In the present research, seismic behaviours of a steel frame equipped with either viscous damper or lead-core rubber bearings (LRB) isolator were evaluated and compared under the effect of near-fault earthquake records. For this purpose, three buildings of 5, 10, and 15 stories equipped with lateral bearing systems composed of steel moment-resisting frames were subjected to 7 near-fault earthquake accelerogram pairs at earthquake hazard levels 1 and 2, so as to evaluate their responses under three scenarios, namely without any energy dissipation system, with viscous damper, and with LRB isolator, using dynamic analysis of time history utilizing PERFORM 3D v5 software. The results were indicative of enhancement in seismic performance of the viscous damper-equipped structures at earthquake hazard level 1, as the corresponding performance level was enhanced from life safety to uninterrupted usability, while no significant seismic performance level enhancement was determined at seismic level 2. Seismic isolator-equipped structures were also associated with seismic performance level enhancement from life safety to uninterrupted usability at both earthquake hazard levels. Relative lateral displacement at floor levels in damper-equipped structures and seismic isolator-installed buildings were found to be about 29% and 68% improved over that of the structure with no energy dissipation system. Results of distribution of shear forces within structures equipped with viscous damper and seismic isolator, as compared against that of the structures with no energy dissipation system, indicted increased and decreased shear forces, respectively..
The Khorjinee connection is a special connection that was widely used in the construction of low-rise steel structures in Iran before the national seismic code was established. However, catastrophic experiences from recent earthquakes have shown that these connections are highly vulnerable to seismic loads. In fact, it is recognised that such connections should be classified as non-seismic, semi-rigid connections with unknown rotational stiffness. As a result of this issue, many structures with these connections may need to be rehabilitated to ensure their safety in future earthquakes. In this study the dynamic properties of a half-scale model of a four-storey steel structure with Khorjinee connections were determined by a forced vibration test. The structure was then separately rehabilitated by means of four different bracing configurations (X bracing, chevron bracing, eccentrically braced frames (EBFs) and knee braces) and the structural responses were investigated. The results, which include the natural frequencies, mode shapes and damping ratios for each case, present a realistic approach to enable engineers to select the best way of strengthening steel structures with Khorjinee connections according to diverse structural limitations. The structural displacement ratios were found to be significantly larger in structures with EBFs and knee bracing systems when the number of braced spans is high compared with the first mode.
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