Seismic collapse and reparability performance of reinforced concrete frames retrofitted with external PBSPC BRBF sub-frame in near-fault regions

Abstract

In this paper, the effectiveness of using newly proposed novel external sub-structures (precast bolt-connected steel-plate reinforced concrete buckling-restrained-brace-frame, PBSPC BRBF) for reducing the collapse as well as post-earthquake demolition risks of old reinforced concrete frames within 20 km near-fault regions is investigated. Two old reinforced concrete frames representing typical school or hospital buildings in western China are selected and retrofitted via PBSPC BRBF. Elaborated numerical simulation models of the retrofitted frames are developed and, two sets of ground motions including 121 near-fault pulse-like ground motions and 40 non-pulse-like ground motions are selected for incremental dynamic analysis (IDA) of the frame models. Effects of the period of velocity pulse on collapse and demolition capacity of the frames are studied. Near-fault probabilistic seismic hazard analysis is conducted for four sites near a representative active fault and, combining with the IDA results, parameterized seismic collapse and demolition fragility surfaces of the frames are calculated. With the fragility surfaces, effects of site to fault distance on failure probabilities of the frames are discussed. Meanwhile, collapse and demolition risks of the frames under two hazard levels, namely design basic earthquake and maximum considered earthquake, are investigated. Results indicate that the median collapse and demolition capacity of the 6-story unretrofitted frame will decrease from 1.03 and 0.97 g to 0.91 and 0.87 g, respectively, when the site to fault distance decrease from 20 km to 5 km. The PBSPC BRBF can effectively increase the collapse and demolition capacity of the old frames. The 6-story frame at 10 km site can get 120.4% and 116.7% increased collapse and demolition capacity after retrofitting. The retrofitted frames can meet the collapse performance requirements and repairable performance requirements at sites larger than 10 km and 5 km, respectively. Research results in this study can provide basis for the seismic designing and decision making for retrofitting of old frames in near-fault regions.