橋梁與橋梁施工方法英語(yǔ)版

BridgesandtheBridgeConstructionTechniques

Part1.Bridges

SimpleTrussBridge

FormanyyearsAmericanbirdge-desigersexercisedtheiringenuityindevisingnewformsoftrussesandgirders,theprincipalobjectoftheirendeavorsbeingtofindformsinvolvingtheuseofthesmallestamountofmetal.Eachformasitappearedwastestedbysubjectingittotheordealofactualuse,whichshowedconclusivelybothitsmeritsanditsdefects;hence,byaprocessofelimination,basedupontheprincipleofthesurvivalofthefittest,afewformshavebeenretainedandothershabebeenrelegatedtothehistoryofbridge-buiding.Asmighthavebeenanticipated,thefewformswhichhavesurvivedarethesimplestofall;andalthoughevenatthepresenttimeonehearsoccasionallyofsomeimprovedformoftruss,theassumedimprovementrarelymaterializes.TheformsofturssthathavebestsurvivedthetestoftimearethePratt,Petit,andmanyothers:

ThePratttruss,isthetypemostcommonlyusedinAmericaforspansunder250feetinlength.Itsadvantagesaresimplicity,economonlyofmetal,andsuitabilityforconnectingtothefloorandlateralsystems.

ThePetittruss,isamodificationofthePratt,andisgenerallyusedforspansexceeding250-300feet.

Itiscomparativelysimple,and,likethePratttruss,itiseconomicalofmetalandlendsitselfreadilytotheconnectionofthefloorandlateralsystems.

CantileverBridge

Acantileverbridgeistheonewhosespanissupportedbycantileverswhichprojectfromthepiersonwhichitrestsandwhichmeetinthecenterofthespan,wheretheyjoinedtogether.Initsdesignthecantilevertakesvariousformsbutallofthesedependontheprincipleofbalanceaboutacommoncenter.

Therearetwomainadvantagesofthisformofbridge.First,withthismethodofbridgebuilding,itispossibletousesmallerandmorecompactpiersforaviaductthanwouldbethecaseifeachpierhadtocarrytheendsoftwoadjacentgirderspans.Secondly,whenarailwayrunsacrossawidewaterwayofwhichthedepthprecludesthesinkingofmanyoranyfoundationsforintermediatepiers,thisformofbridgemaybeadopter.

However,cantileverbridgesshouldneverbeadoptedunlesstheabove-mentioned

conditionsexist,becausetheyareinfeiorinrigiditytosimpleturssbridgesandusuallyrequiremoremetalfortheirconstruction.

ArchBridge

Fordeepgorgeswithrockysides,orforshallowstreamswithrockbottomandnaturalabutments,archesanreminentlyproperandeconomical.

Theadvantagesofthearchareapossibleeconomyincostofmetalandanaestheticappearance,whileitsdiaadvantagesarealackofrigidityand,formosttypes,anduncertainyconcerningthestressesinthemembers.

Whenbridgesfoundationshavetobebuiltonpilesoronanyothermaterialthatisliabletoslightsettlement,orwhentheabutmentscouldpossiblymovelaterallyevenameretrifle,itisnotpropertoadoptanarchsuperstructure;formaysettlementoranymotionwhatsoeverineitherpiersorabutmentswouldupsettheconditionsassumedforthecomputations,andthuscausetobeincreasedtoanrncertainamountsomeofthestressesforwhichthesuperstructurewasproportions.

Cable-StayedBridge

Duringthepastdecadecable-stayedbridgeshavefoundwideapplication,especiallyinWesternEurope,andtolesserextentinotherpartsoftheworle.Cable-stayedbridgesareconstructedalongastructuralsystemwhichconprisesanorthotropicdeckandcontinuousgirderswhicharesupportedbystays,i.einclinedcablespassingoverorattachedtotowerslocatedatthemainpiers.

