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CSMIP 92-02

"Seismic Performance Investigation of the Hayward BART Elevated Section"

by W. Tseng, M. Yang and J. Penzien

September 1993, 84 pp.

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This final report presents the results of a seismic performance investigation of the Hayward BART elevated section, instrumented by the California Division of Mines and Geology under its Strong Motion Instrumentation Program (CSMIP), using the acceleration time-histories recorded during the October 17, 1989 Loma Prieta earthquake. The recorded structural responses are correlated with corresponding theoretically-predicted responses. Adjustments of structural parameters and modelling concepts required to achieve satisfactory correlations are discussed, along with their implications to procedures of standard engineering practice. The results obtained in this study indicate that during the Loma Prieta earthquake, the maximum seismic demand in the pier-columns of this section of the BART elevated structures was at the level of about 50% of the full-yield capacities of the columns; that soil-structure interaction effects play a significant role in controlling the seismic response characteristics in the transverse direction; and that, due to the presence of the continuous rails which are rigidly fastened to the girders, the BART elevated structures are highly coupled in the longitudinal direction; therefore, the single-pier model used for design in this direction may be inappropriate, especially for those elevated sections which have large variations in pier-column heights. Due to the importance of including soil-structure interaction considerations in the seismic modelling for this type of structure, the studies conducted point out an urgent need of instrumentation that allows independent recordings of the foundation response motions at the pier-column bases. Recommendations are made toward improving the arrangement of CSMIP strong-motion instruments at the Hayward BART site to fulfill this need.