建筑結(jié)構(gòu)英語(yǔ)論文

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ArchitectureStructure

Wehaveandthearchitectsmustdealwiththespatialaspectofactivity,physical,andsymbolicneedsinsuchawaythatoverallperformanceintegrityisassured.Hence,heorshewellwantstothinkofevolvingabuildingenvironmentasatotalsystemofinteractingandspaceformingsubsystems.Isrepresentsaple*challenge,andtomeetitthearchitectwillneedahierarchicdesignprocessthatprovidesatleastthreelevelsoffeedbackthinking:schematic,preliminary,andfinal.

Suchahierarchyisnecessaryifheorsheistoavoidbeingconfused,atconceptualstagesofdesignthinking,bythemyriaddetailissuesthatcandistractattentionfrommorebasicconsiderations.Infact,wecansaythatanarchitect’sabilitytodistinguishthemorebasicformthemoredetailedissuesisessentialtohissuccessasadesigner.

Theobjectoftheschematicfeedbacklevelistogenerateandevaluateoverallsite-plan,activity-interaction,andbuilding-configurationoptions.Todosothearchitectmustbeabletofocusontheinteractionofthebasicattributesofthesiteconte*t,thespatialorganization,andthesymbolismasdeterminantsofphysicalform.Thismeansthat,inschematicterms,thearchitectmayfirstconceiveandmodelabuildingdesignasanorganizationalabstractionofessentialperformance-spaceinteractions.Thenheorshemaye*ploretheoverallspace-formimplicationsoftheabstraction.Asanactualbuildingconfigurationoptionbeginstoemerge,itwillbemodifiedtoincludeconsiderationforbasicsiteconditions.

Attheschematicstage,itwouldalsobehelpfulifthedesignercouldvisualizehisorheroptionsforachievingoverallstructuralintegrityandconsidertheconstructivefeasibilityandeconomicofhisorherscheme.Butthiswillrequirethatthearchitectand/oraconsultantbeabletoconceptualizetotal-systemstructuraloptionsintermsofelementaldetail.Suchoverallthinkingcanbeeasilyfedbacktoimprovethespace-formscheme.

Atthepreliminarylevel,thearchitect’semphasiswillshifttotheelaborationofhisorhermorepromisingschematicdesignoptions.Herethearchitect’sstructuralneedswillshifttoappro*imatedesignofspecificsubsystemoptions.Atthisstagethetotalstructuralschemeisdevelopedtoamiddlelevelofspecificitybyfocusingonidentificationanddesignofmajorsubsystemstothee*tentthattheirkeygeometric,ponent,andinteractivepropertiesareestablished.Basicsubsysteminteractionanddesignconflictscanthusbeidentifiedandresolvedintheconte*toftotal-systemobjectives.Consultantscanplayasignificantpartinthiseffort;thesepreliminary-leveldecisionsmayalsoresultinfeedbackthatcallsforrefinementorevenmajorchangeinschematicconcepts.

Whenthedesignerandtheclientaresatisfiedwiththefeasibilityofadesignproposalatthepreliminarylevel,itmeansthatthebasicproblemsofoveralldesignaresolvedanddetailsarenotlikelytoproducemajorchange.Thefocusshiftsagain,andthedesignprocessmovesintothefinallevel.Atthisstagetheemphasiswillbeonthedetaileddevelopmentofallsubsystemspecifics.Heretheroleofspecialistsfromvariousfields,includingstructuralengineering,ismuchlarger,sincealldetailofthepreliminarydesignmustbeworkedout.DecisionsmadeatthislevelmayproducefeedbackintoLevelIIthatwillresultinchanges.However,ifLevelsIandIIarehandledwithinsight,therelationshipbetweentheoveralldecisions,madeattheschematicandpreliminarylevels,andthespecificsofthefinallevelshouldbesuchthatgrossredesignisnotinquestion,Rather,theentireprocessshouldbeoneofmovinginanevolutionaryfashionfromcreationandrefinement(ormodification)ofthemoregeneralpropertiesofatotal-systemdesignconcept,tothefleshingoutofrequisiteelementsanddetails.

