Apparent periods of a building. I: Fourier analysis

This and the companion paper present an analysis of the amplitude and time-dependent changes of the apparent frequency of a seven-story reinforced-concrete hotel building in Van Nuys, Calif. Data of recorded response to 12 earthquakes are used, representing very small, intermediate, and large excitations (peak ground velocity, v(max) = 0.6 - 11, 23, and 57 cm/s, causing no minor and major damage). This paper presents a description of the building structure? foundation, and surrounding soil, the strong motion data used in the analysis, the soil-structure interaction model assumed, and results of Fourier analysis of the recorded response. The results show that the apparent frequency changes form one earthquake to another. The general trend is a reduction with increasing amplitudes of motion. The smallest values (measured during the damaging motions) are 0.4 and 0.5 Hz for the longitudinal and transverse directions. The largest values are 1.1 and 1.4 Hz, respectively, determined from response to ambient noise after the damage occurred. This implies 64% reduction of the system frequency, or a fatter approximate to3 change, from small to large response amplitudes, and is interpreted to be caused by nonlinearities in the soil.