A set of 14 acceleration records was obtained from a 42-storey steel-f
rame building, the Chevron Building, in San Francisco during the M(s)
= 7.1 Loma Prieta earthquake of 17 October 1 989. Data were analysed u
sing a system identification method based on the discrete-time linear
filtering, and the least-squares estimation techniques. The results sh
ow that the response of the building is dominated by two modes: a tran
slational mode in the weaker (southwest-northeast) principal direction
of the building at 0.1 6 Hz with 5% damping, and a translational-tors
ional mode along the east-west diagonal of the building's cross-sectio
n at 0.20 Hz with 7% damping. There are significant contributions from
higher modes at 0.54 Hz, 0.62 Hz, 1.02 Hz and 1.09 Hz. All the modes
incorporate some torsion, but the amplitudes of torsional components a
re small, about 10% of translational amplitudes. Soil-structure intera
ction influences the vibrations near 1.0 Hz. The contribution of soil-
structure interaction to the peak displacements of the building is sig
nificant, particularly at lower floors.