Reliability-based assessment of percentage combination rules considering the collapse performance of special concentrically braced frames

Abstract

In the seismic design of structures, the combinatorial effects of multiple horizontal ground motion components are often determined using the 100/p% percentage rule (i.e. designs based on 100% in one direction and p% in the other). In this study, a reliability-based methodology is developed to explicitly link building collapse caused by percentage-rule-based-underestimation of the design force demands to the adopted p% value. The methodology is applied to three special concentrically braced frame (SCBF) buildings (4, 8, and 12 stories) with corner columns shared by orthogonal frames. Each building case is designed using the 100/30%, 100/65%, and 100/100% percentage rule. Response history analyses using bidirectional and unidirectional loading are performed on nonlinear structural models to obtain the “true” demands and percentage-rule-based demands, respectively. Models constructed with and without considering SCBF column failure are utilized within the overall reliability-based framework to isolate the effect of underestimated design force demands on collapse performance. A quantitative relationship between the adopted p% value and building structural collapse risk is established. The results show that there is a nonlinear relationship between an increase in p% and the benefits in terms of reduced collapse risk. Also, taller buildings are shown to benefit more from conservative (higher) p% values compared to shorter ones.