A Replacement for the 30%, 40%, and SRSS Rules for Multicomponent Seismic Analysis

A response spectrum rule for combining the contributions from three orthogonal components of ground motion to the maximum value of a response quantity is presented. This rule, denoted CQC3, is compared to the 30% and 40% rules and the square-root-of-sum-of-squares (SRSS) rule currently specified in many design codes. It is shown that these current rules are special cases of the CQC3 rule, when certain conditions regarding the nature of the ground motion or the structural response are satisfied. Because these conditions are not always satisfied, it is argued that the CQC3 rule should be adopted as a general rule for the multicomponent combination problem. The CQC3 rule additionally offers a simple formula for determining the most critical orientation of the ground motion components for each response quantity of interest. The CQC3 rule is computationally simple and easy to implement in standard dynamic analysis codes.