In which case do I use which superposition expression for the modal response spectrum method?
The complete quadratic combination (Complete Quadratic Combination, CQC) is used if in the analysis of spacial models with mixed torsional / translational eigenvectors there are adjacent mode shapes, whose periods differ by less than 10 %.
If this is not the case, the quadratic combination (SRSS) is used.
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Dynamic and seismic analysis including time history analysis and multi-modal response spectrum analysis