How many mode shapes do I need to consider in the dynamic analysis?

Answer

According to EN 1998-1, so many modal forms should be used, that the sum of the effective modal masses is at least 90% of the effective total mass (usually this corresponds to the total mass of the structure). This regulation can be different in other seismic standards.

In the input window of RF-/DYNAM Pro, you can find a tab 'Mode Shapes' in the Dynamic Load Cases where frequencies and effective modal mass factors are summarized in a table. So you can check before the calculation whether you need to determine the required number of eigenvalues or if you need to increase the number.

After the calculation, the effective modal masses and the factors are displayed in the table 5.7 Effective modal mass factors.

This FAQ lists causes for small effective modal mass factors and possible solutions.


Keywords

Modal mass factor Seismicity Mode shape Dynamics Mass

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  • Updated 8 November 2020

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