土木工程专业英语Part.ppt

土木工程专业英语 天津大学尹越 Part1 EarthquakeEngineeringandStructureDynamics TextandReadingMaterial Unit6DynamicAnalysisonStructuresUndampedSystemUnit20EarthquakesandEarthquake ResistantStructuresANewKindofEarthquake ResistantBuilding RelatedBookorJournal AnilK Chopra DynamicsofStructures TheoryandapplicationstoEarthquakeEngineering 清华大学出版社 2005 5EarthquakeEngineeringandStructureDynamics JournalofInternationalAssociationofEarthquakeEngineering IAEE JohnWiley Sons Inc TextDynamicAnalysisonStructures Itisnotalwayspossibletoobtainrigorousmathematicalsolutionsforengineeringproblems Infact analyticalsolutionscanbeobtainedonlyforcertainsimplifiedsituations dynamic static TextDynamicAnalysisonStructures Forproblemsinvolvingcomplexmaterialproperties loading andboundaryconditions theengineerintroducesassumptionsandidealizationsdeemednecessarytomaketheproblemmathematicallymanageablebutstillcapableofprovidingsufficientlyapproximatesolutionsandsatisfactoryresultsfromthepointofviewofsafetyandeconomy TextDynamicAnalysisonStructures materialpropertydensity elastic Young s modulus poisson sratio yieldstress shearmodulusloadingdeadload liveload windload loaddistribution concentrateload lineload uniformloadsectionpropertyarea momentofinertia sectionmodulus radiusofgyration firstmomentofarea TextDynamicAnalysisonStructures Thelinkbetweentherealphysicalsystemandthemathematicallyfeasiblesolutionisprovidedbythemathematicalmodelwhichisthesymbolicdesignationforthesubstituteidealizedsystemincludingalltheassumptionsimposedonthephysicalproblem TextDynamicAnalysisonStructures DegreeofFreedomInstructuraldynamicsthenumberofindependentcoordinatenecessarytospecifytheconfigurationorthepositionofasystematanytimeisreferredtoasthenumberofdegreesoffreedom SDOF MDOFsystem TextDynamicAnalysisonStructures Ingeneral acontinuousstructurehasaninfinitenumberofdegreesoffreedom Nevertheless theprocessofidealizationorselectionofanappropriatemathematicalmodelpermitsthereductioninthenumberofdegreesoffreedomtoadiscretenumberandinsomecasestojustasingledegreeoffreedom TextDynamicAnalysisonStructures Fig 6 1showssomeexamplesofstructureswhichmayberepresentedfordynamicanalysisasone degree of freedomsystem thatis structuresmodeledassystemswithasingledisplacementcoordinate Fig figure Eq equation table TextDynamicAnalysisonStructures Theseone degree of freedomsystemsmaybedescribedconvenientlybythemathematicalmodelshowninFig 6 2whichhasthefollowingelements 1 amasselementmrepresentingthemassandinertialcharacteristicsofthestructures 2 aspringelementkrepresentingtheelasticrestoringforceandpotentialenergycapacityofthestructure TextDynamicAnalysisonStructures 3 adampingelementcrepresentingthefrictionalcharacteristicandenergylossesofthestructure 4 anexcitationforceF t representingtheexternalforcesactingonthestructuresystem TheforceF t iswrittenthiswaytoindicatethatitisafunctionoftime Internalforce axialforce bendingmoment shear TextDynamicAnalysisonStructures InadoptingthemathematicalmodelinFig 6 2 itisassumedthateachelementinthesystemrepresentsasingleproperty thatis themassmrepresentsonlythepropertyofinertialandnotelasticityorenergydissipation whereasthespringkrepresentsexclusivelyelasticityandnotinertiaorenergydissipation Finally thedamperconlydissipatesenergy TextDynamicAnalysisonStructures Thereadercertainlyrealizesthatsuch pure elementsdonotexistinourphysicalworldandthatmathematicalmodelsareonlyconceptualidealizationsofrealstructures Assuch mathematicalmodelsmayprovidecompleteandaccurateknowledgeofthebehaviorofthemodelitself butonlylimitedorapproximateinformationonthebehavioroftherealphysicalsystem TextDynamicAnalysisonStructures Nevertheless fromapracticalpointofview theinformationacquiredfromtheanalysismayverywellbesufficientforanadequateunderstandingofthedynamicbehaviorofthephysicalsystem includingdesignandsafetyrequirements TextDynamicAnalysisonStructures D alembert sPrincipleAnapproachtoobtaintheequationofthemotionistomakeuseofD Alembert sPrinciplewhichstatesthatasystemmaybesetinastateofdynamicequilibriumbyaddingtotheexternalforcewhichiscommonlyknownastheinertialforce TextDynamicAnalysisonStructures Fig 6 3showsthefreebodydiagramwithinclusionoftheinertialforce Thisforceisequaltothemassmultipliedbytheacceleration andshouldalwaysbedirectednegativelywithrespecttothecorrespondingcoordinate displacement velocity accelerationnegative positive TextDynamicAnalysisonStructures TheapplicationofD Alembert sPrincipleallowsustouseequationsofequilibriuminobtainingtheequationofmotion Forexample inFig 6 3 thesummationofforcesintheydirectiongivesdirectly whichistheequationofmotionforone degree of freedomsystem TextDynamicAnalysisonStructures TheuseofD Alembert sPrincipleinthiscaseappearstobetrivial Thiswillnotbethecaseforamorecomplexproblem inwhichtheapplicationofD Alembert sPrinciple inconjunctionwiththePrincipleofVirtualWork constitutesapowerfultoolofanalysis TextDynamicAnalysisonStructures