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CSMIP 94-01

"Torsional Response Characteristics of Regular Buildings under Different Seismic Excitation Levels"

by H. Sedarat, S. Gupta, and S. Werner

January 1994, 43 pp.

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Torsional response characteristics of three regular buildings in San Jose, and one in Watsonville, California, were studied by analyzing the strong motions recorded in these buildings during three recent earthquakes: 1989 Loma Prieta, 1986 Mt. Lewis, and 1984 Morgan Hill. The story shear forces, torsional moments and dynamic eccentricities in these buildings during the three earthquakes were obtained from an analysis of the recorded motions. The fundamental period of vibrations and damping ratios for these buildings were also estimated for the three earthquakes by using the Fourier Amplitude Spectra of the recorded motions. These results were then compared with the provisions of the 1988 Uniform Building Code. The results of our investigation indicate that the provisions of the 1988 UBC may sometimes not be adequate to realistically account for the torsional response of buildings during earthquakes, especially for steel moment frame buildings. The results of this investigation also indicate that the fundamental building period obtained using Method A in the 1988 UBC may be longer than the actual period of the building during earthquakes, especially for stiffer low to medium-rise buildings. This could result in unconservative estimate of earthquake design forces when using the static force procedure of the 1988 UBC.