欧标 EC4 教程06.docx
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欧标 EC4 教程06.docx
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欧标EC4教程06
StructuralSteelworkEurocodes
Developmentof
aTrans-NationalApproach
Course:
Eurocode4
Lecture6:
Simplysupportedbeams
Summary:
∙Theproceduresfordeterminingthesectionclassificationaresimilartothoseforbaresteelsections,althoughsomemodificationsmaybemade.
Themomentresistanceofclass1and2sectionsiscalculatedusingplasticanalysis,thedetailsdependingontheneutralaxisposition.
Themomentresistanceofclass3sectionsiscalculatedusingelasticanalysis,withdueaccountforcreepandspecialconsiderationofbuildingsusedmainlyforstorage.
Theverticalshearstrengthisbasedonthatofthebaresteelsection.
∙Thedetailsofthelongitudinalshearconnection(numberandtypeofconnector,andslabreinforcement)aredeterminedonthebasisofthelongitudinalforcetransmittedbetweenthesteelsectionandconcreteslab.
∙Whereinsufficientconnectorsareprovided,thebeammaybedesignedonthebasisofpartialinteraction,themomentresistancecalculatedonthebasisofthelongitudinalforcetransmittedbetweenthesteelsectionandconcreteslab.
∙DeflectionlimitsareasstatedinEC3forbaresteelsections.
∙Concretecrackingcanbecontrolledbyensuringaminimumamountofslabreinforcementandlimitingbarsizeandspacing.
Pre-requisites:
Lecture1:
IntroductiontoCompositeConstruction-forthoseunfamiliarwithcompositeconstruction
Lecture2:
IntroductiontoEC4-outlinestheconceptofthecode,andexplainssomeoftheprincipalterminologyandnotation.
Lecture3:
StructuralModelling-providesessentialcoverageoftechnicalissuessuchaseffectivewidth,sectionclassificationinrelationtoglobalanalysis,andanalysisforserviceability.
Notesfortutors:
Thismaterialcomprisesone60minutelecture.
Objectives:
∙Tooutlinethedesigncheckswhicharenecessaryforbothultimateandserviceabilitylimitstates.
Todescribetheproceduresfordeterminingthesectionclassification,andthemodificationswhichmaybemade.
Toexplaintheproceduresforcalculatingtheplasticmomentresistanceofclass1and2sectionsinrelationtoneutralaxisposition.
Toexplaintheproceduresforcalculatingtheelasticmomentresistanceofclass3sectionsinrelationtothemethodofconstruction,withspecialconsiderationofbuildingsusedmainlyforstorage.
Todescribethesimplifiedproceduresforcheckingtheverticalshearstrengthofcompositebeams.
∙Toexplainhowthelongitudinalshearconnectionisdesigned.
∙Tointroducetheconceptofpartialinteraction,anddescribehowitaffectsthecalculatedmomentofresistance.
∙Tooutlinetherequirementsforcontrollingdeflectionsandconcretecrackingattheserviceabilitylimitstate.
References:
∙EC4:
EN1994-1-1:
Eurocode4(Draft2):
DesignofCompositeSteelandConcreteStructuresPart1.1:
Generalrulesandrulesforbuildings.
Contents:
1.Introduction
2.UltimateLimitStateDesign
2.1Designchecks
2.2Sectionclassificationofcompositebeams
2.2.1Classificationofcompressionflange
2.2.2Classificationofweb
2.3PlasticMomentresistanceofClass1or2Sections
2.3.1Plasticneutralaxislocatedintheslabdepth
2.3.2Plasticneutralaxisintheflangeofthesteelbeam
2.3.3Plasticneutralaxisinthewebofthesteelbeam
2.4Elasticmomentresistance(Class3Sections)
2.4.1Proppedcompositebeamsinbuildingsingeneral
2.4.2Proppedcompositebeamsinstoragebuildings
2.4.3Unproppedcompositebeamsinbuildingsingeneral
2.4.4Unproppedcompositebeamsinstoragebuildings
2.5ShearResistance-effectonthemomentofresistance
3.Designofconnectorsforsimplysupportedbeamsclass1or2
3.1Fullinteraction
3.2Partialinteraction
3.2.1Ductileconnectors
3.2.2Partialcompositedesignofbeams
3.3Morecomplexloadcases
4.Connectordesignforclass3or4beams
5.Transversereinforcement
6.ServiceabilityLimitState
6.1General
6.2Deflections
6.3Concretecracking
7.ConcludingSummary
1.Introduction
Acompositebeamisformedbyareinforced(possiblyprestressed)concreteslabattachedtotheupperflangeofahot-rolledorweldedsteelbeambyshearconnectorssothatthetwocomponentsacttogetherasasinglesection.
Aswithsteelbeams,compositebeamsmustbecheckedforbothultimatelimitstateandserviceabilitylimitstate.Thislecturecoverstheprincipalcheckstobeappliedtosimplysupportedbeams.Thisincludescalculationproceduresforthemomentresistance,whichdependonthesectionclassificationandpositionoftheneutralaxis.Thetreatmentofelasticmomentresistancedependsontheconstructionsequenceandwhetherthebuildingisintendedmainlyforstorage,inwhichcasetheloadingispredominantlylongterm.Designchecksforverticalsheararesimilartothoseforbaresteelbeams.Thedesignoftheconnectorsisdiscussedinrelationfullandpartialinteraction,andtherequirementsfortransversereinforcementaredescribed.Serviceabilitydesignisbasedonelasticanalysisandconcernslimitingdeflectionsandcontrollingcrackingintheconcrete.Thecalculationproceduresfortheseareoutlined.
