公路线形设计外文翻译文献Word文件下载.docx
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GeometricDesignofHighways
AAlignmentDesign
ofaroadisshownontheplaneviewandisaseriesofstraightlinescalledtangentsconnectedbycircular.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;
curvedsectionsaresuperProvisionmustbemadeforgradualchangefromonetotheother.Thisusuallyinvolvesmaintainingthecenterlineofeachindividualroadwayatprofilegradewhileraisingtheouteredgeandloweringtheinneredgetoproducethedesiredsuperisattainedsomedistancebeyondthepointofcurve.
Ifavehicletravelsathighspeedonacarefullyrestrictedpathmadeupoftangentsconnectedbysharpcircularcurve,ridingisextremelyuncomfortable.Asthecarapproachesacurve,superbeginsandthevehicleistiltedinward,butthepassengermustremainverticalsincethereisoncentrifugalforcerequiringcompensation.Whenthevehiclereachesthecurve,fullcentrifugalforcedevelopsatonce,andpullstherideroutwardfromhisverticalposition.Toachieveapositionofequilibriumhemustforcehisbodyfarinward.Astheremainingsupertakeseffect,furtheradjustmentinpositionisrequired.Thisprocessisrepeatedinreverseorderasthevehicleleavesthecurve.Wheneasementcurvesareintroduced,thechangeinradiusfrominfinityonthetangenttothatofthecircularcurveiseffectedgraduallysothatcentrifugalforcealsodevelopsgradually.Bycarefulapplicationofsuperalongthespiral,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)addedwidthbecauseoftheslantedpositionofthefrontofthevehicletotheroadwaycenter.For24-ftroadways,theaddedwidthissosmallthatitcanbeneglected.Onlyfor30mphdesignspeedsandcurvessharperthan22°
doestheaddedwidthreach2ft.Fornarrowerpavements,however,wideningassumesimportanceevenonfairlyflatcurves.RecommendedamountsofandproceduresforcurvewideningaregiveninGeometricDesignforHighways.
BGrades
Theverticalalignmentoftheroadwayanditseffectonthesafeandeconomicaloperationofthemotorvehicleconstituteoneofthemostimportantfeaturesofroaddesign.Theverticalalignment,whichconsistsofaseriesofstraightlinesconnectedbyverticalparabolicorcircularcurves,isknownasthe“gradeline.”Whenthegradelineisincreasingfromthehorizontalitisknownasa“plusgrade,”andwhenitisdecreasingfromthehorizontalitisknownasa“minusgrade.”Inanalyzinggradeandgradecontrols,thedesignerusuallystudiestheeffectofchangeingradeonthecenterprofile.
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,maximumgradestypicallyrangefrom7to12percent,dependingontopography.
Whereverlongsustainedgradesareused,thedesignershouldnotsubstantiallyexceedthecriticallengthofgradewithouttheprovisionofclimbinglanesforslow-movingvehicles.Criticalgradelengthsvaryfrom1700ftfora3percentgradeto500ftforan8percentgrade.
Longsustainedgradesshouldbelessthanthemaximumgradeonanyparticularsectionofahighway.Itisoftenpreferredtobreakthelongsustaineduniformgradebyplacingsteepergradesatthebottomandlighteningthegradenearthetopoftheascent.Dipsintheprofilegradeinwhichvehiclesmaybehiddenfromviewshouldalsobeavoided.
Maximumgradeforhighwayis9percent.Standardssettingminimumgradesareofimportanceonlywhensurfacedrainageisaproblemaswhenwatermustbecarriedawayinagutterorroadsideditch.InsuchinstancestheAASHTOsuggestsaminimumof0.35%.
CSightDistance
Forsafevehicleoperation,highwaymustbedesignedtogivedriversasufficientdistanceorclearversionaheadsothattheycanavoidunexpectedobstaclesandcanpassslowervehicleswithoutdanger.Sightdistanceisthelengthofhighwayvisibleaheadtothedriverofavehicle.Theconceptofsafesightdistancehastwofacets:
“stopping”(or“nopassing”)and“passing”.
Attimeslargeobjectsmaydropintoaroadwayandwilldoseriousdamagetoamotorvehiclethatstrikesthem.Againacarortruckmaybeforcedtostopinthetrafficlaneinthep