STRUCTURAL RESPONSE DEPENDENT DURATION OF STRONG MOTION

Abstract

The duration of ground motion is generally not given the same importance in assessing the structural response and performance as the amplitude and frequency characteristics of ground motion are given. As regards duration, the duration of strong shaking only governs the structural response which may not be related directly to the total duration. The problem of estimation of strong motion duration, which may be related closely to the structural response, is a very challenging task. Several widely used methods for estimation of duration of strong shaking have been reviewed critically to analyze their relative performances in defining the structural response of single-degree-of-freedom systems with widely differing natural periods • and damping ratios. It has been found that many of the methods proposed by different investigators to estimate the strong motion duration do not qualify this test. A more appropriate method for estimating the strong motion duration from a given accelerogram is due to Gupta (1994). This definition ensures the conservation of total energy in the input accelerogram and matching of several peaks_of ground acceleration with highest amplitude. It is thus able to represent the duration of strong shaking in a more physical manner compared to the methods based on matching of the highest peak amplitude. However, this method also is found to associate with quite broad band of percentage root -mean-sq u are error in prediction of the response amplitude over the complete range of natural periods. As an alternative, it is therefore proposed to estimate the strong motion duration from the response time-histories of single-degree-of freedom systems with different natural periods and damping ratios. This provides the spectra of duration similar to the commonly used spectra of structural response amplitude by developing empirical scaling relations in terms of earthquake magnitude, source-to-site distance, and soil site conditions. For the proposed structural response dependent duration, it is possible to estimate the duration for any specific structure with given natural period and damping ratios. Use of this duration in simulation of design ground motion will help in enhancing the confidence level of structural design compared to the current practice of using the same single duration for all the structures.