Mainshock-aftershock effect on the seismic performance of multi-story CBFs equipped with SMA-friction damping braces

This paper aims to understand the effect of mainshock-aftershock earthquake sequences on the seismic performance of multi-story concentrically braced frames (CBFs) equipped with shape memory alloy (SMA) friction damping braces (SMAFDBs), based on a comparison with those equipped with buckling-restrained braces (BRBs). The SMAFDB is a serial connection of SMA damper and friction damper. The friction damper starts to slip and serves as the “fuse” element to protect the SMA damper under destructive earthquake events. Ten ground motion records corresponding to the maximum considered earthquake (MCE) hazard level are defined as the mainshock input, and the aftershock input is generated by appropriately scaling the mainshock input to four seismic hazard levels. To achieve a fair comparison, the considered CBFs are first designed to exhibit identical mean height-wise peak interstory drift ratios under the design-basis earthquake (DBE) ground motions. Then, these two frames are subjected to four suites of mainshock-aftershock earthquake sequences and their seismic responses are compared against each other. It indicates that both the peak and residual interstory drift ratios of the SMAFDB frame (SMAFDBF) are smaller than that of the BRB frame (BRBF) on average during the aftershock earthquakes, which is found primarily attributed to the fact that the former suffers from smaller post-mainshock residual deformation. The higher seismic capacity of the SMAFDBF over the BRBF is also confirmed by conducting fragility analysis. Finally, the recentering capacity under aftershock earthquakes are analyzed for both frames, based on the maximum residual interstory drift ratios.