Tosummarize:AtLevelI,thearchitectmustfirstestablish,inconceptualterms,theoverallspace-formfeasibilityofbasicschematicoptions.Atthisstage,collaborationwithspecialistscanbehelpful,butonlyifintheformofoverallthinking.AtLevelII,thearchitectmustbeabletoidentifythemajorsubsystemrequirementsimpliedbytheschemeandsubstantialtheirinteractivefeasibilitybyappro*imatingkeyponentproperties.Thatis,thepropertiesofmajorsubsystemsneedbeworkedoutonlyinsufficientdepthtoverytheinherentpatibilityoftheirbasicform-relatedandbehavioralinteraction.ThiswillmeanasomewhatmorespecificformofcollaborationwithspecialiststhenthatinlevelI.AtlevelIII,thearchitectandthespecificformofcollaborationwithspecialiststhenthatprovidingforalloftheelementaldesignspecificsrequiredtoproducebiddableconstructiondocuments.

OfcoursethissuccessesfromthedevelopmentoftheStructuralMaterial.

ReinforcedConcrete

Plainconcreteisformedfromahardenedmi*tureofcement,water,fineaggregate,coarseaggregate(crushedstoneorgravel),air,andoftenotheradmi*tures.Theplasticmi*isplacedandconsolidatedintheformwork,thencuredtofacilitatetheaccelerationofthechemicalhydrationreactionlfthecement/watermi*,resultinginhardenedconcrete.Thefinishedproducthashighpressivestrength,andlowresistancetotension,suchthatitstensilestrengthisappro*imatelyonetenthlfitspressivestrength.Consequently,tensileandshearreinforcementinthetensileregionsofsectionshastobeprovidedtopensatefortheweaktensionregionsinthereinforcedconcreteelement.

tisthisdeviationinthepositionofareinforcesconcretesectionfromthehomogeneityofstandardwoodorsteelsectionsthatrequiresamodifiedapproachtothebasicprinciplesofstructuraldesign.Thetwoponentsoftheheterogeneousreinforcedconcretesectionaretobesoarrangedandproportionedthatoptimaluseismadeofthematerialsinvolved.Thisispossiblebecauseconcretecaneasilybegivenanydesiredshapebyplacingandpactingthewetmi*tureoftheconstituentingredientsareproperlyproportioned,thefinishedproductbeesstrong,durable,and,inbinationwiththereinforcingbars,adaptableforuseasmainmembersofanystructuralsystem.

Thetechniquesnecessaryforplacingconcretedependonthetypeofmembertobecast:thatis,whetheritisacolumn,abean,awall,aslab,afoundation.amasscolumns,orane*tensionofpreviouslyplacedandhardenedconcrete.Forbeams,columns,andwalls,theformsshouldbewelloiledaftercleaningthem,andthereinforcementshouldbeclearedofrustandotherharmfulmaterials.Infoundations,theearthshouldbepactedandthoroughlymoistenedtoabout6in.indepthtoavoidabsorptionofthemoisturepresentinthewetconcrete.Concreteshouldalwaysbeplacedinhorizontallayerswhicharepactedbymeansofhighfrequencypower-drivenvibratorsofeithertheimmersionore*ternaltype,asthecaserequires,unlessitisplacedbypumping.Itmustbekeptinmind,however,thatovervibrationcanbeharmfulsinceitcouldcausesegregationoftheaggregateandbleedingoftheconcrete.

Hydrationofthecementtakesplaceinthepresenceofmoistureattemperaturesabove50°F.Itisnecessarytomaintainsuchaconditioninorderthatthechemicalhydrationreactioncantakeplace.Ifdryingistoorapid,surfacecrackingtakesplace.Thiswouldresultinreductionofconcretestrengthduetocrackingaswellasthefailuretoattainfullchemicalhydration.

Itisclearthatalargenumberofparametershavetobedealtwithinproportioningareinforcedconcreteelement,suchasgeometricalwidth,depth,areaofreinforcement,steelstrain,concretestrain,steelstress,andsoon.Consequently,trialandadjustmentisnecessaryinthechoiceofconcretesections,withassumptionsbasedonconditionsatsite,availabilityoftheconstituentmaterials,particulardemandsoftheowners,architecturalandheadroomrequirements,theapplicablecodes,andenvironmentalreinforcedconcreteisoftenasite-constructedposite,incontrasttothestandardmill-fabricatedbeamandcolumnsectionsinsteelstructures.Atrialsectionhastobechosenforeachcriticallocationinastructuralsystem.

