Spectrum analysis of strong motion earthquakes
The problem of the dynamic response of a structure to an earthquake has been formulated in a manner which permits separation of the characteristics of particular structures from the characteristics of the earthquake. The expression involving the characteristics of the earthquake is defined as the "spectrum" of the earthquake and it is shown that the spectrum is simply a plot of the response of a simple oscillator versus the period of the oscillator. Eighty-eight such spectra were computed by means of an electric analog computer and are presented in this report. It is found that damping is a very important parameter in the overall problem; relatively small amounts of damping reduce structural response sharply. It is shown that, when damping is considered, the spectra are consistent with the hypothesis of a distribution about a mean value. It is concluded that the concept of a "dominant ground period" is not valid for the purpose of aseismic structural design. Further research on damping in buildings is recommended, and it is proposed that the mean value of a damped spectrum be used as a quantitative measure of earthquake intensity.
Additional InformationFirst technical Report until Office of Naval Research Contract N6onr-244 Task Order 25 Project Designation NR-081-095. The authors wish to express their appreciation to the staff of the Analysis Laboratory for making the electric analog computations. Thanks are due Mr. D. A. Hausmann for his help in reducing the data and preparing the figures.