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本科毕业设计（论文）

专业外文翻译

专业名称：

土木工程专业（道路与桥梁）

年级班级：

道桥08-5班

学生姓名：

指导教师：

二○一二年五月十八日

GeometricDesignofHighways

Theroadisonekindoflinearconstructionusedfortravel.Itismadeoftheroadbed,theroadsurface,thebridge,theculvertandthetunnel.Inaddition,italsohasthecrossingoflines,theprotectiveprojectandthetrafficengineeringandtheroutefacility.

Theroadbedisthebaseofroadsurface,roadshoulder,sideslope,sideditchfoundations.Itisstonematerialstructure,whichisdesignedaccordingtoroute'

splaneposition.Theroadbed,asthebaseoftravel,mustguaranteethatithastheenoughintensityandthestabilitythatcanpreventthewaterandothernaturaldisasterfromcorroding.

Theroadsurfaceisthesurfaceofroad.Itissingleorcomplexstructurebuiltwithmixture.Theroadsurfacerequirebeingsmooth,havingenoughintensity,goodstabilityandanti-slipperyfunction.Thequalityofroadsurfacedirectlyaffectsthesafe,comfortandthetraffic.

HighwaygeometrydesignstoconsiderHighwayHorizontalAlignment,VerticalAlignmenttwokindsoflinearandcross-sectionalcompositionofcoordination,butalsopayattentiontothesmoothflowofthelineofsight,etc.Determinetheroadgeometry,considerthetopography,surfacefeatures,rationaluseoflandandenvironmentalprotectionfactors,tomakefulluseofthehighwaygeometriccomponentsofreasonablesizeandthelinearcombination.

Design

Thealignmentofaroadisshownontheplaneviewandisaseriesofstraightlinescalledtangentsconnectedbycircular.Inmodernpracticeitiscommontointerposetransitionorspiralcurvesbetweentangentsandcircularcurves.

Alignmentmustbeconsistent.Suddenchangesfromflattosharpcurvesandlongtangentsfollowedbysharpcurvesmustbeavoided;

otherwise,accidenthazardswillbecreated.Likewise,placingcircularcurvesofdifferentradiiendtoend（compoundcurves）orhavingashorttangentbetweentwocurvesispoorpracticeunlesssuitabletransitionsbetweenthemareprovided.Long,flatcurvesarepreferableatalltimes,astheyarepleasinginappearanceanddecreasepossibilityoffutureobsolescence.However,alignmentwithouttangentsisundesirableontwo-laneroadsbecausesomedrivershesitatetopassoncurves.Long,flatcurvesshouldbeusedforsmallchangesindirection,asshortcurvesappearas“kink”.Alsohorizontalandverticalalignmentmustbeconsideredtogether,notseparately.Forexample,asharphorizontalcurvebeginningnearacrestcancreateaseriousaccidenthazard.

Avehicletravelinginacurvedpathissubjecttocentrifugalforce.Thisisbalancedbyanequalandoppositeforcedevelopedthroughcannotexceedcertainmaximums,andthesecontrolsplacelimitsonthesharpnessofcurvesthatcanbeusedwithadesignspeed.Usuallythesharpnessofagivencircularcurveisindicatedbyitsradius.However,foralignmentdesign,sharpnessiscommonlyexpressedintermsofdegreeofcurve,whichisthecentralanglesubtendedbya100-ftlengthofcurve.Degreeofcurveisinverselyproportionaltotheradius.

Tangentsectionsofhighwayscarrynormalcrossslope;

curvedsectionsaresuperelevated.Provisionmustbemadeforgradualchangefromonetotheother.Thisusuallyinvolvesmaintainingthecenterlineofeachindividualroadwayatprofilegradewhileraisingtheouteredgeandloweringtheinneredgetoproducethedesiredsuperelevationisattainedsomedistancebeyondthepointofcurve.

Ifavehicletravelsathighspeedonacarefullyrestrictedpathmadeupoftangentsconnectedbysharpcircularcurve,ridingisextremelyuncomfortable.Asthecarapproachesacurve,superelevationbeginsandthevehicleistiltedinward,butthepassengermustremainverticalsincethereisoncentrifugalforcerequiringcompensation.Whenthevehiclereachesthecurve,fullcentrifugalforcedevelopsatonce,andpullstherideroutwardfromhisverticalposition.Toachieveapositionofequilibriumhemustforcehisbodyfarinward.Astheremainingsuperelevationtakeseffect,furtheradjustmentinpositionisrequired.Thisprocessisrepeatedinreverseorderasthevehicleleavesthecurve.Wheneasementcurvesareintroduced,thechangeinradiusfrominfinityonthetangenttothatofthecircularcurveiseffectedgraduallysothatcentrifugalforcealsodevelopsgradually.Bycarefulapplicationofsuperelevationalongthespiral,asmoothandgradualapplicationofcentrifugalforcecanbehadandtheroughnessavoided.

