Validation of modal analysis: total modal mass ratio

Dynamic analysis troubleshootingmodal analysis > total modal mass ratio

 

Check the total modal mass ratio

Standard validation criteria

Most seismic design codes define validity criteria on the cumulated modal mass ratio for the response spectrum method. Typically, the Eurocode states in EC-EN 1998-1 clause 4.3.3.3.1 that

(2)P The response of all modes of vibration contributing significantly to the global response shall be taken into account.

(3) The requirements specified in paragraph (2)P may be deemed to be satisfied if either of the following can be demonstrated:

  • the sum of the effective modal masses for the modes taken into account amounts to at least 90% of the total mass of the structure;
  • all modes with effective modal masses greater than 5% of the total mass are taken into account.

In most seismic design codes, only the 90% rule is mentioned.

Total modal mass ratio in SCIA Engineer

As of SCIA Engineer 19.0 patch 1, seismic load case settings include a mode filtering feature, which allows to automatically use only the most significant mode shapes. Modes that are not necessary to meet the total modal mass ratio criteria are filtered out (see mode filtering section in " Defining a seismic load case").

The total modal mass ratio can be found in two places in SCIA Engineer results:

Meeting relative modal mass criteria

It is not uncommon, that the 90% condition is not met at first when running the modal analysis. This can happen for various reasons:

Depending on the reason for the lack of relative modal mass, the solution can be either increasing the number of computed modes or modifying the model and calculation parameters to remove unwanted modes from the results.

 

Next step: check for immobilized masses

SCIA web

Downloads

SCIA is part of the Nemetschek group