Learning Forum

[BenjaminStarling]

nSome structures do not require that the total modal mass captured be 100% or close to it. For example, a steel structure sitting atop a very stiff concrete foundation. The concrete foundation is present in the analysis to accurately capture the boundary conditions of the structure. If we are only interested in analysing the structure between 0-30 Hz, and the concrete foundation has no mode shapes before 1000Hz, there is no need to ensure we capture this mode shape, it will not be excited by such low frequencies. But because concrete foundations generally have a mass comparable to the overal steel structure, this will result in a considerable difference in the modal mass captured.nBeyond this, your code currently will not work. Try the following.n/SOLnmodopt,lanb,60000nmxpand,,,,,0.001,YES,,EFFM,nmodseloption,yes,yes,yes,yes,yes,yesnRestarting the modal analysis to select expansion options makes no sense in this context. The code above will only expand modes if they are greater than 0.1% of the total mass, therefore you should only end up with 1000 modes maximum. You may find that you will end up with extraordinarliy high frequencies which may be unnecessary for your spectrum analysis. You may also find that you still don't acheive 100% modal effective mass. nAnother issue not discussed, you may have some loose mesh somewhere, producing many modes with a very small mass participation.n