An Investigation of Characteristic Periods of Seismic Ground Motions

Abstract

The design seismic base shear was obtained from the spectral elastic acceleration Sa divided by a system behavior factor R, accounting for ductility and overstrength. The behavior factor is currently taken as a constant for a given type of structures in various codes regardless of structural periods. In fact, the behavior factor is also a spectrum varying with the natural periods of structures. In order to understand the relationship between the spectral values and the corresponding characteristic periods in these two spectra, Sa and Rμ, this article carries out an investigation into the characteristic periods of 370 seismic ground motions from 4 site types. It is found that the periods Tga at which the peak values appear in the Sa spectra are much less than the periods T gR at which the Rμ spectra take a maximum value. Two characteristic periods are necessary to determine the seismic action if a more elaborate procedure is required in practice. Statistical study on these two periods is carried out for the 370 records, and results are presented. For site types A–D, the ratio of these two periods has a statistically averaged value of 5.5–6.7. The maximum input energy S EI , relative velocity S v , power density P SD , and the Fourier amplitude F S spectra were constructed to determine their characteristic periods, respectively. These four spectra predict similar characteristic periods to T gR . T gR is very close to the characteristic period T gd of the elastic displacement spectra. Analysis of SDOF systems under combined harmonic excitations shows that the Sa spectrum is more sensitive to high-frequency excitations, while the displacement spectrum is more sensitive to long period excitations. For the elastic-plastic Sa spectra, peak values tend to appear at shorter periods even the amplitudes of the longer periods are greater than that of the shorter period. This provides an explanation on different characteristic periods in the Sa and Rμ spectra.