Seismic Response of Eccentrically Braced Tall Building

Spectral analyses and system identification techniques are used to analyze a set of acceleration response records obtained during the October 17, 1989, Loma Prieta earthquake $(M_s=7.1)$ from the 47‐story, moment‐resisting framed and eccentrically braced Embarcadero Building (EMB), located in San Francisco. The EMB was constructed in 1979 based on the 1976 Uniform Building Code requirements and a design response spectra defined by two levels of earthquake performances. The EMB is in the Lower Market area of San Francisco, which is of great interest to the engineering community because of the area's soft‐soil characteristics that amplify ground motions originating at long distances, and because the Embarcadero Freeway (within 100 m of the building) suffered extensive damage during the earthquake and was razed in 1991. The first modal frequencies of the building at approximately 0.19 Hz (north‐south) and 0.16 Hz (east‐west) are identified. The torsional response and rocking motions of the building are insignificant. Discontinuity of stiffness and mass at the 40th‐floor level causes significant response issues above that floor such as excessive drift ratios.