桥梁施工方法英语文章

四川大学锦城学院桥梁施工方法专业：土木工程姓名：傅志翔学号：090310913BridgeConstructionTechniquesThefinalcostofabridgeisthesumofthecostofpermanentmaterials,theproportionatecosttotheprojectofplantandtemporaryworksandthecostoflabor.Thecostofpermanentmaterialscanbeestimatedreasonablycorrectly.Withexperience,abridgecontractorcandealcompletelywithcostofplantandtemporaryworks.Butthelaborcostdoesnotlenditselftoexactanalysis.Recentcompetitivedesignshaveattemptedtointroduceinnovationsinconstructionmethodswithaviewtoeffecteconomyinthecostonlaborbyreducingtemporaryworksandbyminimizingthedurationofsitework.Thesuitabletechniquesofconstructionofbridgesuperstructurewillvaryfromsitetosite,andwilldependonthespansandlengthofthebridge,typeofthebridge,materialsusedandsiteconditions.Forinstance,cast-in-siteconcreteconstructioncouldbeadoptedforshortspansupto40m,iftheriverbedisdryforaconsiderateportionoftheyear,whereasfreecantileverconstructionwithprestressedconcretedeckingwouldbeappropriateforlongspansinriverswithnavigationalrequirements.Thecurrenttrendistowardstheavoidanceofstagingasmuchaspossibleandtouseprecastorprefabricatedcomponentstomaximumextent.Also,constructionmachinerysuchascranesandlaunchinggirdersarecomingintowideruse.Thesearegreatersavingstobeeffectedbypayingattentiontothemethodofconstructionevenfromthedesignstagethanbyattackingpermanentmaterials.ShortSpanBridgesForbridgesinvolvingspansupto40m,thesuperstructuremaybebuiltonstagingsupportedontheground.Alternatively,thegirdersmaybeprecastforthefullspanlengthanderectedusinglaunchinggirdersorcranes,ifthebridgehasmanyequalspans.Inthelatterprocedure,theadditionalcostonerectionequipmentshouldbelessthanthesavinginthecostofformworkandinthelabourcostresultingfromfasterconstruction.LongSpanConcreteBridgesLongspanconcretebridgesareusuallyofpost-tensionedconcreteandconstructedeitherasconditionsbeamstypesorasfreevercantilestructures.Manymethodshavebeendevelopedforcontinuousdeckconstruction.Iftheclearancebetweenthegroundandbottomofthedeckissmallandthesoilisfirm,thesuperstructurecanbebuiltonstaging.Thismethodisbecomingobsolete.Currently,free-cantileverandmovablescaffoldsystemsareincreasinglyusedtosavetimeandimprovesafety.Themovablescaffoldsystememploysmovableformsstiffenedbysteelframes.Theseformsextendonespanlengthandaresupportedbysteelgirderswhichrestonapieratoneendandcanbemovedfromspantospanonasecondsetofauxiliarysteelgirders.Aneconomicalconstructiontechniqueknownasincrementalpush-launchingmethoddevelopedbyBaur-LeonhardteamisshownschematicallyinFigure22.1.Thetotalcontinuousdeckissubdividedlongitudinallyintosegmentsof10to30mlengthdependingonthelengthofspansandthetimeavailableforconstruction.Eachofthesesegmentsisconstructedimmediatelybehindtheabutmentofthebridgeinsteelframedforms,whichremaininthesameplaceforconcretingallsegments.Theformsaresodesignedastobecapableofbeingmovedtransverselyorrotatedonhingestofacilitateeasystrippingaftersufficienthardeningofconcrete.Attheheadofthefirstsegment,asteelnoseconsistingofalighttrussisattachedtofacilitatereachingofthefirstandsubsequentpierswithoutincludingatoolargecanyilevermomentduringconstruction.Thesecondandthefollowingsegmentsareconcreteddirectlyonthefaceofthehardenedportionandthelongitudinalreinforcementcancontinueacrosstheconstructionjoint.Thepushingisachievedbyhydraulicjackswhichactagainsttheabutment.SincethecoefficientoffrictionofTeflonslidingbearingsisonlyabout2percent,lowcapacityhydraulicjackswouldsufficetomovethebridgeevenoverlonglengthsofseveralhundredmetres.Thismethodcanbeusedforstraightandcontinuouslycurvedbridgesuptoaspanofabout120m.Thefree-cantileversystemwaspioneeredbyDyckerhoffandWillmanningermany.Inthissystem,thesuperstructureiserectedbymeansofcantilevertruckinsectionsgenerallyof3.5m.Thecantilevertruck,whosecostisrelativelysmallandwhichisattachedfirmlytopermanentconstruction,ermitsbyrepeatedusetheconstructionoflargebridges.Theavoidanceofscaffoldfrombelow,thespeedofworkandthesavinginlabourcostresultintheconstructionbeingveryeconomicdal.Thefree-cantileversystemisideallysuitedforlaunchedgirderswithalargedepthabovethepiercantileversystemisideallysuitedforlaunchedgirderswithalargedepthabovethepiercantileveringtothemiddleofthespan.Anothertechniqueistheuseofthepneumaticcaisson.Thecaissonisahugecylinderwithabottomedgethatcancutintothewaterbed.Whencompressedarispumpedintoit,thewaterisforcedout.Caissonsmustbeusedwithextremecare.foronething,workerscanonlystayinthecompressionchamberforshortperiodsoftime.Foranother,iftheycomeuptonormalatmosphericpressuretoorapidly,theyaresubjecttothebends,orcaissondiseaseasitisalsocalled,whichisacripplingorevenfatalconditioncausedbyexcessnitrogenintheblood.WhentheEadsBridgeacrosstheMississippiRiveratSt.Louiswasunderconstructionbetween1867and1874,atatimewhenthedangerofworkingincompresedairwasnotfullyunderstood,fourteendeathswascausedbythebends.Whenextrastrengthisnecessaryinthepiers,theysometimeskeyedintothebedrock-thatis,theyareextendeddownintothebedrock.ThismethodwasusedtobuildthepiersfortheGoldenGateBridgeinSanFrancisco,whichissubject