Analytical and Experimental Studies on Seismic Behavior of Mid-Story Isolated Buildings(II)

Analytical and Experimental Studies on Seismic Behavior of Mid-Story Isolated Buildings(II)

This study aims to investigate the discrepancies of dynamic characteristics and seismic responses in base-isolated and mid-story isolated buildings through a series of shaking table tests. The system identification results indicate that a mid-story isolated structure has less fundamental modal quantities, including effective vibration frequency, damping ratio and participation mass ratio, than a base-isolated structure. It is particularly evident when the isolation system is designated at a higher story or when the substructure below the isolation system is not sufficiently stiff. Besides, it can be found from the test results that there exists obviously a phase lag of larger than 90 degrees between the seismic responses acting at the isolated superstructure and substructure when the peak seismic responses occur at the isolation system. Therefore, the influence of higher modes on the seismic responses of mid-story isolated buildings should be paid more attention. Acknowledging the contribution of higher modes to the mid-story isolation design, the peak seismic responses of the test structures are predicted by multiple and simplified modal response spectrum analysis procedures in which a sufficient number of higher modes and only one representative residual mode are taken into account, respectively. Based on the comparison between analytical and experimental results, it is disclosed that for the preliminary mid-story isolation design prior to performing nonlinear response history analyses, the modal response spectrum analysis procedure considering a sufficient number of higher modes is more applicable than that considering only one representative residual mode.