英文翻译英文+中文.docx

英文翻译英文+中文.docx

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英文翻译英文+中文

OVERFLOWSPILLWAY

Anoverflowspillwayisasectionofdamdesignedtopermitwatertopassoveritscrest.Overflowspillwaysarewidelyusedongravity,arch,andbuttressdams.Someearthdamshaveaconcretegravitysectiondesignedtoserveasaspillway.Thedesignofthespillwayfortowdamsisnotusuallycritical,andavarietyofsimplecrestpatternsareused.Inthecaseoflargedamsitisimportantthattheoverflowingwaterbeguidedsmoothlyoverthecrestwithaminimumofturbulence.Iftheoverflowingwaterbreakscontactwiththespillwaysurface,avacuumwillformatthepointofseparationandcavitationsmayoccur.Cavitationsplusthevibrationfromthealternatesmakingandbreakingofcontactbetweenthewaterandthefaceofthedammayresultinseriousstructuraldamage.

Cavitiesfilledwithvapor,air,andothergaseswillforminaliquidwhenevertheabsolutepressureoftheliquidisclosetothevaporpressure.Thisphenomenon,cavitations,islikelytooccurwherehighvelocitiescausereducedpressure.Suchconditionsmayariseifthewallsofapassagearesosharplycurvedastocauseseparationofflowfromtheboundary.Thecavity,onmovingdownstream,mayenteraregionwheretheabsoluteismuchhigher.Thiscausesthevaporinthecavitytocondenseandreturntoliquidwitharesultingimplosion,orcollapse,extremelyhighpressureresult.Someoftheimplosiveactivitywilloccuratthesurfacesofthepassageandinthecrevicesandporesoftheboundarymaterial.Underacontinualbombardmentoftheseimplosions,thesurfaceundergoesfatiguefailureandsmallparticlesarebrokenaway,givingthesurfaceaspongyappearance.Thisdamagingactionofcavitationsiscalledpitting.

Theidealspillwaywouldtaketheformoftheundersideofthenappedofasharp-crestedweirwhentheflowratecorrespondstothemaximumdesigncapacityofthespillway.Moreexactprofilesmaybefoundinmoreextensivetreatmentsofthesubject.Thereversecurveonthedownstreamfaceofthespillwayshouldbesmoothandgradual;Aradiusofaboutone-fourthofthespillwayheighthasprovedsatisfactory.Structuraldesignofanogeespillwayisessentiallythesameasthedesignofaconcretegravitysection.Thepressureexertedonthecrestofthespillwaybytheflowingwaterandthedragforcescausedbyfluidfrictionareusuallysmallincomparisonwiththeotherforcesactingonthesection.Thechangeinmomentumoftheflowinthevicinityofthereversecurvemay,however,createaforcewhichmustbeconsidered.Therequirementsoftheogeeshapeusuallynecessitateathickersectionthantheadjacentnooverflowsections.

Asavingofconcretecanbeeffectedbyprovidingaprojectingcorbelontheupstreamfacetocontroltheflowinoutletconduitsthroughthedam,acorbelwillinterferewithgateoperation.

Thedischargeofanoverflowspillwayisgivenbytheweirequation

WhereQ=discharge,or

L=coefficient

h=headonthespillway（verticaldistancefromthecrestofthespillwaytothereservoirlevel）,m

Thecoefficient

varieswiththedesignandhead.Experimentalmodelsareoftenusedtodeterminespillwaycoefficient.EndcontractionsonaspillwayreducetheeffectivelengthbelowtheactuallengthL.Square-corneredpiersdisturbtheflowconsiderablyandreducetheeffectivelengthbythewidthofthepiersplusabout0.2hforeachpier.

Streamliningthepiersorflaringthespillwayentranceminimizestheflowdisturbance.Ifthecross-sectionalareaofthereservoirjustupstreamfromthespillwayislessthanfivetimestheareaofflowoverthespillway,theapproachvelocitywithincreasethedischargeanoticeableamount.Theeffectofapproachvelocitycanbeaccountedforbytheequation

where

istheapproachvelocity.

PROPERTIESOFCONCRETE

Thecharacteristicsofconcreteshouldbeconsideredinrelationtothequalityforanygivenconstructionpurpose.Theclosestpracticableapproachtoperfectionineverypropertyoftheconcretewouldresultinpooreconomyundermanyconditions,andthemostdesirablestructureisthatinwhichtheconcretehasbeendesignedwiththecorrectemphasisoneachofthevariouspropertiesoftheconcrete,andnotsolelywithaviewtoobtaining,say,themaximumpossiblestrength.

Althoughtheattainmentofthemaximumstrengthshouldnotbethesolecriterionindesign,themeasurementofthecrushingstrengthofconcretecubesorcylindersprovidesameansofmaintainingauniformstandardofquality,and,infact,istheusualwayofdoingso.Sincetheotherpropertiesofanyparticularmixofconcretearerelatedtothecrushingstrengthinsomemanner,itispossiblethatasasinglecontroltestitisstillthemostconvenientandinformative.

