FAQ 000242 EN

Add-on modules RF-DYNAM Pro - Natural Vibrations RF-DYNAM Pro - Equivalent Loads RF-DYNAM Pro - Forced Vibrations

How many mode shapes do I need to consider for an earthquake analysis?

Answer

In accordance to the EN 1998.-1 you need to calculate so many mode shapes that the sum of the effective modal masses is at least 90 % of the effective total mass (this is usually the overall structural mass). This can be regulated differently in other earthquake standards.

In the module RF-DYNAM Pro you find those values together with the frequencies in the Dynamic Load Cases (DLCs) in a tab called 'Mode Shapes', please see attached picture. You can check whether or not you calculated a sufficient number of eigenvalues before starting the calculation, it might be that you have ti increase the number of eigenvalues.

After calculation you find the effective modal masses and the factors in Table 5.7 5.7 Effective Modal Mass Factors.

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