专业英语翻译三

PAGEPAGE11专业英语翻译BridgeconstructionBridgeconstructioncanpresentextraordinarydifficulties.Usuallythefoundationsforthepiersmustrestonthebedrock,andoftenunderwater.Onetechniqueforworkingintheseconditionsisbymeansofacofferdam.Pilesusuallymadeofinterlockingsteelplatesaredrivenintothewaterbed.Thewateristhenpumpedoutfromwithintheareathathasbeenenclosed.Anothertechniqueistheuseofthepneumaticcaisson.Thecaissonishugecylinderwithabottomedgethatcancutintothewaterbed..Whencompressedairispumpedintoit,thewaterisforcedout.Caissonsmustbeusedwithextremecare.Foronething,workerscanonlystayinthecompressionchamberforshortperiodsoftime.Foranother,iftheycomeuptonormalatmosphericpressuretoorapidly,theyaresubjecttothebends,orcaissondiseaseasitisalsocalled,whichisacripplingorevenfatalconditioncausedbyexcessunderconstructionbetween1867and1874,atatimewhenthedangerofworkingincompressedairwasnotfullyunderstood,fourteendeathswerecausedbythebends.Whenextrastrengthisnecessaryinthepiers,theyaresometimeskeyedintothebedrock-thatis,theyareextendeddownintothebedrock.Thismethodwasusedtostrongtidesandhighwinds,andislocatedinanearthquakezone.Thedrillingwascarriedoutunderwaterbydeep-seadivers.Wherebedrockcannotbereached,pilesaredrivenintothewaterbed.Today,thepilesinconstructionareusuallymadeofpre-stressedconcretebeams.Oneingenioustechnique,usedfortheTappanZeeBridgeacrosstheHudsonRiverinNewYork,istorestahollowconcreteboxontopofalayerofpiles.Whentheboxispumpeddry,itbecomesbuoyantenoughtosupportalargeproportionoftheweightofthebridge(seeFig.22.3)Eachtypeofbridge,indeedeachindividualbridge,presentsspecialconstructionproblems.Withsometrussbridges,thespanisfloatedintopositionafterthepiershavebeenerectedandthenraisedintoplacebymeansofjacksorcranes.Archbridgescanbeconstructedoverafalsework,ortemporaryscaffolding.Thismethodisusuallyemployedwithreinforcedconcretearchbridges.Withsteelarches,however,atechniquehasbeendevelopedwherebythefinishedsectionsareheldinplacebywiresthatsupplyacantileversupport.Cranesmovealongthetopofthearchtplacenewsectionsofsteelwhilethetensioninthecablesincreases.Withsuspensionbridges,thefoundationsandthetowersarebuiltfirst.Thenacableisrunfromtheanchorage-aconcreteblockinwhichthecableisfastened-uptothetowerandacrosstotheoppositetowerandanchorage.Awheelthatunwindswirefromareelrunsalongthiscable.Whenthereelreachestheotherside,anotherwireisplacedonit,andthewheelreturnstoitsoriginalpositiontheotherside,anotherwireisplacedonit,andthechinemovesalongthecabletocompactandtobindthem.Constructionbeginsonthedeckwhenthecablesareinplace,withworkprogressingtowardthemiddlefromeachendofthestructure.Inallmethodsofconstruction,itisnecessarytodeterminethestressesanddeformationateverystageofconstruction.ConstructionTechniquesThedecisionofhowabridgeshouldbedependsmainlyonlocalconditions.Theseincludecostofmaterial,availableequipment,allowableconstructiontimeandenvironmentalrestriction.Sinceallthesevarywithlocationandtime,thebestconstructiontechniqueforagivenstructuremayalsovary.IncrementalLaunchingorPush-outMethodInthisformofconstructionthedeskispushedacrossthespanwithhydraulicramsorwinches(fig.22.1).Decksofpre-stressedpost-tensionedpre-castsegments,steelorboxgirdershavebeenerected.Usuallyspansarelimitedto50-60mtoavoidexcessivedeflectionandcantileverstresses,althoughgreaterdistanceshavebeenbridgedbyinstallingtemporarysupporttowers.Typicallythemethodismostappropriateforlong,Multiplanbridgesintherange300-600m,but,muchshorterandlongerbridgeshavebeenconstructed.Unfortunately,thisveryeconomicalmodeofconstructioncanonlybeappliedwhenboththehorizontalandverticalalignmentsofthedeckareperfectlystraight,oralternativelyofconstantradius.Wherepushinginvolvesasmalldownwardgrade(4-5%)thenabrakingsystemshouldbeinstalledtopreventthedeckslippingawayandheavybracingisthisneededattherestrainingpiers.Bridgelaunchingdemandsverycarefulsurveyingandsettingoutwithcontinuousandprecisechecksmadeofdeckdeflections.Alightaluminumorsteellaunchingnoseformstheheadofthedecktoprovideguidanceoverthepier.SpecialTeflonorchrome-nickelsteelplatebearingsareusedtoreduceslidingfrictiontoabout5%oftheweight,thusslenderpierswouldnormallybesupplementedwithbracedcolumnstoavoidcreakingandotherdamage.Thesecolumnswouldgenerallyalsosupportthetemporaryfrictionbearingsandhelpsteerthenose.Inthecaseofpre-castconstruction,ideallysegmentshouldbecastonbedsneartheabutmentsandtransferredbyrailtothepost-tensioningbed,theactualtransportdistanceobviouslybeingkepttotheminimum.Usuallyasegmentiscastagainstthefaceofthepreviouslyconcretedunittoensureagoodfitwhenfinallygluedinplacewithanepoxyresin.Ifthisprocedureisnotadopted,gapsofapproximately500mmshouldbeleftbetweensegmentswiththereinforcementrunningthroughandsubsequentlyfilledwithconcretebeforepost-tensioningbegins.Generallyallthesegmentsarestressedtogethertoformacompleteunit,butwhenaccessorspaceontheembankmentisatapremiumitmaybenecessarytolaunchthedeckintermittentlytoallowsectionstobeaddedprogressively.Thecorrespondingpre-stressingarrangements,bothforthetemporaryandpermanentconditionswouldbemorecomplicatedandcarefulcalculationsneededatallpositions.Theprincipaladvantageofthebridge-launchingtechniqueisthesavinginfalsework,especiallyforhighdecks.Segmentscanalsobefabricatedorprecutinaprotectedenvironmentusinghighlyproductiveequipment.Forconcretesegments,typicallytwosegmentsarelayereachweekendafterpost-tensioningincrementallylaunchedatabout20mperdaydependingonthewinchingequipment.SpecialRequirementsinAdvancedConstructionTechniquesTherearethreeimportantareasthattheengineeringandconstructionteamhastoconsider:Stressanalysisduringconstruction:becausetheloadingsandsupportconditionsofthebridgearedifferentfromthefinishedbridge,stressesineachconstructionstagemustbecalculatedtoensurethesafetyofthestructure.Forthispurpose,realisticconstructionloadsmustbeusedandsitepersonnelmustbeinformedonalltheloadinglimitations.Windandtemperatureareusuallysignificantforconstructionstages.Camberingordertoobtainabridgewiththerightelevation,therequiredcamberofthebridgeateachconstructionstagemustbecalculated.Itisrequiredthatdueconsiderationbegiventocreepandshrinkageoftheconcrete.Thiskindofcalculation,althoughcumbersome,hasbeensimplifiedbytheuseofcomputers.Qualitycontrol:thisisimportantforanymethodofconstruction,butitismoresoforthecomplicatedconstructiontechniques.Curingofconcrete,posttensioning,jointpreparations,etc.aredetrimentaltoasuccessfulstructure.Thedietpersonnelmustbemadeawareoftheminimumconcretestrengthsrequiredforpost-tensioning,formremoval,falseworkremoval,launchingandotherstepsofoperations.Generallyspeakingspeaking,theseadvancedconstructiontechniquesrequiremoreengineeringworkthantheconventionalfalseworktypeconstruction,butthesavingcouldbesignificant.桥梁建设桥建设在目前有一定的困难。通常桩的基础必须放在基岩上而岩石经常在水下。在这些环境条件里起作用的一种技术是通过一个围堰。由钢板的做成的围堰被敲进含水层。水从围堰里被抽出。另一种技术是使用气压沉箱。沉箱是有巨大刃脚的圆筒，它能够进入河床。当压缩的空气被泵送入它时，水被排挤出。在应用沉箱的过程必须非常小心。原因之一，工人只能呆在里面非常短时间。对于另一个来说如果他们太快的达到正常的大气压，他们受到非常大的压强，容易得潜水病也当时也叫沉箱病，有时甚至是致命的。这是由于血管中溶于过多的氮引起的的。在1867和1874之间在桥梁的建设中，当在压缩的空气里工作的危险没被完全了解时，有14起死亡事故是由这种原因引起的。当桥桩在外力作用进入岩层时，，它们有时嵌进那些基岩，它们向下延长进入那些基岩。旧金山的金门大桥就是用这种方法建造的。它能用于大风大浪的区域，且位于一个地震区域。深海潜水员在水下进行操纵这种技术。在基岩不能到达的区域桩被敲进含水层。今天，桩结构上通常由给预加压力的混凝土的梁做成。一种新颖的技术被用在纽约赫德森河上的桥上。将空心的混凝土箱放置在群桩上，箱子里的水被抽干的时候其受到的浮力能够承担起一定比例桥梁的重量。桥的每种类型，实际上每座桥，都有自己的施工方法。一些桁架桥，它们的跨度在桥桩的位置确定时已经确定。通过千斤顶或者起重机建造然后提高到位置之后，桁架被放到相应的位置。拱桥可以通过搭架或者暂时的脚手架上方建造。这种方法通常被用钢筋混凝土拱桥。然而对于刚架拱桥来说，已经发展了一种新技术，这种技术将已装好的部分借助于起着支撑作用的钢索控制就位。（钢拱在安装过程中还没有合拢前，是两个悬臂。需要用钢索拉着这两个悬臂以免倾倒）对于悬索桥，基础和塔是首先建造的。然后悬索是从塔的锚固端的混凝土块被固定到另一个塔的锚固端，。然后线轴沿着悬索移动，当线轴移动到另一端时。另一绳索被放在它上并且车轮返回它的原先的位置，当所有的绳索放置好时，另一种设备开始沿着悬索移动，并且捆他们。当绳索固定好时，桥面建造的工作也开始进行，在所有的建设方法中，确定每个阶段的应力和变形是必要的。施工技术桥梁的建设大都取决于当地的条件，这些条件包括材料的价格，可利用的设备，工期的长短，和当地的地形条件，所有的这些因素都随时间和地点的变化而变化对于同一个结构来说，优秀的施工方法也不止一种。顶推法施工在这种方法中，桥面结构被液压马达或者带液压装置的绞车从桥跨的一端推动到另一端。桥面结构一般是由预应力梁