In case of an earthquake, the actual extent of excitation of the different natural modes is dependent on the direction of the seismic waves (ground accelerations), the corresponding mass participation and on the damping behavior of the structure. The analytic solutions for typical structures and unit impacts are provided in the design codes as relevant response spectra, specifying the relevant proportionality factors for the individual eigenmodes dependent on the natural frequency.
The calculated amplitudes related to the individual natural modes are superimposed using different methods described in literature. The program offers the ABS-method, the SRSS method, and the Complete Quadratic Combination (CQC).
Three separate calculations are provided to consider different possible earthquake directions, transverse, longitudinal and vertical directions. These different cases are combined to get finally the envelope of extreme values.