Quadratic Combinations SRSS and CQC as "Equivalent Linear Combination"

Tips & Tricks

The modal results of a response spectrum analysis are combined with quadratic combination rules. In RF‑/DYNAM Pro, the SRSS and the CQC rule are available. The default setting modifies the quadratic expressions into equivalent linear combinations. The advantage of this option is that corresponding internal forces keep their signs and are often much smaller compared to the standard SRSS or CQC rule. The standard SRSS and CQC rule are on the conservative side and the “equivalent linear combinations” is recommended.

The figure provides the exact formulas and gives a specific example of this option when the SRSS rule is applied. Analogue this is working for the CQC rule.

Quadratic combinations, also in form of the equivalent linear combinations, are available in RFEM and RSTAB and can be applied to any result combination. A license for RF‑/DYNAM Pro is not essentially required.

The “equivalent linear combination” was first published by Prof. Katz in the German journal D‑A‑CH‑Mitteilungsblatt.


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