Development of real/synthetic time histories to match smooth design spectra

Abstract

In modern nuclear plant design there is a definite trend toward time history type analysis of equipment and structural components to determine the peak seismic response and loads. However, the site response spectra remain as the only acceptable description of expected ground motion during a seismic event. Thus, it has become necessary to develop time history accelerograms compatible with smoothed response design spectra established for a site. This paper describes a new computerized technique for generating time history accelerogram traces from existing seismic records to match the smoothed site response spectra. Our method of analysis is particularly attractive in that it minimizes man—machine interaction inherent in previous attempts to generate time histories. The program utilizes existing seismic traces represented in the frequency domain as opposed to the time domain through the use of Fourier transform techniques. By manipulation of both the amplitude and phase of the Fourier transform representation in accordance with criteria formulated from the comparison of a computed response spectra to the smoothed design response spectra, successive new time histories are generated having response spectra converging to the smooth design response spectra. In addition to matching the smooth design response spectra, an additional constraint is imposed on generated time histories in that the peak acceleration of a time history is also established for the site in the form of the zero period ground response. This constraint is also accounted for automatically in the program by revising the time history at each iteration to eliminate peaks greater than the peak site ground response. The authors have utilized the program successfully to match smoothed design response spectra of various damping ratios to generate seismic time history site accelerograms used in the development of floor response spectra and equipment analysis for several nuclear facilities.