土木工程毕业设计外文翻译3.docx
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土木工程毕业设计外文翻译3
7Rigid-FrameStructures
Arigid-framehigh-risestructuretypicallycomprisesparallelororthogonallyarrangedbentsconsistingofcolumnsandgirderswithmomentresistantjoints.Resistancetohorizontalloadingisprovidedbythebendingresistanceofthecolumns,girders,andjoints.Thecontinuityoftheframealsocontributestoresistinggravityloading,byreducingthemomentsinthegirders.
Theadvantagesofarigidframearethesimplicityandconvenienceofitsrectangularunobstructedarrangement,clearofbracingmembersandstructuralwalls,allowsfreedominternallyforthelayoutandexternallyforthefenestration.Rigidframesareconsideredeconomicalforbuildingsofupto'about25stories,abovewhichtheirdriftresistanceiscostlytocontrol.If,however,arigidframeiscombinedwithshearwallsorcores,theresultingstructureisverymuchstiffersothatitsheightpotentialmayextendupto50storiesormore.Aflatplatestructureisverysimilartoarigidframe,butwithslabsreplacingthegirdersAswitharigidframe,horizontalandverticalloadingsareresistedinaflatplatestructurebytheflexuralcontinuitybetweentheverticalandhorizontalcomponents.
Ashighlyredundantstructures,rigidframesaredesignedinitiallyonthebasisofapproximateanalyses,afterwhichmorerigorousanalysesandcheckscanbemade.Theproceduremaytypicallyincludethefollowingstages:
1.Estimationofgravityloadforcesingirdersandcolumnsbyapproximatemethod.
2.Preliminaryestimateofmembersizesbasedongravityloadforceswitharbitraryincreaseinsizestoallowforhorizontalloading.
3.Approximateallocationofhorizontalloadingtobentsandpreliminaryanalysisofmemberforcesinbents.
4.Checkondriftandadjustmentofmembersizesifnecessary.
5.Checkonstrengthofmembersforworstcombinationofgravityandhorizontalloading,andadjustmentofmembersizesifnecessary.
6.Computeranalysisoftotalstructureformoreaccuratecheckonmemberstrengthsanddrift,withfurtheradjustmentofsizeswhererequired.Thisstagemayincludethesecond-orderP-Deltaeffectsofgravityloadingonthememberforcesanddrift..
7.Detaileddesignofmembersandconnections.
Thischapterconsidersmethodsofanalysisforthedeflectionsandforcesforbothgravityandhorizontalloading.Themethodsareincludedinroughlytheorderofthedesignprocedure,withapproximatemethodsinitiallyandcomputertechniqueslater.StabilityanalysesofrigidframesarediscussedinChapter16.
RIGIDFRAMEBEHAVIOR
Thehorizontalstiffnessofarigidframeisgovernedmainlybythebendingresistanceofthegirders,thecolumns,andtheirconnections,and,inatallframe,bytheaxialrigidityofthecolumns.Theaccumulatedhorizontalshearaboveanystoryofarigidframeisresistedbyshearinthecolumnsofthatstory(Fig..Theshearcausesthestory-heightcolumnstobendindoublecurvaturewithpointsofcontraflexureatapproximatelymid-story-heightlevels.Themomentsappliedtoajointfromthecolumnsaboveandbelowareresistedbytheattachedgirders,whichalsobendindoublecurvature,withpointsofcontraflexureatapproximatelymid-span.Thesedeformationsofthecolumnsandgirdersallowrackingoftheframeandhorizontaldeflectionineachstory.Theoveralldeflectedshapeofarigidframestructureduetorackinghasashearconfigurationwithconcavityupwind,amaximuminclinationnearthebase,andaminimuminclinationatthetop,asshowninFig..
Theoverallmomentoftheexternalhorizontalloadisresistedineachstorylevelbythecoupleresultingfromtheaxialtensileandcompressiveforcesinthecolumnsonoppositesidesofthestructure(Fig..Theextensionandshorteningofthecolumnscauseoverallbendingandassociatedhorizontaldisplacementsofthestructure.Becauseofthecumulativerotationuptheheight,thestorydriftduetooverallbendingincreaseswithheight,whilethatduetorackingtendstodecrease.Consequentlythecontributiontostorydriftfromoverallbendingmay,in.theuppermoststories,exceedthatfromracking.Thecontributionofoverallbendingtothetotaldrift,however,willusuallynotexceed10%ofthatofracking,exceptinverytall,slender,,rigidframes.Thereforetheoveralldeflectedshapeofahigh-riserigidframeusuallyhasashearconfiguration.
Theresponseofarigidframetogravityloadingdiffersfromasimplyconnectedframeinthecontinuousbehaviorofthegirders.Negativemomentsareinducedadjacenttothecolumns,andpositivemomentsofusuallylessermagnitudeoccurinthemid-spanregions.Thecontinuityalsocausesthemaximumgirdermomentstobesensitivetothepatternofliveloading.Thismustbeconsideredwhenestimatingtheworstmomentconditions.Forexample,thegravityloadmaximumhoggingmomentadjacenttoanedgecolumnoccurswhenliveloadactsonlyontheedgespanandalternateotherspans,asforAinFig..Themaximumhoggingmomentsadjacenttoaninteriorcolumnarecaused,however,whenliveloadactsonlyonthespansadjacenttothecolumn,asforBinFig..Themaximummid-spansaggingmomentoccurswhenliveloadactsonthespanunderconsideration,andalternateotherspans,asforspansABandCDinFig..
