S4-1NAS122,Section4,April2002SECTION4EFFECTIVEMASS
TABLEOFCONTENTS PagePARTICIPATIONFACTORTHEORY 4-3NASTRANCASECONTROLENTRY 4-6CASESTUDY 4-8APPLICATIONSININDUSTRY 4-18WORKSHOP19–EFFECTIVEMASS 4-19
WehaveseenthattheEigenvectorsthatwecalculateinaNormalModesAnalysisareindependentofeachotherandarearbitrarilyscaled.Untilweapplysomekindofloading–eithertransientorfrequencyresponse,thenitisverydifficulttopredictwhichmodeswillplayadominantpartinastructure.OnewaywecanhelppredictwhataretheimportantmodesistouseatechniquecalledModalParticipationFactor.WeknowthatlinearcombinationsofEigenvectorscanbeassembledtomakearbitraryshapes.InthiscasewesaythattheshapetobemadeisaRigidBodyVectorinthedirectionofresponseweareinterestedin.TheRigidBodyVectorisDRAndweassume whereeisavectorofscalingfactorsontheeigenvectorsF,i.e.asetofParticipationFactors.PARTICIPATIONFACTORTHEORY
PreMultiplybyFTM:WhereMiiisthediagonalmatrixofgeneralizedmassesforeachmodeIThetermFTMDRiscommonlyknownastheParticipationFactor.ThescalingfactoreiisthenascalingonthegeneralizedmassMiitoachievetheParticipationFactor.Wecanalsodefinea‘rigidbody’massMrinasimilarwaytoageneralizedmassasButSoPARTICIPATIONFACTORTHEORY(Cont.)
SothecontributionwhicheachmodeprovidestotherigidbodymassMris asMiiisadiagonalmatrixThisisknownasthemodaleffectivemass.IfweMassNormalizethe soparticipationFactorise,ModalEffectiveMassise2Themodaleffectiveweightismodaleffectivemassfactoredbygintheappropriateunits.PARTICIPATIONFACTORTHEORY(Cont.)
Thecommandhasthefollowingform:Examples:MEFFMASSMEFFMASS(GRID=12,SUMMARY,PARTFAC)Describers MeaningPRINT Writeoutputtotheprintfile.(Default)NOPRINT Donotwriteoutputtotheprintfile.PUNCH Writeoutputtot