.SCIP397containsnumericalexamplesofmemberverifications.
Crosssectionresistance
Memberbending,axialandshearresistancesmustbeverified.Iftheshearoraxialforceishigh,thebendingresistanceisreducedsocombinedshearforceandbendingandaxialforceandbendingresistancesneedtobeverified.Intypicalportalframesneithertheshearforcenortheaxialloadissufficientlyhightoreducethebendingresistance.Whentheportalframeformsthechordofthebracingsystem,theaxialloadintheraftermaybesignificant,andthiscombinationofactionsshouldbeverified.
Althoughallcrosssectionsneedtobeverified,thelikelykeypointsareatthepositionsofmaximumbendingmoment:
InthecolumnattheundersideofthehaunchIntherafteratthesharpendofthehaunchIntherafteratthemaximumsagginglocationadjacenttotheapex.
Memberstability
Thefigureshowsadiagrammaticrepresentationoftheissuesthatneedtobeaddressedwhenconsideringthe
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Diagrammaticrepresentationofaportalframerafter
stabilityofamemberwithinaportalframe,inthisexamplearafterbetweentheeavesandapex.Thefollowingpointsshouldbenoted:
Purlinsprovideintermediatelateralrestrainttooneflange.DependingonthebendingmomentdiagramthismaybeeitherthetensionorcompressionflangeRestraintstotheinsideflangecanbeprovidedatpurlinpositions,producingatorsionalrestraintatthatlocation.
Inplane,nomemberbucklingchecksarerequired,astheglobalanalysishasaccountedforallsignificantinplaneeffects.Theanalysishasaccountedforanysignificantsecondordereffects,andframeimperfectionsareusuallyaccountedforbyincludingtheequivalenthorizontalforceintheanalysis.Theeffectsofinplanememberimperfectionsaresmallenoughtobeignored.
Becausetherearenominoraxismomentsinaportalframerafter,Expression6.62simplifiesto:
Rafterdesignandstability
Intheplaneoftheframeraftersaresubjecttohighbendingmoments,whichvaryfromamaximumhoggingmomentatthejunctionwiththecolumntoaminimumsaggingmomentclosetotheapex.Compressionisintroducedintheraftersduetoactionsappliedtotheframe.Theraftersarenotsubjecttoanyminoraxismoments.Optimumdesignofportalframeraftersisgenerallyachievedbyuseof:
AcrosssectionwithahighratioofIyytoIzzthatcomplieswiththerequirementsofClass1or2under
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Typicalpurlinandrafterstayarrangementforthegravitycombinationofactions
combinedmajoraxisbendingandaxialcompression.Ahaunchthatextendsfromthecolumnforapproximately10%oftheframespan.Thiswillgenerallymeanthatthemaximumhoggingandsaggingmomentsintheplainrafterlengthareofsimilarmagnitude.
Outofplanestability
Purlinsattachedtothetopflangeoftherafterprovidestabilitytothememberinanumberofways:
Directlateralrestraint,whentheouterflangeisincompressionIntermediatelateralrestrainttothetensionflangebetweentorsionalrestraints,whentheouterflangeisintensionTorsionalandlateralrestrainttotherafterwhenthepurlinisattachedtothetensionflangeandusedinconjunctionwithrafterstaystothecompressionflange.
Initially,theoutofplanechecksarecompletedtoensurethattherestraintsarelocatedatappropriatepositionsandspacing.
Gravitycombinationofactions
Thefigureshowsatypicalmomentdistributionforthegravitycombinationofactions,typicalpurlinandrestraintpositionsaswellasstabilityzones,whicharereferredtofurther.
Purlinsaregenerallyplacedatupto1.8mspacingbutthisspacingmayneedtobereducedinthehighmomentregionsneartheeaves.
InZoneA,thebottomflangeofthehaunchisincompression.Thestabilitychecksarecomplicatedbythevariationingeometryalongthehaunch.ThebottomflangeispartiallyorwhollyincompressionoverthelengthofZoneB.InZoneC,thepurlinsprovidelateralrestrainttothetop(compression)flange.
Theselectionoftheappropriatecheckdependsonthepresenceofaplastichinge,theshapeofthebendingmomentdiagramandthegeometryofthesection(threeflangesortwoflanges).Theobjectiveofthechecksistoprovidesufficientrestraintstoensuretherafterisstableoutofplane.
