Complete sets of acceleration records from two extensively instrumented high-rise buildings in the San Francisco Bay Area, the Transamerica Building and the Pacific Park Plaza building, have been obtained during the 18 october 1989 Loma Prieta earthquake. The pyramid-shaped Transamerica Building is 60-stories high with square cross section and steel frame construction, whereas the Y-shaped Pacific Park Plaza building is 30-stories high with three wings and of reinforced concrete moment-resisting frame construction. By using recorded motions, we identify the modal components of the response for each building, determine three-dimensional behavior and coupled modes, and investigate the torsional and soil-structure interaction effects.

The results show that the motion of the Transamerica Building is dominated by a coupled mode at 0.28 Hz. with 5% damping in the SW - NE direction. Another dominant mode at 0.52 Hz. with 4% damping is observed only in the E-W direction. There is also rocking in the building with a frequency of 2.15 Hz and 5% damping.

The dominant mode of the Pacific Park Plaza building is a translation in the SW-NE direction with a frequency of 0.4 Hz, and 13% damping. Other significant modes are at 0.7, 1.94, 3.93, and 4.39 Hz, and all are coupled modes. Relative wing displacements with respect to center core are large, about 50% of the center core displacements. The soil-structure interaction influences the motions at 0.7 Hz, and this is believed to be the frequency of the soil medium surrounding the building.

Both buildings were investigated prior to the earthquake by using ambient and forced vibrations tests and dynamic analyses. We find that identified frequencies in this study are lower and damping ratios are higher than those obtained from the previous investigations.

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