Modeling on collapse behaviour of high-rise concrete-filled steel composite frames under over-design seismic excitations

Abstract

This paper presents a numerical study on seismic collapse capacity of high-rise concrete-filled steel tubular moment resisting frames (CFT-MRFs) under extreme ground motions that are beyond the design levels. The ground motions characterised as flat velocity spectral shape were selected to neglect record-to-record uncertainty induced by the changing of the natural periods of structures. A simple numerical approach was developed using fibre element with stiffness and strength degradation accounted in stress–strain models. Then, incremental dynamic analyses with various parameters were performed to evaluate the P-Delta and degradation effects on local and global collapse mechanisms. Results have shown that a drift concentration at the lower stories triggered the side-sway collapse mode that was dominated by the post-buckling strength degradation of CFT columns. The height of drift concentration indicates the range where the collapse mechanism was formed, and a drift concentration factor was proposed to estimate the extent of drift concentration. Last, a factor indicating the safety margin under collapse was computed based on the probability of earthquake occurrence.