2.Ultimatelimitstatedesign
2.1Designchecks
Theultimatelimitstatedesignchecksforsimplysupportedbeamsrelatetobendingstrength,thestrengthofthelongitudinalshearconnection,thelongitudinalshearstrengthoftheconcreteslab,andtheshearstrengthofthewebincludingbuckling.
Thecompositebeamsectionisbasedonaneffectivewidthofslabactingtogetherwiththesteelbeam.DetailsofhowtheeffectivewidthisdeterminedaregiveninLecture3.
cl.6.2.1
2.2.Sectionclassificationofcompositebeams
Intheanalysisofcompositebeams,itisimportanttoconsiderthepossibilityoflocalbuckling.Thisisdonebydefiningtheclassofsection,asforbaresteelsections.DetailsofhowtheseinfluencetheanalysisareincludedinLecture3,butageneraldescriptionofthedifferentclassesandhowtheyaredeterminedforaparticularcompositesectionaregivenhereforsimplysupportedbeams(inpositivebending).Thegeneraldescriptionofthedifferentclassesisasfollows:
cl.5.3
∙Class1and2:
thesectioniscapableofdevelopingthefullplasticbendingmomentM+pl.Rd;class1sectionscanalsorotateaftertheformationofaplastichinge,butthisisnotimportantforsimplysupportedbeams.
cl.5.3.1
∙Class3:
becauseoflocalbucklinginthepartofthesteelsectionincompression,thefullplasticmomentresistancecannotbeachieved,althoughthestressesintheextremefibresofthesteelsectioncanreachyield.
∙Class4:
localbucklinginthesteelsectionoccursbeforeyieldisreachedintheextremefibres.
InEurocode4,theslendernesslimitsforthecompressionflangeandweb(c/tandd/trespectively)areidenticaltothoseinEurocode3.Asectionisclassifiedaccordingtotheleastfavourableclassofitssteelelementsincompression.Forasimplysupportedcompositebeamthismaybethetopflangeofthesteelsection,andpossiblytheweb.
cl.5.3.2
cl.5.3.3
2.2.1Classificationofcompressionflange
Forsimplysupportedbeamsthecompressionflangeisrestrainedfrombucklingbytheconcreteslabtowhichitisattachedbyshearconnectors.Flangebucklingcanthereforebeassumedtobeeffectivelyprevented,andtheflangemaybedefinedasClass1.Forpartiallyencasedbeams(thatisbeamswithconcreteinfillbetweentheflanges,butwithoutshearconnectiontotheslab)theslendernesslimitsfortheoutstandofthecompressionflangeareasshownbelow(EC4Table5.4).
cl.5.3.2
Table5.4
welded
rolled
Stressdistribution
(compressionpositive)
Class
Type
Limit
1
rolled
welded
c/t10ε
c/t9ε
2
rolled
welded
c/t15ε
c/t14ε
3
rolled
welded
c/t21ε
c/t20ε
1
Rolled
c/t10ε
Welded
c/t9ε
2
Rolled
c/t15ε
Welded
c/t14ε
3
Rolled
c/t21ε
Welded
c/t20ε
Table1.Classificationofsteeloutstandflangesincompressionforpartiallyencasedsections(AfterEC4Table5.4)
c=outstandofflange(cleardistancebetweenwebandtoeofflange)
t=flangethickness
ε=(235/fy)
2.2.2Classificationofweb
Whentheplasticneutralaxisisintheslabortheupperflange,thecompositesectioncanbeconsideredasclass1sincethewebisintensionthroughout.However,iftheplasticneutralaxisisintheweb,theslendernessofthewebshouldbecheckedinaccordancewithTable5.2aofEC3todeterminetheclassificationoftheweb,andhencethecross-section.Thisconditionseldomappliesforsimplysupportedbeams,butisdescribedinlecture7forcontinuousbeams.
EC3
Table5.2a
Eurocode4allowssomemodificationstotheclassificationofthewebwhenthesteelcompressionflangeisclass1or2,asfollows:
∙Aclass3webencasedinconcretecanbetakenasaclass2webofthesamesection;
∙Aclass3webnotencasedinconcretecanbetakenasanequivalentclass2webbyconsideringaneffectiveheightofthewebincompressionmadeupoftwopartsofthesameheight20εt,attheextremitiesofthecompressedzone,whereε=(235/fy).Thisprovidesatransitionbetweenclasses2and3,theclassificationofawebbeingverysensitivetoslightchangesintheareaofthelongitudinalreinforcementortheeffectivewidthoftheslab.
cl.5.3.3.2
2.3Plasticmomentresistanceofclass1or2sections
Thebendingresistanceofaclass1or2sectionisdeterminedbyplasticanalysis.Thefollowingsimplificationsareassumed:
∙Thereisfullinteractionbetweenthesteelbeamandconcreteslab.(Thecaseofreducedplasticresistancemomentduetopartialintera
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