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Typicalpurlinandrafterstayarrangementfortheupliftcondition
GuidanceondetailsoftheoutofplanestabilityverificationcanbefoundinSCIP397.
Theupliftcondition
Intheupliftconditionthetopflangeofthehaunchwillbeincompressionandwillberestrainedbythepurlins.Themomentsandaxialforcesaresmallerthanthoseinthegravityloadcombination.Asthehaunchisstableinthegravitycombinationofactions,itwillcertainlybesointheupliftcondition,beingrestrainedatleastaswell,andunderreducedloads
InZoneF,thepurlinswillnotrestrainthebottomflange,whichisincompression.
Theraftermustbeverifiedbetweentorsionalrestraints.Atorsionalrestraintwillgenerallybeprovidedadjacenttotheapex.Theraftermaybestablebetween
thispointandthevirtualrestraintatthepointofcontraflexure,asthemomentsaregenerallymodestintheupliftcombination.Iftherafterisnotstableoverthislength,additionaltorsionalrestraintsshouldbeintroduced,andeachlengthoftherafterverified.
Inplanestability
Noinplanechecksofraftersarerequired,asallsignificantinplaneeffectshavebeenaccountedforintheglobalanalysis.
Columndesignandstability
Themostheavilyloadedregionoftherafterisreinforcedbythehaunch.Bycontrast,thecolumnissubjecttoasimilarbendingmomentattheundersideofthehaunch,butwithoutanyadditionalstrengthening.
Theoptimumdesignformostcolumnsisusuallyachievedbytheuseof:
AcrosssectionwithahighratioofIyytoIzzthatcomplieswithClass1orClass2undercombinedmajoraxisbendingandaxialcompressionAplasticsectionmodulusthatisapproximately50%greaterthanthatoftherafter.
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Typicalportalframecolumnwithplastichingeatundersideofhaunch
Thecolumnsizewillgenerallybedeterminedatthepreliminarydesignstageonthebasisoftherequiredbendingandcompressionresistances.
Whethertheframeisdesignedplasticallyorelastically,atorsionalrestraintshouldalwaysbeprovidedattheundersideofthehaunch.Thismaybefromasiderailpositionedatthatlevel,orbysomeothermeans.Additionaltorsionalrestraintsmayberequiredbetweentheundersideofthehaunchandthecolumnbasebecausethesiderailsareattachedtothe(outer)tensionflangeunlessrestraintsareprovidedtheinnercompressionflangeisunrestrained.Asiderailthatisnotcontinuous(forexample,interruptedbyindustrialdoors)cannotbereliedupontoprovideadequaterestraint.Thecolumnsectionmayneedtobeincreasedifintermediaterestraintstothecompressionflangecannotbeprovided.
Thepresenceofaplastichingewilldependonloading,geometryandchoiceofcolumnandraftersections.Inasimilarwaytotherafter,outofplanestabilitymustbeverified.
Outofplanestability
Ifthereisaplastichingeattheundersideofthehaunch,thedistancetotheadjacenttorsionalrestraintmustbelessthanthelimitingdistanceLmasgivenbyBSEN199311[11]ClauseBB.3.1.1.
Itmaybepossibletodemonstratethatatorsionalrestraintisnotrequiredatthesiderailimmediatelyadjacenttothehinge,butmaybeprovidedatsomegreaterdistance.Inthiscasetherewillbeintermediatelateralrestraintsbetweenthetorsionalrestraints
Ifthestabilitybetweentorsionalrestraintscannotbeverified,itmaybenecessarytointroduceadditionaltorsionalrestraints.Ifitisnotpossibletoprovideadditionalintermediaterestraints,thesizeofthemembermustbeincreased.
Inallcases,alateralrestraintmustbeprovidedwithinLmofaplastichinge.
Whentheframeissubjecttouplift,thecolumnmomentwillreverse.Thebendingmomentswillgenerallybesignificantlysmallerthanthoseundergravityloadingcombinations,andthecolumnislikelytoremainelastic
Inplanestability
Noinplanechecksofcolumnsarerequired,asallsignificantinplaneeffectshavebeenaccountedforintheglobalanalysis.
Bracing
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Bracinginaportalframe(ImagecourtesyofWilliamHaleyEngineeringLtd.)
Bracingisrequiredtoresistlongitudinalactionsduetowindandcranes,andtoproviderestrainttomembers.