Thedependenceofarigidframeonthemomentcapacityofthecolumnsforresistinghorizontalloadingusuallycausesthecolumnsofarigidframetobelargerthanthoseofthecorrespondingfullybracedsimplyconnectedframe.Ontheotherhand,whilegirdersinbracedframesaredesignedfortheirmid-spansaggingmoment,girdersinrigidframesaredesignedfortheend-of-spanresultanthoggingmoments,whichmaybeoflesservalue.Consequently,girdersinarigidframemaybesmallerthaninthecorrespondingbracedframe.Suchreductionsinsizealloweconomythroughthelowercostofthegirdersandpossiblereductionsinstoryheights.Thesebenefitsmaybeoffset,however,bythehighercostofthemorecomplexrigidconnections.
APPROXIMATEDETERMINATIONOFMEMBERFORCESCAUSEDBYGRAVITYLOADSIMG
Arigidframeisahighlyredundantstructure;consequently,anaccurateanalysiscanbemadeonlyafterthemembersizesareassigned.Initially,therefore,membersizesaredecidedonthebasisofapproximateforcesestimatedeitherbyconservativeformulasorbysimplifiedmethodsofanalysisthatareindependentofmemberproperties.Twoapproachesforestimatinggirderforcesduetogravityloadingaregivenhere.
GirderForces—CodeRecommendedValues
Inrigidframeswithtwoormorespansinwhichthelongerofanytwoadjacentspansdoesnotexceedtheshorterbymorethan20%,andwheretheuniformlydistributeddesignliveloaddoesnotexceedthreetimesthedeadload,thegirdermomentandshearsmaybeestimatedfromTable.ThissummarizestherecommendationsgivenintheUniformBuildingCode[].Inothercasesaconventionalmomentdistributionortwo-cyclemomentdistributionanalysisshouldbemadeforalineofgirdersatafloorlevel.
Two-CycleMomentDistribution[].
Thisisaconciseformofmomentdistributionforestimatinggirdermomentsinacontinuousmultibayspan.ItismoreaccuratethantheformulasinTable,especiallyforcasesofunequalspansandunequalloadingindifferentspans.
Thefollowingisassumedfortheanalysis:
1.Acounterclockwiserestrainingmomentontheendofagirderispositiveandaclockwisemomentisnegative.
2.Theendsofthecolumnsatthefloorsaboveandbelowtheconsideredgirderarefixed.
3.Intheabsenceofknownmembersizes,distributionfactorsateachjointaretakenequalto1/n,wherenisthenumberofmembersframingintothejointintheplaneoftheframe.
Two-CycleMomentDistribution—WorkedExample.Themethodisdemonstratedbyaworkedexample.InFig,,afour-spangirderAEfromarigid-framebentisshownwithitsloading.Thefixed-endmomentsineachspanarecalculatedfordeadloadingandtotalloadingusingtheformulasgiveninFig,.ThemomentsaresummarizedinTable.
Thepurposeofthemomentdistributionistoestimateforeachsupportthemaximumgirdermomentsthatcanoccurasaresultofdeadloadingandpatternliveloading.Adifferentloadcombinationmustbeconsideredforthemaximummomentateachsupport,andadistributionmadeforeachcombination.
ThefivedistributionsarepresentedseparatelyinTable,andinacombinedforminTable.DistributionsainTablearefortheexteriorsupportsAandE.ForthemaximumhoggingmomentatA,totalloadingisappliedtospanABwithdeadloadingonlyonBC.Thefixed-endmomentsarewritteninrows1and2.Inthisdistributiononly.theresultingmomentatAisofinterest.Forthefirstcycle,jointBisbalancedwithacorrectingmomentof-(-867+315)/4=-U/4assignedtoMBAwhereUistheunbalancedmoment.Thisisnotrecorded,buthalfofit,(-U/4)/2,iscarriedovertoMAB.Thisisrecordedinrow3andthenaddedtothefixed-endmomentandtheresultrecordedinrow4.
ThesecondcycleinvolvesthereleaseandbalanceofjointA.Theunbalancedmomentof936isbalancedbyadding-U/3=-936/3=-312toMBA(row5),implicitlyaddingthesamemomenttothetwocolumnendsatA.Thiscompletesthesecondcycleofthedistribution.TheresultingmaximummomentatAisthengivenbytheadditionofrows4and5,936-312=624.ThedistributionforthemaximummomentatEfollowsasimilarprocedure.
DistributionbinTableisforthemaximummomentatB.ThemostsevereloadingpatternforthisiswithtotalloadingonspansABandBCanddeadloadonlyonCD.TheoperationsaresimilartothoseinDistributiona,exceptthattheTfirstcycleinvolvesbalancingthetwoad