Portal Frames - Steelconstruction

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PortalframesFromSteelconstruction.info

Portalframesaregenerallylowrisestructures,comprisingcolumnsandhorizontalorpitchedrafters,connectedbymomentresistingconnections.Resistancetolateralandverticalactionsisprovidedbytherigidityoftheconnectionsandthebendingstiffnessofthemembers,whichisincreasedbyasuitablehaunchordeepeningoftheraftersections.Thisformofcontinuousframestructureisstableinitsplaneandprovidesaclearspanthatisunobstructedbybracing.Portalframesareverycommon,infact50%ofconstructionalsteelusedintheUKisinportalframeconstruction.Theyareveryefficientforenclosinglargevolumes,thereforetheyareoftenusedforindustrial,storage,retailandcommercialapplicationsaswellasforagriculturalpurposes.Thisarticledescribestheanatomyandvarioustypesofportalframeandkeydesignconsiderations.

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

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

Crosssectionshowingaportalframeanditsrestraints

towhichitisattached.

Portalframedstructuresoverview

Typesofportalframes

Manydifferentformsofportalframesmaybeconstructed.Frametypesdescribedbelowgiveanoverviewoftypesofportalconstructionwithtypicalfeaturesillustrated.Thisinformationonlyprovidestypicaldetailsandisnotmeanttodictateanylimitsontheuseofanyparticularstructuralform.

Pitchedroofsymmetricportalframe

GenerallyfabricatedfromUKBsectionswithasubstantialeaveshaunchsection,whichmaybecutfromarolledsectionorfabricatedfromplate.25to35marethemostefficient

PitchedroofsymmetricportalframeLancashireWasteDevelopment

spans.

Portalframewithinternalmezzaninefloor

Officeaccommodationisoftenprovidedwithinaportalframestructureusingapartialwidthmezzaninefloor.TheassessmentofframestabilitymustincludetheeffectofthemezzanineguidanceisgiveninSCIP292.

PortalframewithinternalmezzaninefloorWatersMeetingHealthCentre,Bolton(ImagecourtesyBDStructuresLtd.andASDWestokLtd.)

Craneportalframewithcolumnbrackets

Whereatravellingcraneofrelativelylowcapacity(uptosay20tonnes)isrequired,bracketscanbefixedtothecolumnstosupportthecranerails.Useofatiememberorrigidcolumnbasesmaybenecessarytoreducetheeavesdeflection.Thespreadoftheframeatcraneraillevelmaybeofcriticalimportancetothefunctioningofthecranerequirementsshouldbeagreedwiththeclientandwiththecranemanufacturer.

Tiedportalframe

Inatiedportalframethehorizontalmovementofthe

eavesandthebendingmomentsinthecolumnsandraftersarereduced.Atiemaybeusefultolimitspreadinacranesupportingstructure.Thehighaxialforcesintroducedintheframewhenatieisusednecessitatetheuseofsecondordersoftwarewhenanalysingthisformofframe.

Monopitchportalframe

Amonopitchportalframeisusuallychosenforsmallspansorbecauseofitsproximitytootherbuildings.Itisasimplevariationofthepitchedroofportalframe,andtendstobeusedforsmallerbuildings(upto15mspan).

Proppedportalframe

Wherethespanofaportalframeislargeandthereisnorequirementtoprovideaclearspan,aproppedportalframecanbeusedtoreducetheraftersizeandalsothehorizontalshearatthefoundations.

ProppedportalframeRebottlingPlant,Hemswell(ImagecourtesyofMetsecplc)

Mansardportalframe

Amansardportal

framemaybeusedwherealargeclearheightatmidspanisrequiredbuttheeavesheightofthebuildinghastobeminimised.

Curvedrafterportalframe

Portalframesmaybeconstructedusingcurvedrafters,mainlyforarchitecturalreasons.Becauseoftransportlimitationsrafterslongerthan20mmayrequiresplices,whichshouldbecarefullydetailedforarchitecturalreasons.Thecurvedmemberisoftenmodelledforanalysisasaseriesofstraightelements.GuidanceonthestabilityofcurvedraftersinportalframesisgiveninSCIP281.Alternatively,theraftercanbefabricatedasaseriesofstraightelements.Itwillbenecessarytoprovidepurlincleatsofvaryingheighttoachievethecurvedexternalprofile.

