Cumulative Masses result from eigenvalue analysis

[zthaer]

hi everyone,

i m trying to understand why is the eigenvalue analysis result show a stable and constant cumulative masses values at 71.151765% with almost no change after the forth mode of vibration in x-axis direction , but reach a 93% in y - direction in the same mode of vibration.

please help me answering this question.

regards;

[huffte]

I may be misinterpreting your question, but it doesn't seem out of the ordinary. Can you plot the mode shapes and see if, in fact, they make sense.

[zthaer]

hi huffte ;
pleasure hearing from you again; what i m trying to say is that i need to capture 90 percent of the structure mass which is in my case seems impossible in the x-axis .
here is the Cumulative Masses result with its associated mode of vibrations:

1- https://drive.google.com/file/d/0BzZ5tJ ... sp=sharing
2- https://drive.google.com/file/d/0BzZ5tJ ... sp=sharing

and this is the total base reaction resulting from implementing the static analyses:

https://drive.google.com/file/d/0BzZ5tJ ... sp=sharing

[huffte]

Hi zthaer. Perhaps you simply need to solve for more modes. It is certainly possible that a significant portion of the mass could be effective at a period less than 0.087 seconds. Why don't you try increasing the number of eigenvalues from the default of 10 to, say 15 or 20 and see how much the x-direction mass participation increases? Of course, it is always a good idea to plot and animate your mode shapes to see if everything makes sense, and if not, then why.

[zthaer]

dear huffte;
what is the way to eliminate such unexpected mode shape?
ive just increase the number of eigenvalues but still got the same number each time.

[seismosoft]

Hi Zthaer,

Increase the number to eigenmodes to a really large value, e.g. 50. You might alos need to change the eigensolver algorith (from Lanczos to Ritz or the opposite)
Regards,
Seismosoft Support