Itiscommontousehollowsectionsasbracingmembers.
Bracingarrangementinatypicalportalframe
Verticalbracing
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Commonbracingsystems
Theprimaryfunctionsofverticalbracinginthesidewallsoftheframeare:
Totransmitthehorizontalloadstotheground.ThehorizontalforcesincludeforcesfromwindandcranesToprovidearigidframeworktowhichsiderailsandcladdingmaybeattachedsothattherailscaninturnprovidestabilitytothecolumnsToprovidetemporarystabilityduringerection.
Thebracingmaybelocated:
AtoneorbothendsofthebuildingWithinthelengthofthebuildingIneachportionbetweenexpansionjoints(wheretheseoccur).
Wherethesidewallbracingisnotinthesamebayastheplanbracingintheroof,aneavesstrutisessentialtotransmittheforcesfromtheroofbracingintothewallbracing.Aneavesstrutisalsorequired:
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Longitudinalstabilityusingportalisedbays
Additionalbracingintheplaneofthecranegirder
ToensurethetopsofthecolumnsareadequatelyrestrainedinpositionToassistinduringtheconstructionofthestructureTostabilisethetopsofthecolumnsifafireboundaryconditionexists
Portalisedbays
Whereitisdifficultorimpossibletobracetheframeverticallybyconventionalbracing,itisnecessarytointroducemomentresistingframesintheelevationsinoneormorebays.
Inadditiontothegeneralserviceabilitylimitondeflectionofh/300,wherehistheheightoftheportalisedbayitissuggestedthat:
Thebendingresistanceoftheportalisedbay(notthemainportalframe)ischeckedusinganelasticframeanalysisDeflectionundertheequivalenthorizontalforcesisrestrictedtoh/1000,wheretheequivalenthorizontalforcesarecalculatedbasedonthewholeoftheroofarea.
Bracingtorestrainlongitudinalloadsfromcranes
Ifacraneisdirectlysupportedbytheframe,thelongitudinalsurgeforcewillbeeccentrictothecolumnandwilltendtocausethecolumntotwist,unlessadditionalrestraintisprovided.Ahorizontaltrussatthelevelofthecranegirdertopflangeor,forlightercranes,ahorizontalmemberontheinsidefaceofthecolumnflangetiedintotheverticalbracingmaybeadequatetoprovidethenecessaryrestraint.
Forlargehorizontalforces,additionalbracingshouldbeprovidedintheplaneofthecranegirder.
Planbracing
Planbracingislocatedintheplaneoftheroof.Theprimaryfunctionsoftheplanbracingare:
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Planviewshowingbothendbaysbraced
TotransmitwindforcesfromthegablepoststotheverticalbracinginthewallsTotransmitanyfrictionaldragforcesfromwindontherooftotheverticalbracingToprovidestabilityduringerectionToprovideastiffanchorageforthepurlinswhichareusedtorestraintherafters.
Inordertotransmitthewindforcesefficiently,theplanbracingshouldconnecttothetopofthegableposts.
Restrainttoinnerflanges
Restrainttotheinnerflangesofraftersorcolumnsisoftenmostconvenientlyformedbydiagonalstrutsfromthepurlinsorsheetingrailstosmallplatesweldedtotheinnerflangeandweb.Pressedsteelflattiesarecommonlyused.Whererestraintisonlypossiblefromoneside,therestraintmustbeabletocarrycompression.Intheselocationsanglesectionsofminimumsize4040mmmustbeused.Thestayanditsconnectionsshouldbedesignedtoresistaforceequalto2.5%ofthemaximumforceinthecolumnorraftercompressionflangebetweenadjacentrestraints.
Connections
Themajorconnectionsinaportalframearetheeavesandapexconnections,whicharebothmomentresisting.Theeavesconnectioninparticularmustgenerallycarryaverylargebendingmoment.Boththeeavesandapexconnectionsarelikelytoexperiencereversalincertaincombinationsofactionsandthiscanbeanimportantdesigncase.Foreconomy,connectionsshouldbearrangedtominimiseanyrequirementforadditionalreinforcement(commonlycalledstiffeners).Thisisgenerallyachievedby:
Makingthehaunchdeeper(increasingtheleverarms)Extendingtheeavesconnectionabovethetopflangeoftherafter(anadditionalboltrow)AddingboltrowsSelectingastrongercolumnsection.
