专业英语605 197203.docx
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专业英语605 197203.docx
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专业英语605197203
Chapter6SeismicResistanceofStructures197
6.5DuctileDesignofConcreteFrameStructures
6.5.1DuctilityIksignPhilosophy
Theseismicdesignphilosophyreliesonprovidingsufficientductilitytothestructurebywhichthestructurecandissipateseismicenergy.Areinforcedconcretestructurewithsufficientductilityhasthefollowingadvantages:
(a)Aductilereinforcedconcretestructuremaytakecareofoverloading,loadreversals,impactandsecondarystressesduetodifferentialsettlementoffoundation.
(b)Aductilereinforcedconcretestructuregivestheoccupantsufficienttimetovacatethestructurebyshowinglargedeformationbeforeitsfinalcollapse.Accordingly,thelossoflifeisminimizedwiththeprovisionofsufficientductility.
(c)Properlydesignedductilejointsarecapableofresistingforcesanddeformationsattheyieldingofsteelreinforcement.Therefore,thesesectionscanreachtheirrespectivemomentcapacities,whichisoneoftheassumptionsinthedesignofreinforcedconcretestructuresbylimitstatemethod.
Theductiledesignofreinforcedconcretemembersshouldensurebothstrengthand
ductility.Strengthofmemberscanbeassuredbyproperdesignofthesectionsfollowinglimitstat^method.Toachieveanappropriatedegreeofductility,thefollowingfailuremodeareexpected:
(a)Brittlefailuremodesshouldbesuppressed;and(b)beamsshouldfailbeforecolumns.
Flexuralyieldingofbendingmembersisductile.Shearfailureisbrittleandshouldbeavoidedunderseismicloading.Topreventshearfailureoccurringbeforebendingfailureitisgoodpracticetodesignsothattheflexuralsteelinamemberyieldswhiletheshearreinforcementisworkingatastresslessthanyield(say90%).
Thcstructuralductilityisachievedinthefomiofinelasticrotationsinreinforcedconcretemembers.Theinelasticrotationsspreadoverdefiniteregionscalledasplastichinges.Duringinelasticdeformations,theactualmaterialpropertiesarebeyondelasticrangeandhencedamagesintheseregionsareobvious.Theplastichingesare“expected”locationswherethestructuraldamagecanbeallowedtooccurduetoinelasticactionsinvolvinglargedefomuitions.Hence,inseismicdesign»thedamagesintheformofplastichingesareacceptedtobeformedinbcamsratherthanincolumnsasshowninFigure6.18.Mechanismwithbeamyieldingischaracteristicofstrongcolumn-weak-beambehaviorinwhichtheimposedinelasticrotationaldemandscanbe
198
土木工程专业英语
achievedreasonablywellthroughproperdetailingpracticeinbeams.
Figure6.18Framemechanism
Therefore,inthismodeofbehavior,itispossibleforthestructuretoattainthedesiredinelasticresponseandductility.Ontheotherhand,ifplastichingesareallowedtoformincolumns,theinelasticrotationaldemandsimposedareveryhighthatitisverydifficulttobecateredwithanypossibledetailing.Themechanismwithsuchafeatureiscalledcolumnyieldingorstoreymechanism.
Oneofthebasicrequirementsofdesignthatthecolumnsaboveandbelowthe
Mc\
jointshouldhavesufficientflexuralstrengthwhentheadjoiningbeamsdevelopflexuralover-strengthattheirplastichinges.Thiscolumntobeamflexuralstrengthratioisanimportantparametertoensurethatpossiblehingingoccursinbcamsratherthanincolumns.
Themomentcapacitiesofbeamsandcolumnsaresuchthatthecolumnmoments
、乂岣2FiRure6.19Colum^and-beamassembly
opposethebeammomentsasshowninFigure6.19.Toobtainastrong-column-weak-beamdesign,GB50011-2001requiresthatthedesignflexuralstrengthofthe
columnsframingintoajointexcecdthedesignflexuralstrengthofthebeamsframingintothejoint.Therefore
SM«*-=^YlMixtm(6.6.1)
where.
Chapter6SeismicResistanceofStructures199
y~!
A^umn=sumofmomentsatjointfacescorrespondingtodesignflexuralstrengthofcolumnsframingintojoint
y^jMbfam=sumofmomentsatjointfacescorrespondingtodesignflexuralstrengthofbeamsframingintojoint
jjt=columntobeamflexuralstrengthratio,takenas1.4forGrade1,takenas
1.2forGrade2,andtakenas1.1forGrade3.ThesumofmomentcapacityofthelowerendofcolumnatthefirststoreyofGrade1,2and3shallbemultipliedbyanamplifyingfactorof1.5,1.25and1.15
6.5.2DuctilityDetailing
Forensuringductility,specificrecommendationsaretobefollowedregardingthematerials,dimensions,minimumandmaximumpercentagesofreinforcement.
1.FrameBeams
Beamsinframesmusthaveaclearspan-to-effectivedepthratioofatleast4,awidth-to-depthratioofatleast0.25,andawebwidthofnotlessthan200mm.Theminimumclearspan-to-effectivedepthratiohelpsensurethatflexuralratherthanshearstrengthdominatesmemberbehaviorunderinelasticloadreversals.Minimumwebdimensionshelpprovideadequatecopfinementfortheconcrete,whereasthcwidthofbeamrelativetothecolumnislimitedtoprovideadequatemomenttransferbetweenbearnsandcolumns.
