Combination Rules Used to Account for Orthogonal Seismic Effects: State-of-the-Art Review

During an earthquake, buildings are simultaneously excited by multiple (horizontal and vertical) ground motion components. In the design process, the combinatorial effects of multicomponent horizontal loading are commonly determined using simplified methods such as the percentage (e.g., 100%–30%, 100%–40%) and square root sum of squares (SRSS) rules. In this paper, the state of the art in assessing the effectiveness of combination rules for orthogonal seismic effects is first reviewed. Prior studies are categorized based on the adopted analysis method (e.g., response spectrum versus response history), type of structural system, presence of structural irregularities, and considered demand parameters. A recent attempt to evaluate the combination studies using strong motion data is also discussed. Subsequently, the adoption of combination rules in popular codes and standards is presented and discussed including the conditions that trigger their implementation. Based on the review of prior research on the topic, it is concluded that the structural engineering community has not reached a consensus on the relative effectiveness of the various orthogonal combination methods. The paper concludes with a comprehensive discussion that covers specific challenges, knowledge gaps, and suggestions for future research.