Are you familiar with the Modal Analysis add-on for RFEM 6 / RSTAB 9 ? It allows you to easily determine natural frequencies and mode shapes.
In contrast, with the Response Spectrum Analysis add-on for RFEM 6 / RSTAB 9, you can perform seismic analyses using the multi-modal response spectrum analysis. Among others, the Chinese standard GB 50011 is implemented here.
Modal Analysis Add-on for RFEM 6 / RSTAB 9 Response Spectrum Analysis Add-on for RFEM 6 / RSTAB 9
