SEISMIC BEHAVIOUR OF BUILDINGS WITH STIFFNESS AND STRENGTH ECCENTRICITIES"

Abstract

Asymmetric distribution of mass, stiffness and strength, asymmetric yielding of members or presence of rotational component in ground motions can induce torsion in buildings subjected to seismic events. Significant research effort in the past has been devoted to address the issue of torsion in seismic response of buildings and adequacy of code provisions for considering torsion. Various national seismic codes have placed restrictions on the design of buildings with irregular layouts and prescribed torsional provisions in order to deal with unfavorable torsion, but high cost and scarcity of land due to increased urbanization has pressurized the builders to come up with irregular plan configurations of the multi-storey buildings for efficient utilization of land area. This work is an attempt to understand the seismic behavior of asymmetric buildings. The influence of torsion on the seismic response of multi-storey buildings in elastic and inelastic range is studied through analytical investigations. A brief review on the existing literature concerning the elastic as well as inelastic torsional response of buildings is presented. A six storey RC frame mass eccentric building system is used to study the effect of torsion on its elastic response. Parametric study is carried out by varying eccentricity. Both unidirectional and bidirectional mass eccentric systems are investigated by subjecting them to a set of recorded earthquake time histories. A linear relationship between the ratio of maximum to average storey-drifts and the ratio of eccentricity to torsional radius is obtained. Results from mass eccentric systems are close to those obtained with stiffness eccentric systems studied previously. However, the response obtained in case of stiffness eccentric systems are 25% higher. It is observed that IS 1893 (Part 1): 2002 and ASCE/SEI 7-05:2005 code provisions for considering the building as irregular are more stringent as compared to NZS 1170-5:2004 and EUROCODE: 8. In order to study the inelastic torsional seismic behaviour of multi-storey, RC buildings both inherently asymmetric and artificially generated eccentric building models are considered. Two categories of artificially generated models are considered in this study. In the first category of models, mass eccentricity is introduced in otherwise symmetric models. In the second category, groups of mass symmetric III models having strength, stiffness and both strength and stiffness eccentricities are considered. The building models are subjected to semi artificially generated ground motions to closely match the design spectrum of IS 1893 (Part 1): 2002 for Zone-Von hard rock. Building systems are modelled in SAP2000 V 14.2.4 as 3D space frame with lumped plasticity. Non-linear time history analysis is employed to study the response of different frames. The evaluation is made using the peak rotational ductility demand of beams of the different frames as measures of their inelastic response. From the results of first category of artificially generated mass symmetric systems it is observed that the inelastic response depends both on the stiffness and strength eccentricities, but the response can be better co-related with the strength eccentricity. The results from the second category of systems not designed for torsion indicate that there is significant variation in the beam ductility demands of different frames. Unidirectional eccentric systems not designed for torsion subjected to unidirectional seismic action show significant demand in the other direction. The systems which are designed for torsion indicate a shift in the center of strength towards the center of mass and exhibit almost uniform ductility demand in beams of various frames. Also, the systems subjected to bidirectional ground motions are found to have qualitatively similar ductility demands when compared to systems •subjected to unidirectional ground motion. iv