Answer:
There can be many reasons for small effective modal mass factors. This can often be observed in the case of large structures. In most cases, the reason for this problem is the fact that only local mode shapes occur. The following text describes how you can handle this:
- You should recognize from the result graphic whether the local mode shapes are really there. If the individual members or surfaces have a very low natural frequency, these occur first.
- If you still want these local eigenvectors to be included in the calculation, you should increase the number of mode shapes to be calculated.
- If the local mode shapes occur on surfaces, the masses of the affected surfaces can be neglected. This feature is described in this technical article.
- For local mode shapes on members, we recommend deactivating the FE mesh division on members.
If you have taken all of these tips into account, only global mode shapes of the structure should appear which also activate a high mass.
