Study on secondary collapse of the bottom frame masonry structure in semi-ruined state based on FEM–FDEM

Abstract

The bottom frame masonry structure (BFMS) in a semi-ruined state is vulnerable to secondary collapse under strong aftershocks, posing a significant risk to rescuers during post-earthquake operations. Therefore, investigating the mechanisms and characteristics of secondary collapse of BFMS in a semi-ruined state (BFMS-SR) under aftershocks is critical. This paper proposes a numerical modeling framework for BFMS under mainshock-aftershock conditions, utilizing a combination of the finite element method (FEM) and the finite discrete element method (FDEM). The validation demonstrates that FEM–FDEM can effectively reproduce the transition of BFMS from an intact state to a semi-ruined state, ultimately leading to a secondary collapse state. Subsequently, the mechanisms and structural response characteristics of BFMS-SR under aftershocks are analyzed. The secondary collapse of the BFMS-SR under aftershocks is primarily governed by column hinge development. Furthermore, according to the time-history curve of absolute vertical velocity, the secondary collapse of BFMS-SR exhibits four distinct stages: stabilization, structural response development, secondary collapse, and post-collapse. The end of the stabilization phase is proposed as the early-warning threshold for secondary collapse of BFMS-SR under aftershocks, aiding in post-earthquake rescue operations.