土木工程类外文文献翻译 (6)

外文文献翻译
建筑工程学院土木工程系
土木084班
院系:_________________________
张绍云
班级:_________________________
付佳丽韩有民
姓名:_________________________
指导教师:_________________________
2012年2月20日
2外文翻译
2.1ReinforcedConcrete
Plainconcreteisformedfromahardenedmixtureofcement,water,fineaggregate,coaraggregate(crushedstoneorgravel),air,andoftenotheradmixtures.Theplasticmixisplacedandconsolidatedintheformwork,thencuredtofacilitatetheaccelerationofthechemicalhydrationreactionlfthecement/watermix,resultinginhardenedconcrete.Thefinishedproducthashighcompressivestrength,andlowresistancetotension,suchthatitstensilestrengthisapproximatelyonetenthlfitscompressivestrength.Conquently,tensileandshearreinforcementinthetensileregionsofctionshastobeprovidedtocompensatefortheweaktensionregionsinthereinforcedconcreteelement.
Itisthisdeviationinthecompositionofareinforcesconcretectionfromthehomogeneityofstandardwoodorsteelctionsthatrequiresamodifiedapproachtothebasicprinciplesofstructuraldesign.Thetwocomponentsoftheheterogeneousreinforcedconcretectionaretobesoarrangedandproportionedthatoptimaluismadeofthematerialsinvolved.Thisispossiblebecauconcretecaneasilybegivenanydesiredshapebyplacingandcompactingthewetmixtureoftheconstituentingredientsareproperlyproportioned,thefinishedproductbecomesstrong,durable,and,incombinationwiththereinforcingbars,adaptableforuasmainmembersofanystructuralsystem.
Thetechniquesnecessaryforplacingconcretedependonthetypeofmembertobecast:thatis,whetheritisacolumn,abean,awall,aslab,afoundation.amasscolumns,oranextensionofpreviouslyplacedandhardenedconcrete.Forbeams,columns,andwalls,theformsshouldbewelloiledaftercleaningthem,andthereinforcementshouldbeclearedofrustandotherharmfulmaterials.Infoundations,theearthshouldbecompactedandthoroughlymoistenedtoabout6in.indepthtoavoidabsorptionofthemoistureprentinthewetconcrete.Concreteshouldalwaysbeplacedinhorizontallayerswhicharecompactedbymeansofhighfrequencypower-drivenvibratorsofeithertheimmersionorexternaltype,asthecarequires,unlessitisplacedbypumping.Itmustbekeptinmind,however,thatovervibrationcanbeharmfulsinceitcouldcaugregationoftheaggregateandbleedingoftheconcrete.
Hydrationofthecementtakesplaceintheprenceofmoistureattemperaturesabove50
°F.Itisnecessarytomaintainsuchaconditioninorderthatthechemicalhydrationreactioncantakeplace.Ifdryingistoorapid,surfacecrackingtakesplace.Thiswouldresultinreductionofconcretestrengthduetocrackingaswellasthefailuretoattainfullchemicalhydration.
Itisclearthatalargenumberofparametershavetobedealtwithinproportioningareinforcedconcreteelement,suchasgeometricalwidth,depth,areaofreinforcement,steelstrain,concretestrain,steelstress,andsoon.Conquently,trialandadjustmentisnecessaryinthechoiceofconcretections,withassumptionsbadonconditionsatsite,availabilityoftheconstituentmaterials,particulardemandsoftheowners,architecturalandheadroomrequirements,theapplicablecodes,andenvironmentalreinforcedconcreteisoftenasite-constructedcomposite,incontrasttothestandardmill-fabricatedbeamandcolumnctionsinsteelstructures.
Atrialctionhastobechonforeachcriticallocationinastructuralsystem.Thetrialctionhastobeanalyzedtodetermineifitsnominalresistingstrengthisadequatetocarrytheappliedfactoredload.Sincemorethanonetrialisoftennecessarytoarriveattherequiredction,thefirstdesigninputstepgeneratesintoariesoftrial-and-adjustmentanalys.
Thetrial-and–adjustmentproceduresforthechoiceofaconcretectionleadtotheconvergenceofanalysisanddesign.Henceeverydesignisananalysisonceatrialctionischon.Theavailabilityofhandbooks,charts,andpersonalcomputersandprogramssupportsthisapproachasamoreefficient,compact,andspeedyinstructionalmethodcomparedwiththetraditionalapproachoftreatingtheanalysisofreinforcedconcreteparatelyfrompuredesign.