Thetestingofthehardenedconcreteinprefabricatedunitspresentsnodifficulty,sincecompleteunitscanbeselectedandbrokenifnecessaryintheprocessoftesting.Samplescanbetakenfromsomepartsofafinishedstructurebycuttingcores,butatconsideronecostandwithapossibleweakeningofthestructure.Itiscustoms,therefore,toestimatethepropertiesoftheconcreteinthestructureontheoasisofthetestsmadeonspecimensmoundedfromthefreshconcreteasitisplaced.ThesespecimensarecompactedandcuredinastandardmannergiveninBS1881in1970asinthesetworespectsitisimpossibletosimulateexactlytheconditionsinthestructure.Sincethecrushingstructureisalsoaffectedbythesizeandshapeofaspecimenorpartofastructure,itfollowsthatthecrushingstrengthofacubeisnotnecessarythesameasthatofthemassofexactlythesameconcrete.

Crushingstrength

Concretecanbemadehavingastrengthincompressionofuptoabout80N/

orevenmoredependingmainlyontherelativeproportionsofwaterandcement,thatis,thewater/cementratio,andthedegreeofcompaction.Crushingstrengthsofbetween20and50N/

at28daysarenormallyobtainedonthesitewithreasonablygoodsupervision,formixesroughlyequivalentto1:

2:

4ofcement:

sand:

coarseaggregate.Insometypesofprecastconcretesuchasrailwaysleepers,strengthsrangingfrom40to65N/

at28daysareobtainedwithrichmixeshavingalowwater/cementratio.

Thecrushingstrengthofconcreteisinfluencedbyanumberoffactorsinadditiontothewater/cementratioandthedegreeofcompaction.Themoreimportantfactorsare

Typeofcementanditsquality.Boththerateofstrengthgainandtheultimatestrengthmaybeaffected.

Typeandsurfacetextureofaggregate.Thereisconsiderableevidencetosuggestthatsomeaggregatesproduceconcreteofgreatercompressiveandtensilestrengthsthanobtainedwithsmoothrivergravels.

Efficiencyofcuring.Alossinstrengthofuptoabout40percentmayresultfromprematuredryingout.Curingisthereforeofconsiderable,importancebothinthefieldandinthemakingoftests.ThemethodofcuringconcretetestcubesgiveninBS1881should,forthisreason,bestrictlyadheredto.

TemperatureIngeneral,therateofhardeningofconcreteisincreasedbyanincreasetemperature.Atfreezingtemperaturesthecrushingstrengthmayremainlowforsometime.

AgeUndernormalconditionsincreaseinstrengthwithage,therateofincreasedependingonthetypeofcementwithage.Forinstance,highaluminacementproducesconcretewithacrushingstrengthat21hoursequaltothatofnormalPortlandcementconcreteat28days.Hardeningcontinuesbutatamuchslowerrateforanumberofyears.

Theabovereferstothestaticultimateload.Whensubjectedtorepeatedloadsconcretefailsataloadsmallerthantheultimatestaticload,afatigueeffect.Anumberofinvestigatorshaveestablishedthatafterseveralmillioncyclesofloading,thefatiguestrengthincompressionis50-60percentoftheultimatestaticstrength.

Tensileandflexuralstrength

Thetensilestrengthofconcretevariesfromone-eighthofthecompressivestrengthatearlyagestoaboutone-twentiethlater,andisnotusuallytakenintoaccountinthedesignofreinforcedconcretestructures.Thetensilestrengthis,however,ofconsiderableimportanceinresistingcrackingduetochangesinmoisturecontentortemperature.Tensilestrengthtestsareusedforconcreteroadsandairfields.

Themeasurementofthestrengthofconcreteindirecttensionisdifficultandisrarelyattempted.Twomorepracticalmethodsofassessingtensilestrengthareavailable.Onegivesameasureofthetensilestrengthinbending,usuallytermedtheflexuralstrength.BS1881:

1970givesdetailsconcerningthemakingandcuringofflexuretestspecimens,andofthemethodtest.Thestandardsizeofspecimenis150

×150

×750

longforaggregateofmaximumsize40

.Ifthelargestnominalsizeoftheaggregateis20

specimens100

×100

×750

longmaybeused.

Aloadisappliedthroughtworollersatthethirdpointsofthespanuntilthespecimenbreaks.Theextremefiberstresses,thatis,compressiveatthetopandtensileatthebottom,canthenbecomputedbytheusualbeamformulae.Thebeamwillobviouslyfailintensionsincethetensilestrengthismuchlowerthanthecompressivestrength.FormulaeforthecalculationofthemodulusofrupturearegiveninBS1881:

1970.

Testspecimensistheformofbeamsaresometimesusedtomeasurethemodulusofruptureorflexuralstrengthquicklyonthesite.Thetwohalvesofthespecimenmaythenbecrushedsothatbesidestheflexuralstrengththecompressivestrengthcanbeapproximatelydeterminedonthesamesample.ThetestisdescribedinBS1881:

1970.

Valuesofthemodulusofruptureareutilizedinsomemethodsofdesignofunreinforcedconcreteroadsandrunways,inwhichrelianceisplacedonthef