Easementcurveshavebeenusedbytherailroadsformanyyears,buttheiradoptionbyhighwayagencieshascomeonlyrecently.Thisisunderstandable.Railroadtrainsmustfollowtheprecisealignmentofthetracks,andthediscomfortdescribedherecanbeavoidedonlybyadoptingeasementcurves.Ontheotherhand,themotor-vehicleoperatorisfreetoalterhislateralpositionontheroadandcanprovidehisowneasementcurvesbysteeringintocircularcurvesgradually.However,thisweavingwithinatrafficlane（butsometimesintootherlanes）isdangerous.Properlydesignedeasementcurvesmakeweavingunnecessary.Itislargelyforsafetyreasons,then,thateasementcurveshavebeenwidelyadoptedbyhighwayagencies.

Forthesameradiuscircularcurve,theadditionofeasementcurvesattheendschangesthelocationofthecurvewithrelationtoitstangents;

hencethedecisionregardingtheiruseshouldbemadebeforethefinallocationsurvey.TheypointofbeginningofanordinarycircularcurveisusuallylabeledthePC（pointofcurve）orBC（beginningofcurve）.ItsendismarkedthePT（pointoftangent）orEC（endofcurve）.Forcurvesthatincludeeasements,thecommonnotationis,asstationingincreases:

TS（tangenttospiral）,SC（spiraltocircularcurve）,CS（circularcurvetospiral）,andST（spiralgotangent）.

Ontwo-lanepavementsprovisionofawilderroadwayisadvisableonsharpcurves.Thiswillallowforsuchfactorsas

（1）thetendencyfordriverstoshyawayfromthepavementedge,

（2）increasedeffectivetransversevehiclewidthbecausethefrontandrearwheelsdonottrack,and（3）addedwidthbecauseoftheslantedpositionofthefrontofthevehicletotheroadwaycenterline.For24-ftroadways,theaddedwidthissosmallthatitcanbeneglected.Onlyfor30mphdesignspeedsandcurvessharperthan22°

doestheaddedwidthreach2ft.Fornarrowerpavements,however,wideningassumesimportanceevenonfairlyflatcurves.RecommendedamountsofandproceduresforcurvewideningaregiveninGeometricDesignforHighways.

2.Grades

Theverticalalignmentoftheroadwayanditseffectonthesafeandeconomicaloperationofthemotorvehicleconstituteoneofthemostimportantfeaturesofroaddesign.Theverticalalignment,whichconsistsofaseriesofstraightlinesconnectedbyverticalparabolicorcircularcurves,isknownasthe“gradeline.”Whenthegradelineisincreasingfromthehorizontalitisknownasa“plusgrade,”andwhenitisdecreasingfromthehorizontalitisknownasa“minusgrade.”Inanalyzinggradeandgradecontrols,thedesignerusuallystudiestheeffectofchangeingradeonthecenterlineprofile.

Intheestablishmentofagrade,anidealsituationisoneinwhichthecutisbalancedagainstthefillwithoutagreatdealofborroworanexcessofcuttobewasted.Allhaulsshouldbedownhillifpossibleandnottoolong.Thegradeshouldfollowthegeneralterrainandriseandfallinthedirectionoftheexistingdrainage.Inmountainouscountrythegrademaybesettobalanceexcavationagainstembankmentasacluetowardleastoverallcost.Inflatorprairiecountryitwillbeapproximatelyparalleltothegroundsurfacebutsufficientlyaboveittoallowsurfacedrainageand,wherenecessary,topermitthewindtocleardriftingsnow.Wheretheroadapproachesorfollowsalongstreams,theheightofthegradelinemaybedictatedbytheexpectedleveloffloodwater.Underallconditions,smooth,flowinggradelinesarepreferabletochoppyonesofmanyshortstraightsectionsconnectedwithshortverticalcurves.

Changesofgradefromplustominusshouldbeplacedincuts,andchangesfromaminusgradetoaplusgradeshouldbeplacedinfills.Thiswillgenerallygiveagooddesign,andmanytimesitwillavoidtheappearanceofbuildinghillsandproducingdepressionscontrarytothegeneralexistingcontoursoftheland.Otherconsiderationsfordeterminingthegradelinemaybeofmoreimportancethanthebalancingofcutsandfills.

Urbanprojectsusuallyrequireamoredetailedstudyofthecontrolsandfineradjustmentofelevationsthandoruralprojects.Itisoftenbesttoadjustthegradetomeetexistingconditionsbecauseoftheadditionalexpenseofdoingotherwise.

Intheanalysisofgradeandgradecontrol,oneofthemostimportantconsiderationsistheeffectofgradesontheoperatingcostsofthemotorvehicle.Anincreaseingasolineconsumptionandareductioninspeedareapparentwhengradesareincreaseingasolineconsumptionandareductioninspeedisapparentwhengradesareincreased.Aneconomicalapproachwouldbetobalancetheaddedannualcostofgradereductionagainsttheaddedannualcostofvehicleoperationwithoutgradereduction.Anaccuratesolutiontotheproblemdependsontheknowledgeoftrafficvolumeandtype,whichcanbeobtainedonlybymeansofatrafficsurvey.

Whilemaximumgradesvaryagreatdealinvariousstates,AASHTOrecommendationsmakemaximumgradesdependentondesignspeedandtopography.Presentpracticelimitsgradesto5percentofadesignspeedof70mph.Foradesignspeedof30mph,maximumgradesty