عنوان مقاله [English]
Progressive collapse is defined as a local failure that may occur due to the various factors in structural members then it can spread to adjacent members and ultimately can result in total collapse of structure or a large portion of it. Though the abnormal loads could cause into the progressive collapse, many structures have experienced progressive collapse due to seismic actions in our modern history. Recently some code specifications and guidelines requirements such as Unified Facilities Criteria (UFC) introduce different analysis methods for assessment progressive collapse in buildings based on increasing strength, ductility and continuity. Many research works have been conducted in relation to the phenomenon of progressive collapse almost considering the gravity loads and in the recent years seismic progressive collapse has attracted much attention and is an open research area for researchers. Lead rubber bearing (LRB) is considered as one of the most conventional isolation systems that have been studied and examined theoretically and developed widely in practice.
This study investigates the potential for LRB base isolation under progressive collapse. For this purpose, the behavior of intermediate steel moment frames in two cases: fixed and with the LRB seismic isolator with 4, 8 and 12 number of stories under progressive collapse are compared using nonlinear static and dynamic analysis in different situation of column removal. At first, tow fixed and isolated 3D structures were designed by SAP2000 software according to Iranian codes then analysis was performed under gravity load (consist of nonlinear static and dynamic analysis) according to UFC guild-lines and seismic loading (by nonlinear time history analysis) using Perform-3D software. The base isolation are modeled with isolator element in the Perform-3D software and these separators provide hysteresis damping through the sink of lead core.
Adding seismic base isolation system to structures averagely reduces 61 percent the response of the frames under earthquakes. The progressive collapse potential of fixed and base isolated structures in the middle and corner column removal conditions is the same as the results of nonlinear static and dynamic analysis according to loading UFC instructions. Furthermore the results of the progressive collapse analysis show that increasing the number of structural members lead to reduction of the progressive failure potential. It is observed that the use of base isolation system has a significant impact on localization the failures under seismic loads and preventing their expansion in the structure.