Aswillbeexplainedlatter thePrincipleofVirtualWorkisdirectlyapplicabletoanysysteminequilibrium Itfollowsthenthatthisprinciplemayalsobeapplicabletothesolutionofdynamicproblems providingthatD Alembert sPrincipleisusedtoestablishthedynamicequilibriumofthesystem TextEarthquakesandEarthquake ResistantStructures Destructiveearthquakesoccurasaresultofsuddenreleaseofenergystoredintheearth scrustinweakzonesknownasgeologicfaults Uponthereleaseoftheenergy arupturedevelopsthatmayextendforseveralhundredkilometersalongthefault kN kilo Newton kg kilogram km kilometer TextEarthquakesandEarthquake ResistantStructures Therupturemayoccurinasufficientlydeeplocationfromthegroundsurfacesothatitmaynotappearonthesurface Whentheoppositesidesoftherupturedzonemovelongitudinallyagainsteachother theearthquakemotioniscalled strike slip whereas whenthedislocationofthesidesoccursacrossthewidthoftherupturedzone themotionis dip slip longitudinal lateral vertical horizontal TextEarthquakesandEarthquake ResistantStructures Earthquakesareoftenmeasuredbytheirmagnitudes ThemagnitudeisameasureofenergyreleaseandisdefinedwiththeRichterscale Magnitudesof4ormorearebelievedtobesignificanttotheengineeringdesign Themagnitudeisaconstantvaluethatdescribeshowlargeanearthquakeis TextEarthquakesandEarthquake ResistantStructures Inengineeringdesign however theearthquakeintensity ratherthanthemagnitude isused Theintensityisameasureofearthquake sdestructiveness Theintensitydependsprimarilyonthemagnitudeandthedistancebetweentheepicenterandthelocationwheretheintensityisevaluated TextEarthquakesandEarthquake ResistantStructures ThecommonmeasurefortheintensityistheModifiedMercalliIntensityScale However inengineeringdesign thegroundaccelerationisoftenusedastheintensityandappliedtothestructureasthebaseexcitationintensitylevel Empiricalrelations knownasattenuationequation relatetheintensityatadesiredlocationtothemagnitudeanddistance TextEarthquakesandEarthquake ResistantStructures Earthquakedamagetoastructuredependsmainlyontheresponseofthestructuretothedynamicforcesarisingfromthegroundshaking Theresponseofthestructuredependsonmanyfactorsincludingthegroundmotionpeakvalueaswellasitsfrequencyandduration shakingtabletest TextEarthquakesandEarthquake ResistantStructures Also thenaturalfrequencyofthebuilding soilconditionatthesiteofthebuilding constructionpractice andoveralldesignofthebuildinginfluencetheresponsetotheearthquakeloads Naturalfrequencyofthebuilding inturn dependsonitsgeometryandthetypesofconstructionmaterialused TextEarthquakesandEarthquake ResistantStructures Becauseofthevarietyoffactorsinvolvedintheresponseofabuildingtotheearthquakeloads itisdifficulttogeneralizethetypeofdamagethatmayoccurtothebuilding Rather thepotentialearthquakedamagehastobeinvestigatedonacase by casebasis TextEarthquakesandEarthquake ResistantStructures Damagetobuildingsvariestoagreatextentwiththebuilding sabilitytodissipateenergyanddampenthevibrationduringanearthquake Asdescribedearly thenaturalfrequencyofabuildingandthefrequencyandthedurationofthegroundshakingalsoplayimportantrolesinthedamage TextEarthquakesandEarthquake ResistantStructures Thebuilding sgeometryandthestructuralmaterialinfluenceitsductility dampingability andnaturalfrequencyofvibration Thus dependingonthetypeofbuilding thedamagecanrangefromonlyafewcrackstomajorcracksand ortotalcollapseofthestructure TextEarthquakesandEarthquake ResistantStructures Buildingswithhighnaturalfrequencyofvibration i e shortperiodofvibration exhibitmuchstifferbehaviorduringanearthquake Thisisespeciallytrueforlow risebuildings suchasresidentialbuildings shoppingcenters etc whichoftenpossessanaturalperiodofvibrationof0 006to0 25sec i e thatistosay inotherwordsetc etcetera TextEarthquakesandEarthquake ResistantStructures Thestifferbehaviormaycausethebuildingtosuffermoredamageduringanearthquake Insuchacase however iftheductility i e theabilityofthestructuretodeformastheloadapplies isincreased thebuildingbecomeslessstiffandthuslessvulnerabletopotentialearthquakedamage TextEarthquakesandEarthquake ResistantStructures Whenthenaturalperiodofvibrationofthebuildingisshort thevibrationtendstoquicklyreachthestationarystage Asaresult thedamagetothebuildingwillprimarilydependonthedurationofthegroundshaking Withlongerduration thebuildingwillbemorelikelytosufferdamage TextEarthquakesandEarthquake ResistantStructures Buildingswithhigherductilityexhibitlongerperiodsofvibration Asaresult theyarelesslikelytosufferdamage Thisisespeciallytrueifthenaturalperiodofvibrationofthebuildingismuchlongerthantheperiodofthegroundvibration Intermsofindividualstructuralmembers amemberwithmoreductilitydeformsmoreandthusdelaysreachingthecriticalfailurestage TextEarthquakesandEarthquake ResistantStructures Thisisthetypeofbehaviorinmetalstructuresand tocertainextent intimberstructures However concreteandmasonrystructuresarelessductileandneedtobedesignedwithanappropriateamountofreinforcemen