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DevelopmentofageotechnicalcentrifugeinHongKongC.K.Shen,X.S.Li,C.W.W.NgandP.A.VanLaakHongKongUniversityofScienceandTechnology,ClearWaterBay,Kowloon,HongKongB.L.KutterUniversityofCalifornia,DavisUSAK.CappelandR.C.TauscherTeamCorporation,USAABSTRACTAnadvanced400gtongeotechnicalcentrifugeisbeingdevelopedfortheHongKongUniversityofScienceandTechnologyHKUST.Thenominal8metrediameterbeamcentrifugewillbeinstalledatHKUSTinearly1998.Afeatureuniquetothiscentrifugeisitscapabilitytosimulatedynamicproblemsintwohorizontaldirectionsusinganinflightbiaxialhydraulicshaker.Forstatictests,thecentrifugecanbeoperatedatupto150g,whereasthebiaxialshakerisdesignedforshakingtestsatupto75g.1INTRODUCTIONThedecadeofthe90shaswitnessedamarkedincreaseincentrifugemodellingofgeotechnicalstructuresforresearchandengineeringpractice.Centrifugemodellingconstitutesapowerfultoolforthestudyofmanypreviouslyintractablegeotechnicalproblems,andisusefulfordevelopinganunderstandingofthebasicmechanicalbehaviouroflargescalegeotechnicalsystems,boththroughdirectphysicalanalogysimilitudeandscalinglawsandthroughverificationandcalibrationofcomputerprogramsusedforsubsequentanalysisofprototypesystems.Centrifugemodellingalsofindsutilityasasupplementtoconventionaldesignandanalysistechniques.Significantprogresshasbeenmadeinrecentyearstoimprovetheartandscienceofgeotechnicalcentrifugemodelling.HKUSTisdevelopingitsnewcentrifugefacilitytotakeadvantageoftheserecentadvancesininstrumentationandmodellingtechniquesand,inanefforttocontributetothecontinuingevolutionofcentrifugetechnology,hasundertakenthedevelopmentofanewbiaxialcentrifugeshaker.Withtheintentofdevelopingastateoftheartcapability,thecentrifugehasbeendesignedtomeetthefollowingperformancegoalsaccommodationoflargemodelcontainerstopermitmoredetailedmodellingandtomakeroomforextensiveinstrumentationwithinthemodel,provisionforextensiveonarminstrumentationfordetailedmeasurementofmodelparametersandsimulationoffieldloadingandconstructionprocesses,improvementofearthquakeloadingsimulationandtheabilitytogeneratebiaxialshakingintheplaneperpendiculartothegravityfield.ThedesignandperformancespecificationsfortheHKUSTcentrifugewereconceivedaftertakingtheseobjectivesintoconsideration.Inthispaper,ageneraldescriptionofthecentrifugefacilityalongwithspecificationsanddetailsofthecentrifugeandthebiaxialearthquakehydraulicshakeraregiven.FundingfortheGeotechnicalCentrifugeFacilitywasprovidedjointlybytheResearchGrantCouncilandbytheUniversity.ThecentrifugeandbiaxialshakerweredesignedandsuppliedbyTeamCorporation,BurlingtonWashington,USA.ThiscompanyhasmanyyearsofexperienceinservohydraulicsandhasparticipatedinthedevelopmentofseveralcentrifugeshakersKetcham,1991Kutter,1994VanLaak,1996.ThisfacilityisdedicatedtoservenotonlytheUniversitybutalsothegeotechnicalcommunityinHongKongandaroundtheworld.2THECENTRIFUGEATHKUSTThecentrifugeisinstalledinanewbuilding,locatedashortdistancefromthemaincampus.ThebuildinghousestheGeotechnicalCentrifugeFacilityGCFandtheWindandWaveTunnelFacilityWWTF.Forsafetyreasons,thecentrifugeishousedundergroundinacircularchamber.Anetworkofsteelconduitsisattachedtotheinnersurfaceofthechamberwall,throughwhichchilledwateriscirculatedtocoolthechamberasnecessaryduringcentrifugeoperation.AnelevationandfloorplanofthecentrifugelaboratoryareshowninFigures1and2.Thecentrifugefacilityhasatotalof255m2ofofficeandgenerallaboratoryspace.Inthemainlaboratoryarea,a20tonnecapacityoverheadgantrycraneisavailabletomovetheprecastconcretepanelsabovethecentrifugeenclosureandtoloadandunloadthecentrifugemodelcontainers.Thecraneisalsousedtointerchangethestaticplatformandshakerwhenrequired.Theunusedplatformorshakerisstoredinarecessinthefloorofthecentrifugeenclosure.ThecentrifugeismonitoredusingCCTVcamerasandmicrophones,andanintercomisusedtocommunicatebetweenthecentrifugechamberandcontrolroomduringmodelcheckout.Thehydraulicpowersupplyislocatedbelowthemainlaboratoryareainaroomadjacenttothecentrifuge.Table1summariessomekeyspecificationsforthecentrifuge.Forstatictests,thecentrifugecanbeoperatedatupto150gwhereasfordynamictests,thebiaxialshakerisdesignedtooperateatupto75g.Intotal,threeswingingplatformshavebeenmanufactured.Twoplatformsareidenticalandaredesignedfornonshakingtests.Eachofthesestaticplatformscanaccommodateamodelofupto1.5mx1.5mx1minsizeandupto40,000Ninweight.Thethirdplatformcomprisesthebiaxialshakerandassociatedstructuralsupports,hydraulicmanifoldsandreactionmass.Theshakersliptablecanaccommodatepayloadsofupto0.6mx0.6mx0.4mandupto3000Ninweight.Duetothelargemassofthedynamicplatform,forstatictestsathigherthan75gthedynamicplatformmustbereplacedwithastaticplatform.2.1MainbearinganddriveFigure3showsanelevationsectionofthecentrifuge.ThecentrifugearmissupportedonaconventionalshaftrunninginapairofpreloadedTimkentaperedrollerbearings.Thecentrifugeisdrivenbyahydraulicradialpistonmotordirectlycoupledtothelowerendoftheverticaldriveshaft.Thisdrivewasselectedbecausethetorqueappliedtotheendofthearmbytangentialshakinggreatlyexceedstherequireddrivetorque.Ageareddrivewouldberapidlydamagedbysuchanoscillatingtorqueunlessanexpensivehydraulicclutchwereprovidedtodisconnectthegearboxduringshaking.Inordertopreventdamagetothehydraulicmotorandshaftcoupling,oilflowtothedrivemotorisbypassedduringthebriefperiodinwhichtheshakerisoperated,ineffectpermittingthecentrifugetofreewheel.Uponcompletionofshaking,flowtothedrivemotorisreestablished.Themaximumpermissibleimbalanceat150gis222kN,correspondingtoaweightimbalanceof149kg.Balancingofthecentrifugeisaccomplishedusingweightsofvarioussizesplacedontheplatformnotusedforcarryingthemodelcontainer.CarefulbookkeepingisusedtoensurethecentrifugeiscloselybalancedtomaintainstresseswithinFigure1ElevationviewofthecentrifugefacilityatHKUSTTable1.Technicalspecificationsforthecentrifuge.KeyitemSpecificationPayloadcapacity400gtonsArmradius3.82mtothecentreoftheswingingplatformMaximumacceleration150gPayloadsize1.5mx1.5mx1mforstatictests,0.6mx0.6x0.4mfordynamictestsFigure2Planviewstructurallimitsandtoprolongbearinglife.Straingaugesmountedonthecentrifugeareusedtomonitorquasistaticstructuralloadsarisingfromcentrifugalforcesandpayloadimbalanceaswellasdynamicloadsduringshaking.2.2StructuralarmandmountingThebaseofthecentrifugeisataperedsteelweldmentusedtotransmitthestaticanddynamicloadsfromtherotatingarmtothebuildingfoundation.Thestructuralarmisasteelweldmentrigidlyattachedtothemainshaft.Itsmainfunctionistosupportthefourtensionstrapsandtotransmitthedrivetorquefromthemainshafttotheplatforms.Thetensionstrapsarefixedtothestructuralarmatoneendonlyandarerestrainedagainstradialdisplacementsbybearingsplacedalongtheirlength.Withthisarrangement,anyunbalancedloadsfromthetwoswingingbucketsaretransmittedtothemainshaftthroughthestructuralarm,whileintheabsenceofunbalanceforcesi.e.aperfectlybalancedconditionthelargecentrifugalforcesbetweenthepayloadandthecounterweightaretransmittedentirelythroughthesetoffourtensionstraps,andthebendingmomentappliedtothemainshaftiszero.Apartfromdynamicforcesgeneratedduringshaking,themajorbendingmomentonthemainshaftisduetotheimbalanceforces.2.3SuspensionofplatformsToavoiddeleteriousmaterialpropertychangesandstressraisers,thehighlystressedtensionstrapsaremachinedfromhighstrengthsteelwithoutwelding.Therelianceonunweldedloadpathsiscontinuedintheconnectionbetweenthetensionstrapsandtheplatformstheswingingplatformiscarriedbyamultiplicityofhighstrengthrodsinsertedthroughpivotblocksthroughwhichpassthetrunnionsattheendsofthetensionstraps.Eachoftheserodsisloadedbynomorethan13.3kN,whichgivesafactorofsafetyofabout3sincetheultimatestrengthofeachisover40kN.Thisstructuralsubsystemisverytolerantofslightmisalignment,suchasthatcausedbysmallrotationsoftheplatformstructure.Foursetsof20rodseachareprovided.Toaccommodatetherequirementoftangentialexcitationoftheshaketable,therecoiloftheplatformrequiresanextradegreeoffreedomofrotationinthesuspension,achievedbytheinsertionofasphericaljointbetweenthepivotblockinthemaintrunnion,andthetwopairsofhingedblocksattheedgesoftheplatform.Eachplatformisthenallowedtwodegreesofrotationalfreedom,inthetwoorthogonaldirectionsofshakerdisplacement.Thedynamicmomenttransmittedfromtheshakertothecentrifugearmisthereforedeterminedbytheslidingfrictionintheplatformsuspensionpivots.Allslidingsurfacesaretreatedwithaproprietarycoatingwhichresultsinafrictioncoefficientof0.03.Figure3TheHKUST400gtonCentrifugeSincethetensionrodsarenotsuitablefortransmissionoftheshearforcesrequiredtoovercomepivotfriction,theyareencasedinrectangularstructuraltubingmemberswhichareslideablyattachedtothepivotblocks,butnottotheplatform,sothattheyarenotstressedintensionwhentherodsstretchunderload.2.4CentrifugeControlSystemThecentrifugedriveusesavariablevolumepressurecompensatedpumpdrivenby200hpelectricmotor.Pressurizedoilfromthemainpumpiscirculatedthroughthehydraulicradialpistonmotorinaclosedhydrauliccircuit.Thevolumeflowrateoftheoilinthiscircuitiscontrolledbyaservovalvewhichcontrolstheangleofthepumpswashplateanddeterminestherotationalspeedofthecentrifuge.Themaindrivepumpforthecentrifugeoperatesatanominalpressureof21Mpa,whilethepumpusedforpoweringtheshakerandforcontrollingthedrivepumpswashplateoperatesat35Mpa.ThehydraulicpumpsforthecentrifugedriveandshaketableshareacommonreservoirandarelocatedinthecentrifugepowerroomFig.1.Ananalogsignalproportionaltothedesiredrotationalspeedisappliedtotheservovalveinthehighpressurehydrauliccircuitattachedtothemainpumpswashplate.Inadditiontomodulatingtheflowrateofoiltothecentrifugedrivemotor,theswashplateanglealsodeterminestheoilpressurewithinthemaindrivecircuit.Sincetheswashplatehasaveryhighslewratewithfulltravelachievablewithinafewmilliseconds,thepotentialforoverpressurizationofthemaindriveloopexistsunlesstheentirehydraulicdrivesystemiscarefullycontrolled.Inordertominimizethetimerequiredtoreachasteadystate,ashapedspeedcommandratherthanalinearrampisused.Thispermitsthespeedtobeincreasedquicklywhenthecentrifugeisrotatingslowlyandthedragforcesaresmall,andlimitsderivativesinspeedunderhighdragconditions,preventingexcessiveoperatingtorquesandhydraulicpressures.Additionally,hydraulicpressureinthedrivecircuitismeasuredandusedtoattenuatethedrivesignaltopreventexcessivepressure.Acomputerprogramisusedtoimplementthecontroldescribedabove.Besidescontrollingthespeedofthecentrifuge,thecomputerisusedtomonitoroperationalparameterssuchasimbalanceforcesinthearm,temperatureofthemainbearings,hydraulicfluidtemperatureandpressure,andstatusofsafetyinterlocks.Thecomputerisalsousedtoimplementthesequenceofvalvingoperationsrequiredforoperatingtheshakersystemduringshakingtests.Inputdataandmeasuredparametersfromeachrunareautomaticallyloggedtothecomputerharddisktofacilitatelongtermmonitoringofmachineperformanceandschedulingofroutinemaintenance.3.BIAXIALSHAKERASSEMBLYTheHKUSTcentrifugeincorporatesabiaxialhydraulicshaker,tobeusedforsimulatedseismicexcitation.Inconsiderationofthefactsthatearthquakemotionsaremultidirectionalinnatureandthatmanyunidirectionalcentrifugeearthquakesimulatorsarealreadyavailable,HKUSTdecidedtodevelopabiaxialshakerinordertosimulateearthquakemotionsintwohorizontaldirectionssimultaneously.Becauselargeshakingforcesintwodirectionsarepossiblewiththisshaker,developmentofthecentrifugeandtheshakerwascarriedoutsimultaneously,withtheshakerdesignedasanintegralpartofthecentrifuge.Thisintegratedapproachtothedesignwasadoptedinordertoisolatetheshakingforcesfromthecentrifugetoaslargeanextentaspossible,andledtothearticulatedsuspensiondesigndescribedabove.Tofacilitateinstallationandmaintenance,andtopermitoperationofthecentrifugeataccelerationsgreaterthan75gforstatictests,theshaketableanditsbucketformasingleassemblythatisremovablefromthesuspensionarmsandreplaceablebyanoptionalstaticbucket.Withapayloadweightof3000N,thetotalmovingweightpayloadandshaketablehardwareisabout10,000N.Tooptimizethedynamicbehaviouroftheinflightshaker,alargereactionmass4000kghasbeenincorporatedintothedesign.Theshaketableissupportedbyhydrostaticselfaligningpadbearings.Comparedwithrubbershearpads,thehydrostaticbearingsprovidehighercompressivestiffness.Rubbershearpadslocatedunderneaththesliptableareonlyusedtoprovideanominalcenteringforcetothetable.Theshakerutilizestwopairsofservoactuatorsforeachoftheshakingdirectionsonepairinthetangentialdirectionofthecentrifugerotationandtheotherinthedirectionofbucketswingup.Eachpairofactuatorsislocatedonoppositesidesoftheshakingplatformandcorrespondingpairsaredesignedtoactasaunit,applyingidenticalforcestoeachsideofthesliptable.Themotionofthesliptableisthenostensiblyasuperpositionoftranslationsinthetwoorthogonaldirections,withnorotationsintheplaneofshaking.Hydrostaticpadbearingsareusedbetweeneachactuatorpistonandthesliptable.Thesebearingsproducealowfrictioninterfaceformotionsofthesliptabletransversetothepistonaxis,whileprovidingaverystiffconnectionbetweenthepistonandsliptablewhenloadedintheaxialdirection.
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