"EFFECTS OF TORSION IN SEISMIC RESPONSE OF BUILDINGS",

Abstract

During seismic excitation, the buildings can be subjected to torsion due to asymmetric configuration, asymmetric yielding or rotational component of ground motion. Torsional seismic response of buildings has attracted significant research effort in the past. Different national seismic design codes deal with torsion by placing restrictions on the design of buildings with irregular layouts. High cost andscarcity of land pressurizes the builders to utilize maximum land area and therefore irregular plan configurations of the multi-storey buildings are common. In this work, an attempt has been made to understand the elastic as well as inelastic torsional response of asymmetric buildings through analytical investigations. The effects of torsion in the elastic seismic response of buildings have been studied using eight-storied RC frame stiffness-eccentric building model. The eccentricity at each floor level in the model has been generated by modifying the in-plane stiffness of the peripheral frames keeping the overall stiffness unchanged. Two types of models with uni-directional and bi-directional stiffness eccentricity have been used and subjected to different real earthquake ground motions. The effect of varying the eccentricity ratio on their elastic seismic response has been studied. A linear relationship between the ratio of maximum to average storey-drifts and the ratio of the static eccentricity to the torsional radius has also been obtained. This study also shows that the criteria for the buildings being extreme torsionally irregular as per ASCE/SEI 7-05 and torsionally irregular as per NZS 1170.5:2004, and EUROCODE: 8, are similar, while IS 1893 (Part 1): 2002, and ASCE/SEI 7-05 code provisions for considering the buildings to be torsionally irregular are more stringent. It has been found that although, the overall responses of both uni-directional and bi-directional stiffness-eccentric models are quite similar, but the effect of eccentricity has been found to be more pronounced in case of bi-directionally iii eccentric model. Due to bi-directional eccentricity the response has been observed to increase up to 25%. It has also been shown that the code provisions for taking into account the possibility of increase in seismic response due to dynamic effects are adequate. The in-elastic seismic response