Cellularbeamportalframe

Raftersmaybefabricatedfromcellularbeamsforaestheticreasonsorwhenprovidinglongspans.Where

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,

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

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

ImposedloadsonroofsRoofslope, qk(kN/m)

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.

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

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.

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

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

Diagrammaticrepresentationofaportalframerafter

stabilityofamemberwithinaportalframe,inthisexamplearafterbetweentheeavesandapex.Thefollowingpointsshouldbenoted:

Purlinsprovideintermediatelateralrestrainttooneflange.DependingonthebendingmomentdiagramthismaybeeitherthetensionorcompressionflangeRestraintstotheinsideflangecanbeprovidedatpurlinpositions,producingatorsionalrestraintatthatlocation.

Inplane,nomemberbucklingchecksarerequired,astheglobalanalysishasaccountedforallsignificantinplaneeffects.Theanalysishasaccountedforanysignificantsecondordereffects,andframeimperfectionsareusuallyaccountedforbyincludingtheequivalenthorizontalforceintheanalysis.Theeffectsofinplanememberimperfectionsaresmallenoughtobeignored.

Becausetherearenominoraxismomentsinaportalframerafter,Expression6.62simplifiesto:

Rafterdesignandstability

Intheplaneoftheframeraftersaresubjecttohighbendingmoments,whichvaryfromamaximumhoggingmomentatthejunctionwiththecolumntoaminimumsaggingmomentclosetotheapex.Compressionisintroducedintheraftersduetoactionsappliedtotheframe.Theraftersarenotsubjecttoanyminoraxismoments.Optimumdesignofportalframeraftersisgenerallyachievedbyuseof:

AcrosssectionwithahighratioofIyytoIzzthatcomplieswiththerequirementsofClass1or2under

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.

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.

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

Bracinginaportalframe(ImagecourtesyofWilliamHaleyEngineeringLtd.)

Bracingisrequiredtoresistlongitudinalactionsduetowindandcranes,andtoproviderestrainttomembers.

Itiscommontousehollowsectionsasbracingmembers.

Bracingarrangementinatypicalportalframe

Verticalbracing

Commonbracingsystems

Theprimaryfunctionsofverticalbracinginthesidewallsoftheframeare:

Totransmitthehorizontalloadstotheground.ThehorizontalforcesincludeforcesfromwindandcranesToprovidearigidframeworktowhichsiderailsandcladdingmaybeattachedsothattherailscaninturnprovidestabilitytothecolumnsToprovidetemporarystabilityduringerection.

Thebracingmaybelocated:

AtoneorbothendsofthebuildingWithinthelengthofthebuildingIneachportionbetweenexpansionjoints(wheretheseoccur).

Wherethesidewallbracingisnotinthesamebayastheplanbracingintheroof,aneavesstrutisessentialtotransmittheforcesfromtheroofbracingintothewallbracing.Aneavesstrutisalsorequired:

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:

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

EavesconnectionApexconnection

Haunchedconnections

Columnbases

Inthemajorityofcases,anominallypinnedbaseisprovided,becauseofthedifficultyandexpenseofprovidingarigidbase.Arigidbasewillinvolveamoreexpensivebasedetail,butmoresignificantly,thefoundationmustalsoresistthemoment,whichincreasescostssignificantlycomparedtoanominallypinnedbase.

Ifacolumnbaseisnominallypinned,itisrecommendedthatthebasebemodelledasperfectlypinnedwhenusingelasticglobalanalysistocalculatethemomentsandforcesintheframeunderULSloading.

Thestiffnessofthebasemaybeassumedtobeequaltothefollowingproportionofthecolumnstiffness:

10%whenassessingframestability20%whencalculatingdeflectionsunderserviceabilityloads.

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

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

Retrievedfrom"http://www.steelconstruction.info/Portal_frames"Category:Design

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