2.2Earthwork
Becauearthmovingmethodsandcostschangemorequicklythanthoinanyotherbranchofcivilengineering,thisisafieldwheretherearerealopportunitiesfortheenthusiast.In1935mostofthemethodsnowinuforcarryingandexcavatingearthwithrubber-tyredequipmentdidnotexist.Mostearthwasmovedbynarrowrailtrack,nowrelativelyrare,andthemainmethodsofexcavation,withfaceshovel,backacter,ordraglineorgrab,thoughtheyarestillwidelyudareonlyafewofthemanycurrentmethods.Tokeephisknowledgeofearthmovingequipmentuptodateanengineermustthereforespendtinestudyingmodernmachines.Generallytheonlyreliableup-to-dateinformationonexcavators,loadersandtransportisobtainablefromthemakers.
Earthworksorearthmovingmeanscuttingintogroundwhereitssurfaceistoohigh(cuts),anddumpingtheearthinotherplaceswherethesurfaceistoolow(fills).Toreduceearthworkcosts,thevolumeofthefillsshouldbeequaltothevolumeofthecutsandwhereverpossiblethecutsshouldbeplacedneartofillsofequalvolumesoastoreducetransportanddoublehandlingofthefill.Thisworkofearthworkdesignfallsontheengineerwholaysouttheroadsinceitisthelayoutoftheearthworkmorethananythingelwhichdecidesitscheapness.Fromtheavailablemapsahdlevels,theengineeringmusttrytoreachasmanydecisionsaspossibleinthedrawingofficebydrawingcrossctionsoftheearthwork.Onthesitewhenfurtherinformationbecomesavailablehecanmakechangesinjisctionsandlayout,butthedrawinglfficeworkwillnothavebeenlost.Itwillhavehelpedhimtoreachthebestsolutionintheshortesttime.
Thecheapestwayofmovingearthistotakeitdirectlyoutofthecutanddropitasfillwiththesamemachine.Thisisnotalwayspossible,butwhenitcanbedoneitisideal,beingbothquickandcheap.Draglines,bulldozersandfaceshovelsandothis.Thelargestradiusisobtainedwiththedragline,andthelargesttonnageofearthismovedbythebulldozer,thoughonlyovershortdistances.Thedisadvantagesofthedraglinearethatitmustdigbelowitlf,itcannotdigwithforceintocompactedmaterial,itcannotdigonsteepslopws,anditsdumpinganddiggingarenotaccurate.
Faceshovelsarebetweenbulldozersanddraglines,havingalargerradiusofactionthanbulldozersbutlessthandraglines.Theyareanletodigintoaverticalclifffaceinawaywhichwouldbedangeroustorabulldozeroperatorandimpossibleforadragline.Eachpieceofequipmentshouldbeleveloftheirtracksandfordeepdigsincompactmaterialabackacterismostuful,butitsdumpingradiusisconsiderablylessthanthatofthesameescavatorfittedwithafaceshovel.
Rubber-tyredbowlscrapersareindispensableforfairlyleveldiggingwherethedistanceoftransportistoomuchtoradraglineorfaceshovel.Theycandigthematerialdeeply(butonlybelowthemlves)toafairlyflatsurface,carryithundredsofmetersifneedbe,thendropitandlevelitroughlyduringthedumping.Forharddiggingitisoftenfoundeconomicaltokeepapushertractor(wheeledortracked)onthediggingsite,topusheachscraperasitreturnstodig.Assoonasthescraperisfull,thepushertractorreturnstothebeginningofthedigtoheoptohelpthenestscraper.
Bowlscrapersareoftenextremelypowerfulmachines;manymakersbuildscrapersof8cubicmetersstruckcapacity,whichcarry10m³heaped.Thelargestlf-propelledscrapersareof19
m³struckcapacity(25m³heaped)andtheyaredrivenbyatractorengineof430hor-powers.
Dumpersareprobablythecommonestrubber-tyredtransportsincetheycanalsoconvenientlybeudforcarryingconcreteorotherbuildingmaterials.Dumpershavetheearthcontaineroverthefrontaxleonlargerubber-tyredwheels,andthecontainertipsforwardsonmosttypes,thoughinarticulateddumpersthedirectionoftipcanbewidelyvaried.Thesmallestdumpershaveacapacityofabout0.5m³,andthelargeststandardtypesareofabout4.5m³.Specialtypesincludethelf-loadingdumperofupto4m³andthearticulatedtypeofabout0.5m³.Thedistinctionbetweendumpersanddumptrucksmustberemembered.dumperstipforwardsandthedriversitsbehindtheload.Dumptrucksareheavy,strengthenedtippinglorries,thedrivertravelsinfrontlftheloadandtheloadisdumpedbehindhim,sotheyaresometimescalledrear-dumptrucks.
