SEISMIC OVERSTRENGTH IN REINFORCED-CONCRETE FRAMES

Seismic overstrength of multistory reinforced concrete frames has been assessed by means of nonlinear pseudostatic analysis on four-bay, three-, six-, and nine-story frames designed for seismic zones I to V as per Indian codes. In pseudostatic analysis, the lateral force profile corresponding to the first mode of elastic vibration is applied monotonically. It is seen that the overstrength varies with seismic zone, number of stories, and with design gravity load. The dependence on seismic zone is the strongest. The average overstrength of these frames in zones V and I is 2.84 and 12.7, respectively. The overstrength increases as the number of stories decreases; overstrength of the three-story frame is higher than the nine-story frame by 36% in zone V and 49% in zone I. Further, interior frames have 17% (zone V) to 47% (zone I) higher overstrength as compared to the exterior frames of the same building. These observations are significant for seismic design codes which, at present, do not take into account the variation in overstrength.