ThedesignofmomentresistingconnectionsiscoveredindetailinSCIP398.
Typicalportalframeconnections
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EavesconnectionApexconnection
Haunchedconnections
Columnbases
Inthemajorityofcases,anominallypinnedbaseisprovided,becauseofthedifficultyandexpenseofprovidingarigidbase.Arigidbasewillinvolveamoreexpensivebasedetail,butmoresignificantly,thefoundationmustalsoresistthemoment,whichincreasescostssignificantlycomparedtoanominallypinnedbase.
Ifacolumnbaseisnominallypinned,itisrecommendedthatthebasebemodelledasperfectlypinnedwhenusingelasticglobalanalysistocalculatethemomentsandforcesintheframeunderULSloading.
Thestiffnessofthebasemaybeassumedtobeequaltothefollowingproportionofthecolumnstiffness:
10%whenassessingframestability20%whencalculatingdeflectionsunderserviceabilityloads.
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Typicalnominallypinnedbase
References
1. ^BSEN1991,Eurocode1:Actionsonstructures,BSI2. ^2.02.12.22.3BSEN1990:2002,EurocodeBasisofstructuraldesign,BSI3. ^BSEN199111:2002Eurocode1:Actionsonstructures.Generalactions.Densities,selfweight,
imposedloadsforbuildings,BSI4. ^NAtoBSEN199111:2002,UKNationalAnnextoEurocode1.Actionsonstructures.General
actions.Densities,selfweight,imposedloadsforbuildings,BSI5. ^5.05.1BSEN199113:2003Eurocode1.Actionsonstructures.Generalactions.Snowloads,BSI6. ^NAtoBSEN199113:2003,UKNationalAnnextoEurocode1.Actionsonstructures.General
actions.Snowloads,BSI7. ^BSEN199114:2005+A1:2010Eurocode1.Actionsonstructures.Generalactions.Windactions,BSI8. ^NAtoBSEN199114:2005+A1:2010UKNationalAnnextoEurocode1.Actionsonstructures.
Generalactions.Windactions,BSI9. ^BSEN199117:2006Eurocode1.Actionsonstructures.Generalactions.Accidentalactions,BSI
10. ^NAtoBSEN1990:2002+A1:2005UKNationalAnnexforEurocode.Basisofstructuraldesign,BSI11. ^11.011.111.211.311.411.511.6BSEN199311:2005,Eurocode3:Designofsteelstructures.General
rulesandrulesforbuildings,BSI12. ^NAtoBSEN199311:2005,UKNationalAnnextoEurocode3:Designofsteelstructures.General
rulesandrulesforbuildings,BSI
Furtherreading
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SteelDesigners'Manual7thEdition.(http://shop.steelsci.com/products/231steeldesignersmanual7thedition.aspx)EditorsBDavison&GWOwens.TheSteelConstructionInstitute2012,Chapters3and4
Resources
SCIP292InplaneStabilityofPortalFramestoBS59501:2000,2001SCIP281DesignofCurvedSteel,2001SCIP391StructuralRobustnessofSteelFramedBuildings,SCI,2001SCIP362SteelBuildingDesign:ConciseEurocodes,2009SCIP394WindActionstoBSEN199114,SCI,2013SCIP397ElasticDesignofSinglespanSteelPortalFrameBuildingstoEurocode3,2013SCIP398JointsinSteelConstruction:MomentresistingJointstoEurocode3,2013SCIP313SingleStoreySteelFramedBuildingsinFireBoundaryConditions,2002SCIP400Interimreport:DesignofportalframestoEurocode3:AnoverviewforUKdesigners,2013
Seealso
ThermalperformanceIntroductiontoacousticsSteelworkspecificationSteelconstructionproductsDesigncodesandstandardsMemberdesignConceptdesignFabricationBracedframesAllowingfortheeffectsofdeformedframegeometryModellingandanalysisStructuralrobustnessStructuralfireresistancerequirementsSinglestoreybuildingsinfireboundaryconditionsMomentresistingconnectionsContinuousframesSinglestoreyindustrialbuildingsRetailbuildingsBuildingenvelopesDesignsoftwareandtools
ExternallinksCSC(http://www.cscworld.com/Regional/UK.aspx)
CPD
Analysisanddesignofportalframes
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