Probabilistic evaluation of combination rules that account for orthogonal seismic effects in torsionally irregular structural design

Buildings are simultaneously subjected to multidirectional excitations of earthquakes. Combination rules are commonly used to account for orthogonal seismic effects in building structural design. Still, current seismic design codes have conflicts on whether the orthogonal seismic effects should be considered for torsionally irregular structures. In this study, the influence of implementing combination rules in seismic design of torsionally irregular structures on their collapse risk is investigated. The associated stipulations in modern design codes are reviewed at first. Three special concentrically braced frame (SCBF) buildings with different height and torsional irregularity are designed in accordance with ASCE 7 where the orthogonal seismic effects are not considered. For comparison purposes, torsional-irregular SCBF building cases are redesigned with consideration of orthogonal seismic effects. Incremental dynamic analyses are performed and the effectiveness of combination rules are probabilistically evaluated. The results show that implementation of combination rules will lead to conservative designs for torsional irregular structures.