INELASTIC SEISMIC RESPONSE OF R.C. FRAMES

Abstract

Under the current philosophy of earthquake resistant design, a structure should be capable of withstanding substantial inelastic deformations. An adequate knowledge of inelastic behaviour ,is, therefore, necessary for a safe and economical aseismic design. The aim of present research work is to study the inelastic behaviour of reinforced concrete frames and to asses the validity of inelastic response spectra for aseismic design. Frames of different heights i.e. 4 storey, 6 storey and 10 storey,designed using different proportions,have been selected. Performance of frames in the inelastic range has been studied under monotonically increasing lateral loads and two strong motion earthquakes namely El Centro, 1940 and Mexico, 1985. It is observed that the perfor-mance of frames designed without incorporation of strong „column - weak girder design philosophy is considerably poor as they require very high column rotational ductilities. ■ The response of frames under El Centro earthquake as obtained from the inelastic time history analysis has been compared with the response obtained by performing the inelastic response spectra based SRSS modal superimposition method. It is found that the displacement ductility demand of various. frames obtained by inelastic time history analysis are about 0.5 to 0.85 times the design ductility intended by using the inelastic response spectra method. Thus, the inelastic response spectra method is found to give a conservative design as compared to the time history method which is the most accurate method of analysis. It is, therefore, concluded that this proposed inelastic response spectra method is a very effective and convenient design method for use in design offices. It is preferable over the I.S. codal method which does not give any idea of the ductility requirement of the structure under design.