Thetrialsectionhastobeanalyzedtodetermineifitsnominalresistingstrengthisadequatetocarrytheappliedfactoredload.Sincemorethanonetrialisoftennecessarytoarriveattherequiredsection,thefirstdesigninputstepgeneratesintoaseriesoftrial-and-adjustmentanalyses.

Thetrial-and–adjustmentproceduresforthechoiceofaconcretesectionleadtotheconvergenceofanalysisanddesign.Henceeverydesignisananalysisonceatrialsectionischosen.Theavailabilityofhandbooks,charts,andpersonalputersandprogramssupportsthisapproachasamoreefficient,pact,andspeedyinstructionalmethodparedwiththetraditionalapproachoftreatingtheanalysisofreinforcedconcreteseparatelyfrompuredesign.

Earthwork

Becauseearthmovingmethodsandcostschangemorequicklythanthoseinanyotherbranchofcivilengineering,thisisafieldwheretherearerealopportunitiesfortheenthusiast.In1935mostofthemethodsnowinuseforcarryingande*cavatingearthwithrubber-tyredequipmentdidnote*ist.Mostearthwasmovedbynarrowrailtrack,nowrelativelyrare,andthemainmethodsofe*cavation,withfaceshovel,backacter,ordraglineorgrab,thoughtheyarestillwidelyusedareonlyafewofthemanycurrentmethods.Tokeephisknowledgeofearthmovingequipmentuptodateanengineermustthereforespendtinestudyingmodernmachines.Generallytheonlyreliableup-to-dateinformationone*cavators,loadersandtransportisobtainablefromthemakers.

Earthworksorearthmovingmeanscuttingintogroundwhereitssurfaceistoohigh(cuts),anddumpingtheearthinotherplaceswherethesurfaceistoolow(fills).Toreduceearthworkcosts,thevolumeofthefillsshouldbeequaltothevolumeofthecutsandwhereverpossiblethecutsshouldbeplacedneartofillsofequalvolumesoastoreducetransportanddoublehandlingofthefill.Thisworkofearthworkdesignfallsontheengineerwholaysouttheroadsinceitisthelayoutoftheearthworkmorethananythingelsewhichdecidesitscheapness.Fromtheavailablemapsahdlevels,theengineeringmusttrytoreachasmanydecisionsaspossibleinthedrawingofficebydrawingcrosssectionsoftheearthwork.Onthesitewhenfurtherinformationbeesavailablehecanmakechangesinhissectionsandlayout,butthedrawingofficeworkwillnothavebeenlost.Itwillhavehelpedhimtoreachthebestsolutionintheshortesttime.

Thecheapestwayofmovingearthistotakeitdirectlyoutofthecutanddropitasfillwiththesamemachine.Thisisnotalwayspossible,butwhenitcanbedoneitisideal,beingbothquickandcheap.Draglines,bulldozersandfaceshovelsandothis.Thelargestradiusisobtainedwiththedragline,andthelargesttonnageofearthismovedbythebulldozer,thoughonlyovershortdistances.Thedisadvantagesofthedraglinearethatitmustdigbelowitself,itcannotdigwithforceintopactedmaterial,itcannotdigonsteepslopes,anditsdumpinganddiggingarenotaccurate.

Faceshovelsarebetweenbulldozersanddraglines,havingalargerradiusofactionthanbulldozersbutlessthandraglines.Theyareabletodigintoaverticalclifffaceinawaywhichwouldbedangeroustorabulldozeroperatorandimpossibleforadragline.Eachpieceofequipmentshouldbeleveloftheirtracksandfordeepdigsinpactmaterialabackacterismostuseful,butitsdumpingradiusisconsiderablylessthanthatofthesameescavatorfittedwithafaceshovel.

Rubber-tyredbowlscrapersareindispensableforfairlyleveldiggingwherethedistanceoftransportistoomuchtoradraglineorfaceshovel.Theycandigthematerialdeeply(butonlybelowthemselves)toafairlyflatsurface,carryithundredsofmetersifneedbe,thendropitandlevelitroughlyduringthedumping.Forharddiggingitisoftenfoundeconomicaltokeepapushertractor(wheeledortracked)onthediggingsite,topusheachscraperasitreturnstodig.Assoonasthescraperisfull,thepushertractorreturnstothebeginningofthedigtohelpthenestscraper.

