Seismic performance and fragility analysis of infilled steel frame structures using a new multi-strut model

Abstract

Whether UnReinforced Masonry (URM) infills should be employed as structural members in frame structures is a debatable subject in performance-based earthquake engineering. This paper presents a detailed assessment of the seismic response of infilled moment-resisting steel frames. A recently proposed multi-strut model is applied here to consider both the local and global effects of URM infills. The capability of the employed model in predicting the local and global effects of URM panels has previously been verified by the authors of this study. Two structures, a four-story building and a two-story building, were designed using seismic design codes, and one of the outer frames of each structure was selected for the numerical models. Based on the conventional layouts of URM panels in buildings, three different configurations are considered: the bare frame (model in which the URM panels are isolated from the surrounding frame), the infilled frame, and the soft-first story frame. Fifteen ground motion records have been used to perform nonlinear Incremental Dynamic Analyses (IDA), which were then employed to derive the fragility curves of the studied structures for the Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) limit states. The results show that in lower limit states, the masonry infill leads to some beneficial effects. However, at more severe limit states, these beneficial effects diminish and at some point, revert to adverse effects. This is evident in the CP limit state, wherein the masonry infill weakens the seismic performance of the structure.