2.3SafetyofStructures
Theprincipalscopeofspecificationsistoprovidegeneralprinciplesandcomputationalmethodsinordertoverifysafetyofstructures.The“safetyfactor”,whichaccordingtomoderntrendsisindependentofthenatureandcombinationofthematerialsud,canusuallybedefinedastheratiobetweentheconditions.Thisratioisalsoproportionaltotheinveroftheprobability(risk)offailureofthestructure.
Failurehastobeconsiderednotonlyasoverallcollapofthestructurebutalsoasunrviceabilityor,accordingtoamorepreci.Commondefinition.Asthereachingofa“limitstate”whichcaustheconstructionnottoaccomplishthetaskitwasdesignedfor.Therearetwocategoriesoflimitstate:
(1)Ultimatelimitsate,whichcorrespondstothehighestvalueoftheload-bearingcapacity.Examplesincludelocalbucklingorglobalinstabilityofthestructure;failureofsomectionsandsubquenttransformationofthestructureintoamechanism;failurebyfatigue;elasticorplasticdeformationorcreepthatcauasubstantialchangeofthegeometryofthestructure;andnsitivityofthestructuretoalternatingloads,tofireandtoexplosions.
(2)Servicelimitstates,whicharefunctionsoftheuanddurabilityofthestructure.Examplesincludeexcessivedeformationsanddisplacementswithoutinstability;earlyorexcessivecracks;largevibrations;andcorrosion.
Computationalmethodsudtoverifystructureswithrespecttothedifferentsafetyconditionscanbeparatedinto:
(1)Deterministicmethods,inwhichthemainparametersareconsideredasnonrandomparameters.
(2)Probabilisticmethods,inwhichthemainparametersareconsideredasrandomparameters.
Alternatively,withrespecttothedifferentuoffactorsofsafety,computationalmethodscanbeparatedinto:
(1)Allowablestressmethod,inwhichthestresscomputedundermaximumloadsarecomparedwiththestrengthofthematerialreducedbygivensafetyfactors.
(2)Limitstatesmethod,inwhichthestructuremaybeproportionedonthebasisofitsmaximumstrength.Thisstrength,asdeterminedbyrationalanalysis,shallnotbelessthanthatrequiredtosupportafactoredloadequaltothesumofthefactoredliveloadanddeadload(ultimatestate).
Thestresscorrespondingtoworking(rvice)conditionswithunfactoredliveanddeadloadsarecomparedwithprescribedvalues(rvicelimitstate).Fromthefourpossiblecombinationsofthefirsttwoandcondtwomethods,wecanobtainsomeufulcomputationalmethods.Generally,twocombinationsprevail:
(1)deterministicmethods,whichmakeuofallowablestress.
(2)Probabilisticmethods,whichmakeuoflimitstates.
Themainadvantageofprobabilisticapproachesisthat,atleastintheory,itispossibletoscientificallytakeintoaccountallrandomfactorsofsafety,whicharethencombinedtodefinethesafetyfactor.probabilisticapproachesdependupon:
(1) Randomdistributionofstrengthofmaterialswithrespecttotheconditionsoffabricationanderection(scatterofthevaluesofmechanicalpropertiesthroughoutthestructure);
(2) Uncertaintyofthegeometryofthecross-ctionsandofthestructure(faultsandimperfectionsduetofabricationanderectionofthestructure);
(3) Uncertaintyofthepredictedliveloadsanddeadloadsactingonthestructure;
(4)Uncertaintyrelatedtotheapproximationofthecomputationalmethodud(deviationoftheactualstressfromcomputedstress).
Furthermore,probabilistictheoriesmeanthattheallowableriskcanbebadonveralfactors,suchas:
(1) Importanceoftheconstructionandgravityofthedamagebyitsfailure;
(2)Numberofhumanliveswhichcanbethreatenedbythisfailure;
(3)Possibilityand/orlikelihoodofrepairingthestructure;
(4) Predictedlifeofthestructure.
Allthefactorsarerelatedtoeconomicandsocialconsiderationssuchas:
(1) Initialcostoftheconstruction;
(2) Amortizationfundsforthedurationoftheconstruction;
(3) Costofphysicalandmaterialdamageduetothefailureoftheconstruction;
(4) Adverimpactonsociety;
(5) Moralandpsychologicalviews.
Thedefinitionofalltheparameters,foragivensafetyfactor,allowsconstructionattheoptimumcost.However,thedifficultyofcarryingoutacompleteprobabilisticanalysishastobetakenintoaccount.Forsuchananalysisthelawsofthedistributionoftheliveloadanditsinducedstress,ofthescatterofmechanicalpropertiesofmaterials,andofthegeometryofthecross-ctionsandthestructurehavetobeknown.Furthermore,itisdifficulttointerprettheinteractionbetweenthelawofdistributionofstrengthandthatofstressbecaubothdependuponthenatureofthematerial,onthecross-ctionsand