Evaluation of deflection amplification factor for steel buckling restrained braced frames

In the common force-based design method prescribed in seismic codes, displacements of a structure under reduced seismic forces are amplified by a deflection amplification factor, Cd, to estimate inelastic displacements. The present study evaluates Cd for steel buckling restrained braced frames (BRBFs). For this purpose, six low-to mid-rise structures comprising 2- to 12-story BRBFs are considered, and nonlinear dynamic analyses are performed. Results indicate that the recommended Cd by ASCE 7 (i.e., Cd = 5.0) underestimates the inelastic maximum interstory drift ratio (IMIDR) of lower stories in BRBFs under the design earthquake. Furthermore, estimation of IMIDR in upper stories of mid-rise structures using Cd = 5.0 leads to acceptable results. Therefore, by applying the particle swarm optimization (PSO) algorithm, a new equation is proposed for Cd to more realistically estimate IMIDR in steel BRBFs under the design earthquake. In the case of roof displacement, the results show that for low-rise structures the recommended value of Cd = 5.0 significantly underestimates the inelastic maximum roof drift ratio (IMRDR). On the other hand, for taller structures Cd = 5.0 overestimates the IMRDR. Thus, another new equation is proposed using the PSO algorithm for Cd Roof in order to better estimate the IMRDR. Moreover, effects of variation in strain hardening ratio on Cd and Cd Roof are investigated. In the last part of this study, variations of the ratios of Cd and Cd Roof to response modification coefficient (R) with ductility reduction factor (Rμ) are investigated.