Probabilistic Seismic Performance Assessment of Setback Buildings under Bidirectional Excitation

In this paper, the seismic performance of one-side setback structures is investigated with a probabilistic approach. The effects of the simultaneous action of two orthogonal ground motion components on the performance of structures with different setback ratios are assessed by studying (1)the limit-state capacities, (2)the mean annual frequencies of exceeding performance levels, and (3)the confidence levels in meeting performance objectives. The results show that the simultaneous influence of ground motion components on the seismic performance of these structures is significant over the entire range of structural responses from elasticity to global instability. In addition, the assessment of code-designed setback structures by incremental dynamic analysis demonstrates the poorer performance of these torsionally coupled structures relative to regular structure, depending on the setback ratio values. Therefore, the revision of seismic codes provisions for geometric vertical irregularities seems to be essential to stipulate more restrictive limits or apply more accurate analytical procedures to predict the seismic performance of setback structures under the simultaneous orthogonal seismic excitations, especially for structures with critical setback ratios.