InaccordancewithseismicdesigncodeGB50011-2001,bothtopandbottomminimumflexuralsteelisrequired.ThcminimqmtensionreinforcementratioshouldnotbelessthanthatgivenbyTable6.6.I,withaminimumoftworeinforcingbars,topandbottom,throughoutthemember.Inaddition,thepositivetonegativemomentresistanceratioshallnotbelessthan0.5forframesassignedtoGrade1and0.3forframesassignedGrade2orGrade3,norbelessthanthecalculatednecessaryratio.Neitherthenegativenorthepositivemomentresistanceatanysectionalongthememberlengthshallbelessthanone-quarterofthemaximummomentresistanceprovidedatthefaceofeitherendjoint.Thesecriteriaaredesignedtoprovideforductilebehaviorthroughoutthemember,althoughtheminimumoftworeinforcingbarsonthetopandbottomisbasedprincipallyonconstructionrequirements.Amaximumreinforcementratioof0.025issettolimitproblemswithsteelcongestionandtoensureadequatemember•sizeforcarryingshearthatisgovernedbytheflexuralcapacityofthemember.
200土木工程专业英语
Table6.6.1Minimumtensionreinforcementratio(%)inframebeams
AseismicGrade
PositioninBeam
Support
Inspan
GradcI
Grcatcrof0.4and80f%/fy
Greaterof0.3and65fi/fr
GradeH
Greaterof0.3and65/•//,
Greaterof0.25and55f%/fy
Gradeffl/W
Greaterof0.25and55fx/fy
Grcatcrof0.2and45/,/fy
Toobtainductileperformance,thelocationofsplicesislimitedTheymaybenotbeusedwithinjoints*withinthecloselyspacedtransversereinforcementareaofbeams.Lapsplicemustbeenclosedbyhoopsorspirals,withamaximumofIOOmmor5timesthediameterofthesmallersplicedbar.Weldedandmechanicalconnectionsmaybeused,providedthattheyarenotusedwithinthecloselyspacedtransversereinforcementarea.
Transversereinforcementisrequiredthroughoutbeamsinframesresistingearthquake-inducedforce.Closelyspacedtransversereinforcementintheformofhoopsmustbeusedoveralengthfromthejointfacetowardmid-span,atthebothendsofthebeam.Thesehoopsareintendedtopreventbucklingofthelongitudinalbarsinthecompressionzoneinplastichingeregionswhereboththetopandbottomreinforcementcanbesubjectedtoyieldingintensionandcompressionduetoreversedcyclicflexure.Barsthatbuckleincompressionandaresubsequentlystressedtoyieldintensionusuallyrupture.Thelengthofcloselyspacedtransversereinforcementarea,themaximumspacingofthehoopsandtheminimumdiameterofthehoopbarsshouldbetakenaccordingtoTable6.6.2;whenthetensionreinforcementratioattheendofthcbeamisgreaterthan2%»theminimumdiameterofhoopbarslistedinthetableshouldbeincreasedby2mm.
Table6.6.2Constructionrequirementsforcloselyspacedtransversereinforccmentareaatbeamendsinframes
AseismicGrade
IwngthofClosclySpaccdTran5verseReinforccmcntArca/mm
Max.SpacingofIioops/mm
MinimumDiameterofHoopBars/mm
GradcI
Grcatcrof2hiand500
Smallesto(hy,/A%
6dand100
10
Graden
Greatcrof^band500
SmallestofAb/4.8dand100
8
GradeQl
Greaterofhhand500
Smalle5tofAb/4.
8dand100
8
Grade汉
Grcatcrof1.5kiand500
SmallestofAb/4»
8dand100
8
NOTES:
hiisthcbeamdepth;Jisthediameterofthesmallestlongitudinalbar.
2.FrameColumn
Tohelpensureconstructabilityandadequateconfinementoftheconcrete
Chapter6SeismicResistanceofStructures201
seismicdesigncodeGB50011-2001requiresthatcolumnsinframeshave(a)aminimumcross-sectiondimensionofatleast300mm,(b)aratioofthelargestcross-sectiondimensiontotheperpendiculardimensionofnomorethan3.and
(c)theshear-to-spanratioofnogreaterthan2.
Theaxialcompressiveforceinthecolumnduetodesignloadeffectsshallnotexceed,nAcfc,wherenistakenas0.7forGrade1,takenas0.8forGrade2,andtakenas0.9forGrade3.
Thecolumnreinforcementratiohasedonthegrosssectionshallnotexceed
1.05.Weldedsplicesandmechanicalconnectionsincolumnsmustsatisfythesamerequirementspecifiedforframebeams*whereaslappedsplicesmustbedesignedfortensionandarepermittedonlywithinthecenterhalfofcolumn.
GB50011-2001specifiestheuseofminimumtransversereinforcementoverthelengthofthecloselyspacedtransversereinforcementareafromeachjointface.ThemaximumspacingandtheminimumdiametercfhoopsshouldsatisfythestipulationinTable6.6.3.Thelengthofthecloselyspacedtransversereinforcementareamaynotbelessthan(a)thelargercross-sectiondimensionofthecolumn,(b)one-sixththecolumnheight,or(c)500mm.Asforthecornercolumns
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