Seismic Performance Evaluation of Non-ductile RC Frames Retrofitted with BRB and SMA Braces

Abstract

Around the world, a large percentage of reinforced concrete (RC) frame buildings constructed prior to the implementation of modern building codes tend to exhibit non-ductile behavior under earthquakes. Thus, there is a significant need of modern retrofit techniques for these non-ductile RC frames in order to reduce the extensive loss and casualties during future earthquakes. This thesis conducted seismic performance evaluation of an old non-ductile RC frame building before and after retrofit with buckling restrained braces (BRBs) and shape memory alloy (SMA) braces. In detail, this study includes contents as follows: (1) A beam-column joint model was developed to address the deficiencies in the non-ductile RC joints and was incorporated in the numerical model of a non-ductile RC frame using the Open System for Earthquake Engineering Simulation (OpenSees). The joint and frame models are validated using the results of an experimental joint test and a reference model. (2) To evaluate seismic performance of the as-built and retrofitted frames, a series of dynamic time history analyses using the Frequently Occurred Earthquake (FOE), Design Basis Earthquake (DBE) and Maximum Considered Earthquake (MCE) ground motions were performed. Analyses results show that the BRB and SMA bracing systems can both improve the performance of the non-ductile RC frame, with reduced peak and residual story drifts. It is found that the SMA braced frames are capable of achieving the same level of seismic response control as the BRB frame in terms of peak story drifts. Furthermore, the SMA braced frames have much smaller residual story drifts than the BRB frame. (3) The performance of the as-built and retrofitted frames was studied using the probabilistic evaluation approach. Incremental dynamic analysis was performed to obtain the structural demands, and regression analyses were conducted to determine the relationship between earthquake intensity and structural responses. Using fragility function and results of incremental dynamic analysis, seismic fragility curves of various performance levels were derived. The seismic fragility analyses, combined with seismic hazard analysis, are used to determine the annual probability of exceedance for various performance levels.