Seismic upgrading of multistory steel moment-resisting frames by installing shape memory alloy braces: Design method and performance evaluation

The installation of a damping bracing system is regarded as an effective means to upgrade the seismic capacity of steel moment-resisting frames (MRFs), a common structural form for buildings. However, conventional damping braces are incapable of eliminating residual deformation demand. Shape memory alloy (SMA) braces present an emerging seismic upgrading solution to this problem. However, the corresponding design method is yet to be developed. Therefore, this study proposes a simple and straightforward design method for seismic upgrading of MRFs through the installation of SMA braces. The maximum and residual interstory drift ratios are selected as performance target. Two benchmark frames with three or six stories are adopted to validate the proposed design method. A group of seismic ground motions corresponding to the design basis earthquake level are input to the frames with or without SMA braces. Seismic analysis results indicate that the frames equipped with properly designed SMA braces exhibit responses that satisfy the design target satisfactory. To quantify the seismic upgrading effect, peak and residual interstory drift ratios and peak floor acceleration demands are selected as critical factors to build single- or multiple-parameter indices. Based on the relevant performance indices, comparisons between the original and upgraded frames show that the overall seismic performance of MRFs can be improved by 10%-30% when SMA braces are installed. In addition, the reduction of residual deformation has a significant impact on the evaluation result because of the evident benefit to seismic performance.