Theideaofusingcablestosupportbridgespansisbynomeansnew,andanumberofexamplesofthistypeofconstructionwererecordedalongtimeago.Unfortunately,thesystemingeneralmetwithlittlesuccess,duetothefacethatthestaticswerenotfullyunderstoodandthatunsuitablematerialssuchasbarsandchainswereusedtoformtheinclinedsupportsofstays.

Wideandsuccessfulapplicationofcable-stayedsystemswasrealizedonlyrecently,withtheintroductionofhigh-strengthsteels,orthotropictypedecks,developmentofweldingtechniquesandprogressinstructualanalysis.Thedevelopmentandapplicationofelectroniccomputersopenedupnewandpracticallyunlimitedpossibilitiesfortheexactsolutionofthesehighlystaticallyindeterminatesystemsandforprecisestaticalanalysisoftheirthree-dimensionalperformance.

Theintroductionofthecable-stayedsysteminbridgeengineeringhasresultedinthecreationofnewtypesofstructureswhichpossesmanyexcellentcharacteristicsandadvantages.Outstandingamongthesearetheirstructuralcharacteristics,effciencyandwiderangeofapplication.

Thebasicstructuralcharacteristicsandreasonsfortherapiddevelopmentandsuccessofcable-stayedbridgesareasoutlinedbelow.

Cable-stayedbridgespresentaspacesystem,consistingofstiffeninggirders,steelorconcretedeckandsupportingpartsastowersactinginconpressionandinclinedcablesintension.Bytheirstructuralbehaviorcable-stayedsystemsoccupyamiddlepossitionbetweenthegirdertypeandsuspensiontypebridges.

Themainstructuralcharacteristicofthissystemistheintegralactionofthestiffeninggirdersandprestressedorpost-tensionedinclinedcables,whichrunfromthetowertopsdowntotheanchorpointsatthestiffeninggirders.Horizontalcompressiveforcesduetothecableactionaretakenbygirdersangdnomassiveanchoragesarerequired.Thesubstrucrure,threrfore,isveryeconomical.

Introductionoftheorthotropicsystemhasresultedinthecreationofnewtypesofsuperstructurewhichcaneasilycarrythehorizontalthrustofstaycableswithalmostnoadditionalmaterial,evenforverylongspans.

Inoldtypesofconventionalsuperstructuresthealab,stringers,floorbeamsandmaingirderswereconsideredasactingindependently,Suchsuper-structureswerenotsuitableforcable-stayedbridges.Withtheorthotropictypedeck,however,thestiffenedplatewithitslargecross-sectionalareaactsnotonlyastheupperchordofthemaingirdersbutalsoasthehorizontalplategeideragainstwindforces,givingmodernbridgesmuchmorela-teralstiffnessthanthewindbracingusedinoldsystems,Infact,inrothotropicsystems,allelementsoftheroadwayandsecondarypartsofthesuperstructurepraticipateintheworkofmainbridgesystem.Thisresultsinreductionofthedepthofthegirdersandeconomyinthesteel.

Anotherstructuralcharacteristicofthissystemisthatitiasgeometricallyunchangeableunderanyloadpositiononthebridge,andallcablesarealwaysinastateoftension.Thischaracteristicofthecable-stayedsystemspermitthentobebuiltfromrelativelylightflexibleelements-cables.

Theimportantcharacteristicsofsuchathree-dimensionalbridgeisthefullparticipationofthetransversestructuralpartintheworkofthemainstructureinthelongitudinaldirection.Thismeansaconsiderableincreaseinthemonmentoftheinertiaoftheconstruction,whichpermitsareductionofthedepthofthegirdersandaconsequentsavinginsteel.

Theorthotropicsystemprovidesthecontinuityofthedeckstructureatthetowersandintheventerofthemainapan.Thecontinuityofthebridgesuperstructureovermanyspanshasmanyadvantagesandisactuallynecessaryforagoodcable-stayedbridge.

