Time Variations of Structural Properties During Strong Ground Shaking

After modeling the system as a linear time variant single degree-of-freedom system, a systematic methodology for the analysis of such behavior from a knowledge of the basement and roof records has been developed. By utilizing such noisy records, and incorporating any data available from pre- and post-earthquake vibration tests, the time varying coefficients of the differential equation, used to model the structure, have been identified using a minimum variance sequential filter. The technique has been used to analyze the response of a nine-story reinforced concrete structure during the San Fernando, Calif., earthquake of February 9, 1971. The results indicate that significant changes in the stiffness and damping characteristics, relative to those obtained during small amplitude vibration tests, could occur during the actual progress of strong ground shaking and that a proper knowledge of the pre-earthquake characteristics may be very useful in identification using strong ground motion records.