Response Spectrum Analysis

Response Spectrum Analysis

Response Spectrum Analysis (RSA) is used to calculate a system’s structural response to short, transient dynamic events, for example earthquakes. RSA is also used for DDAM shock analyses (

RSA is a type of restart analysis as it uses modal superposition to calculate the system’s response, after a modal analysis has been completed on the same structure.

RSA uses a design response spectrum as the input load, which is given as load (e.g. ground acceleration) vs frequency. RSA is defined in the frequency domain, rather than the time domain, and hence suitable for applications where the exact time history of the load applied is not known, thus making it impossible to perform a time domain analysis (transient analysis). Sometimes, design response spectra are provided in terms of period rather than frequency. Since one is the inverse of the other, the graph is just a mirror image when plotting on a logarithmic scale. For more information on frequency and period, please read: (

Design spectra usually consist of three input curves, one for each co-ordinate direction. A separate RSA is conducted for each direction. The final result of the RSA is a superposition of these three result sets. There are different mathematical methods for this superposition. The applicable building code usually specifies which method to apply.

RSA is a linear elastic dynamic analysis that cannot include any non-linearities in the FE model. For more information on non-linear behaviour, please look at our blog post (

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