Elastic Design Spectrum Development for Indian Context and its Comparison with IS: 1893–2002 (Part-I)

Abstract

Earthquake is one of the most destructive natural hazards that claim human lives and cause damages to almost all manmade structures. It is important to estimate seismic demand pose by an earthquake on structures in order to present damages. Seismic codes of various countries provide Design Spectrum or Response Spectrum to estimate seismic demand on structures. However, for critical facilities, site specific response spectrum should be developed to estimate seismic demand more accurately. Response spectrum is a central and widely accepted concept in earthquake engineering to estimate lateral force on the structure. It is an envelope of maximum response of various Single Degree of Freedom (SDOF) systems to a specified earthquake ground excitation. The response spectrum can’t be used uniformly to design any structure since it is very much site dependent. Apart, it is very much jagged in nature and required to be smoothen out. In order to utilize them for designing the structure uniformly it is converted to Elastic Spectrum. The elastic spectrum is globally known as Design Spectrum in seismic code of various countries. The design spectrum is based on statistical analysis of the response spectra for the ensemble of ground motions. The elastic design spectrum can be readily used for designing new as well assessing existing structure for seismic capacity. For Indian subcontinent, seismic demand is estimated as per design spectrum given in IS:1893 (Part I)-2002, whose basis are unknown. In the present paper, elastic response spectrum is developed for Indian subcontinent. About 184 earthquake ground motion records are compiled. Out of this 67 earthquake ground motions are established as Strong Ground Motion based on Duration of motion, RMS acceleration and PGA to develop design spectrum. A response spectrum is generated by classifying strong motion excitations to four zones, namely, North, East, West and South-East regions as per their locations. The response spectrums are converted to elastic spectrum through statistical analysis. Appropriate amplification factors are determined from first principle. Comparison among developed design spectrum for East and North region and design spectrum specified in IS:1893 (Part-I) - 2002 is carried out through numerical example. Comparison shows that mean response spectrum yields base shear value lower than the IS based design spectrum for highest seismic zone V. However, the IS based design spectrum yields lower base shear value when compared with mean plus one standard deviation design spectrum. Comparison among developed mean response spectrum for West and South-East region shows the later yields lower base shear value as compared to IS based design spectrum while former yields higher base shear value.