Seismic retrofit design method for RC buildings using buckling-restrained braces and steel frames

Existing RC buildings conforming to relatively older regulations and codes may lack in seismic resistance. This is especially true for public buildings such as schools or hospitals, which demand efficient retrofitting to ensure safety during and after a seismic event. Although retrofit design with conventional braces (CB) has been practiced for decades, the unbalanced hysteresis behavior of CBs tends to result in damage concentrated in specific stories. Buckling-restrained braces (BRBs), a new generation of bracing system, may increase structural integrity and at the same time reduce seismic response in a building via energy absorption. However, when the maximum story drift exceeds the yield point of RC frame, all structural elements including BRBs lose horizontal stiffness, which may result in both damage at a specific story and residual deformation effects occurring after an earthquake. Therefore, BRBs may be applied to such buildings using elastically designed steel frames (SF).This paper discusses the evaluation of damage distribution and self-centering functions of the elastic steel frames that connect BRBs to RC frames. In addition, we propose a simplified method based on equivalent linearization to design the required amount of BRB and elastic SF capacity for retrofitting existing RC buildings. The results were confirmed by nonlinear time-history analysis using high-intensity seismic waves. The results show that RC buildings retrofit with BRBs respond as predicted by the proposed method and target story drift is obtained. The story drifts of the BRB retrofit model is significantly reduced relative to both the original building and the building retrofit with CBs. In addition, because BRBs are attached to an existing building by elastically designed steel frames, the significant effect of SF on the reduction of residual displacements was also shown and discussed.