Seismic retrofit of torsionally coupled RC soft-storey building using short yielding core BRBs

Abstract

This study presents seismic retrofitting of medium-rise torsionally coupled Reinforced-Concrete (RC) non-ductile building with soft ground storey. An irregular RC building with T-shaped plan is considered for the present study. In this sample building the presence of plan irregularity results in the torsional coupling of lateral modes. The location of staircase in the sample building significantly enhances the plan irregularity. Therefore, the contribution of stiffness of the staircase slabs is considered in the overall stiffness of the numerical model. Buckling Restrained Braces (BRBs) with short yielding cores are adopted to improve the seismic performance of the sample building. Short yielding core BRBs are installed in the ground storey to control the seismic lateral response induced by the soft-storey and torsional coupling of lateral modes. Displacement-based design method is adopted for the design of short yielding core BRBs to enhance the stiffness and energy dissipation of the structure so that it can endure the considered earthquake without exceeding the target drift level. Nonlinear dynamic analysis of the sample building is carried out before and after retrofit under bidirectional horizontal ground excitations applied simultaneously. The seismic performance is assessed in terms of maximum inter-storey drift, torsional response, plastic hinge distribution, residual displacement. Maximum axial force distribution of BRBs, maximum ductility and cumulative ductility demand of BRBs are discussed for a critical ground motion record. Analysis of the results reveals that the installation of BRBs decouples the torsionally coupled lateral modes of the sample building; and decreases inter-storey drift, torsional response and residual displacement in the soft ground storey. Thus, the retrofit design considered in this study, enhances the overall structural performance of the sample building reducing the lateral response due to the combined effect of the soft-storey and torsional coupling of lateral modes.