When calculating mode shapes in RF‑/DYNAM Pro, I obtain effective modal mass factors that are all approximately 0. What is the reason?

Answer

There can be many reasons for the 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.
  • In the case of including these local eigenvectors in the calculation anyhow, 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.
  • In the case of the local mode shapes on members, it is recommended to deactivate the FE mesh division on members.
If you have paid attention to all of these notes, the global mode shapes of the structure should only be activated, which also activate a high mass.

Keywords

Effective modal mass Modal mass factor Mode shape Dynamic analysis Mass

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