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In which case and which superposition expression should I use 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.

More information can be found in the literature mentioned below.


quadratic combination superposition expression square root of the sum of the squares SRSS CQC


[1]   Meskouris, K. (1999). Baudynamik, Modelle, Methoden, Praxisbeispiele. Berlin: Ernst & Sohn.
[2]   Eurocode 8: Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings; EN 1998‑1:2004/A1:2013


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