A Technique for Seismic Rehabilitation of Damaged Steel Moment Resisting Frames

Abstract

Moment resisting frames as one of the conventional lateral load resisting systems in buildings suffer from some limitations including code limitations on minimum span-to-depth ratio to ensure the formation of plastic hinges with adequate length at beam ends. According to seismic codes, in ordinary steel moment resisting frames the minimum span-to-depth ratios should be limited to 5 and in special steel moment resisting frames this ratio should not be less than 7, which is typically difficult to achieve in some cases. For instance, framed-tube structures typically have moment resisting frames with span-to-depth ratios lower than the above mentioned ranges. Therefore, existing buildings with low span-to-depth ratios may exhibit poor seismic performance when subjected to seismic excitation. In this paper, a method is presented to rehabilitate such moment resisting frames. The novelty of this rehabilitation method is that it can be used not only for intact structures, but also for damaged moment resisting frames after earthquakes to enhance their remaining strength and ductility capacity. While most of the available rehabilitation methods focus on improving the system strength and stiffness, the proposed rehabilitation in this paper is based on the weakening the mid span of the beam which causes the formation of the shear plastic hinge in middle of the beam instead of the two beam ends. Numerical evaluation is conducted to show the efficacy of this method, and the results indicate that the use of the proposed rehabilitation method significantly increase the ductility capacity of the system during subsequent earthquakes.