Bowlscrapersareoftene*tremelypowerfulmachines;manymakersbuildscrapersof8cubicmetersstruckcapacity,whichcarry10m3heaped.Thelargestself-propelledscrapersareof19m3struckcapacity(25m3heaped)andtheyaredrivenbyatractorengineof430horse-powers.

Dumpersareprobablythemonestrubber-tyredtransportsincetheycanalsoconvenientlybeusedforcarryingconcreteorotherbuildingmaterials.Dumpershavetheearthcontaineroverthefronta*leonlargerubber-tyredwheels,andthecontainertipsforwardsonmosttypes,thoughinarticulateddumpersthedirectionoftipcanbewidelyvaried.Thesmallestdumpershaveacapacityofabout0.5m3,andthelargeststandardtypesareofabout4.5m3.Specialtypesincludetheself-loadingdumperofupto4m3andthearticulatedtypeofabout0.5m3.Thedistinctionbetweendumpersanddumptrucksmustberemembered.dumperstipforwardsandthedriversitsbehindtheload.Dumptrucksareheavy,strengthenedtippinglorries,thedrivertravelsinfrontlftheloadandtheloadisdumpedbehindhim,sotheyaresometimescalledrear-dumptrucks.

SafetyofStructures

Theprincipalscopeofspecificationsistoprovidegeneralprinciplesandputationalmethodsinordertoverifysafetyofstructures.The“safetyfactor〞,whichaccordingtomoderntrendsisindependentofthenatureandbinationofthematerialsused,canusuallybedefinedastheratiobetweentheconditions.Thisratioisalsoproportionaltotheinverseoftheprobability(risk)offailureofthestructure.

Failurehastobeconsiderednotonlyasoverallcollapseofthestructurebutalsoasun-serviceabilityor,accordingtoamoreprecise.mondefinition.Asthereachingofa“l(fā)imitstate〞whichcausestheconstructionnottoacplishthetaskitwasdesignedfor.Therearetwocategoriesoflimitstate:

Ultimatelimitsate,whichcorrespondstothehighestvalueoftheload-bearingcapacity.E*amplesincludelocalbucklingorglobalinstabilityofthestructure;failureofsomesectionsandsubsequenttransformationofthestructureintoamechanism;failurebyfatigue;elasticorplasticdeformationorcreepthatcauseasubstantialchangeofthegeometryofthestructure;andsensitivityofthestructuretoalternatingloads,tofireandtoe*plosions.

Servicelimitstates,whicharefunctionsoftheuseanddurabilityofthestructure.E*amplesincludee*cessivedeformationsanddisplacementswithoutinstability;earlyore*cessivecracks;largevibrations;andcorrosion.

putationalmethodsusedtoverifystructureswithrespecttothedifferentsafetyconditionscanbeseparatedinto:

(1)Deterministicmethods,inwhichthemainparametersareconsideredasnonrandomparameters.

(2)Probabilisticmethods,inwhichthemainparametersareconsideredasrandomparameters.

Alternatively,withrespecttothedifferentuseoffactorsofsafety,putationalmethodscanbeseparatedinto:

Allowablestressmethod,inwhichthestressesputedunderma*imumloadsareparedwiththestrengthofthematerialreducedbygivensafetyfactors.(2)Limitstatesmethod,inwhichthestructuremaybeproportionedonthebasisofitsma*imumstrength.Thisstrength,asdeterminedbyrationalanalysis,shallnotbelessthanthatrequiredtosupportafactoredloadequaltothesumofthefactoredliveloadanddeadload(ultimatestate).

Thestressescorrespondingtoworking(service)conditionswithun-factoredliveanddeadloadsareparedwithprescribedvalues(servicelimitstate).Fromthefourpossiblebinationsofthefirsttwoandsecondtwomethods,wecanobtainsomeusefulputationalmethods.Generally,twobinationsprevail:

deterministicmethods,whichmakeuseofallowablestresses.

Probabilisticmethods,whichmakeuseoflimitstates.

Themainadvantageofprobabilisticapproachesisthat,atleastintheory,itispossibletoscientificallytakeintoaccountallrandomfactorsofsafety,babilisticapproachesdependupon:

(1)Randomdistributionofstrengthofmaterialswithrespecttotheconditionsoffabricationanderection(scatterofthevaluesofmechanicalpropertiesthroughoutthestructure);

(2)Uncertaintyofthegeometryofthecross-sectionsandofthestructure(faultsandimperfectionsduetofabricationanderection