INELASTIC RESPONSE SPECTRA ITS APPLICATIONS

Abstract

Earthquake resistant construction of structures has, been of a great concern to civil engineers due to the hazards caused by failure of structures during earthquakes. It is now well recognised that the structures designed to resist intense earthquake ground motion should be capable of with-standing substantial inelastic deformation. The Inelastic Response Spectra Method, for last few years is being consi-dered as a practical approach in aseismic design. The present study aims at developing a suitable procedure for computation of inelastic response spectra, to investigate the sources of invariability of inelastic response spectra e.g. strong motions, ductility ratio, load deformations model etc.; and to assess the validity of application of inelastic response spectra for aseismic design. A computer program 'IRS' has been developed to analyse the inelastic response of viscously damped single degree of freedom systems to. seismic excitation. It allows the user to conveniently obtain detailed information regarding response of a system and to study systematically the effect of varia-tions in the mechanical and dynamic characteristics of the system and of different excitations. Another computer program 'PLOTRS' has been developed to plot the output from program 'IRS' on a CALCOMP plotter for the above study. The method of analysis is described in Chapter 2. Three types of models namely elasto plastic, bilinear and • stiffness degrading.modellhave been used in the analysis. These are described in Chapter 3. The salient features of the computer programs are described in Chapter 4. Elastic as well as inelastic response spectra are computed for El Centro Earth-quake of May 18, 1940 (NS Component), Koyna Earthquake of Dec 11, 1967 (Longitudinal Component) end artificial earthquake Type B. The reduction factors have been proposed to construct inelastic response spectra from the elastic one. The varia-tions in reduction factors has been discussed for different models and earthquakes. It is found that the load deforma-tion models do not have a very significant effect on inelastic response spectra. The results obtained from elasto plastic model are found to be quite comparable with bilinear and stiffness degrading model. To study the suitability of application of inelastic response spectra for aseismic design, a six storey, two bay reinforced concrete frame was selected. The response of this frame has been compared by using inelastic time history method and inelastic response spectra method. It is found that the maximum ductility demand obtained by inelastic time history analysis are about 0.3 to 0.6 times the design ductility intended by using the inelastic response spectra method. Therefore, inelastic response spectra method is found to give a conservative design as compared to inelastic time history method.