Part2.BridgeConstructionTechniques

Thefinalcostofabridgeisthesumofthecostofpermanentmaterials,theproportionatecosttotheprojectofplantandtemporaryworksandthecostoflabor.Thecostofpermanentmaterialscanbeestimatedreasonablycorrectly.Withexperience,abridgecontractorcandealcompletelywithcostofplantandtemporaryworks.Butthelaborcostdoesnotlenditselftoexactanalysis.Recentcompetitivedesignshaveattemptedtointroduceinnovationsinconstructionmethodswithaviewtoeffecteconomyinthecostonlaborbyreducingtemporaryworksandbyminimizingthedurationofsitework.

Thesuitabletechniquesofconstructionofbridgesuperstructurewillvaryfromsitetosite,andwilldependonthespansandlengthofthebridge,typeofthebridge,materialsusedandsiteconditions.Forinstance,cast-in-siteconcreteconstructioncouldbeadoptedforshortspansupto40m,iftheriverbedisdryforaconsiderateportionoftheyear,whereasfreecantileverconstructionwithprestressedconcretedeckingwouldbeappropriateforlongspansinriverswithnavigationalrequirements.Thecurrenttrendistowardstheavoidanceofstagingasmuchaspossibleandtouseprecastorprefabricatedcomponentstomaximumextent.Also,constructionmachinerysuchascranesandlaunchinggirdersarecomingintowideruse.Thesearegreatersavingstobeeffectedbypayingattentiontothemethodofconstructionevenfromthedesignstagethanbyattackingpermanentmaterials.

ShortSpanBridges

Forbridgesinvolvingspansupto40m,thesuperstructuremaybebuiltonstagingsupportedontheground.Alternatively,thegirdersmaybeprecastforthefullspanlengthanderectedusinglaunchinggirdersorcranes,ifthebridgehasmanyequalspans.Inthelatterprocedure,theadditionalcostonerectionequipmentshouldbelessthanthesavinginthecostofformworkandinthelabourcostresultingfromfasterconstruction.

LongSpanConcreteBridges

Longspanconcretebridgesareusuallyofpost-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,whichissubjecttostrongtidiesandhighwinds,andislocatedinanearthquakezone.Thedrillingwascarriedoutunderwaterbydeep-seadivers.

Wherebedrockcannotbereached,pilesaredrivenintothewaterbed.Today,thepilesinconstructionareusuallymadeofprestressedconcretebeams.Oneingenioustechnique,usedfortheTappanZeeBridgeacrosstheHudsonRiverinNewYork,istorestahollowconcreteboxontopofalayerofpiles.Whentheboxispumpeddry,itbecomesbuoyantenoughtosupportalargeproportionoftheweightofthebridge(seeFig.22.3).

Eachtypeofbridge,indeedeachindividualbridge,presentsspecialconstructionproblems.Withsometrussbridges,thespanisfloatedintopositionafterthepiershavebeenerectedandthenraisedintoplacebymeansofjacksorcranes.Archbridgescanbeconstructedoverafalsework,ortemporaryscaffolding.Thismethodisusuallyemployedwithreinforcedconcretearchbridges.Withsteelarches,however,atechniquehasbeendevelopedwherebythefinishedsectionsareheldinplacebywiresthatsupplyacantileversupport.Cranesmovealongthetopofthearchtoplacenewsectionsofsteelwhilethetensioninthecablesincreases.

Withsuspensionbridges,thefoundionsandthetowersarebuiltfirst.Thenacableisrunfromtheanchorage-aconcreteblockinwhichthecableisfastened-uptothetowerandacrosstotheoppositetowerandanchorage.Awheelthatunwindswirefromareelqunsalongthiscable.Whenthereelreachestheotherside,anotherwireisplacedonit,andthewheelreturnstoitsoriginalposition.Whenallthewireshavebeenputinplace,anothermachinemovesalongthecabletocampactandtobindthem.Constructionbeginsonthedeckwhenthecablesareinplace,withworkprogressingtow