Question:
How many mode shapes do I need to consider in the dynamic analysis?
Answer:
According to EN 1998‑1, enough 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.
The input window of RF‑/DYNAM Pro contains the "Mode Shapes" tab under Dynamic Load Cases, where the frequencies and effective modal mass factors are summarized in a table. This allows you to check before the calculation whether it is necessary to determine the required number of eigenvalues or to increase the number.
After the calculation, the effective modal masses and the factors are displayed in Table 5.7 Effective Modal Mass Factors.
This FAQ lists the causes of small effective modal mass factors and the possible solutions.