Seismic performance and fragility evaluation of assembly blind-bolt CFST composite frames under mainshock-aftershock sequences

This paper highlights the importance of considering semi-rigid characteristics of blind-bolt connections when examining the seismic performance and fragility of the assembly concrete-filled steel tube (CFST) composite frame structure, with a particular focus on the influence of mainshock-aftershock sequences. The design of a nine-story blind-bolt CFST composite frame based on the energy balance-based plastic design method was first presented. The fiber-based numerical model of the frame was developed and validated against experimental results, which was used for the assessment of the rationality and robustness of the designed structure. It indicated that the frame achieved predefined seismic performance targets in terms of the inter-story drift, residual drift and joint rotation. Following this, the seismic fragility curves of the frame subjected to mainshock-aftershock sequences were developed. The results showed that the structural seismic probabilities for inter-story drift exceeding pre-designated limits of 0.2 %, 1.0 % and 2.0 % were respectively 0.54, 0.44 and 0.32, under mainshocks only scaled to frequently occurred earthquake, design basis earthquake and maximum considered earthquake levels. While the probabilities of exceeding a certain damage limit of the frame will be significantly improved, under the identical hazard levels of mainshocks followed with aftershocks. It showed the importance of considering the extra damages induced from aftershocks during the design of the blind-bolt CFST composite frame structure.