MITIGATION OF DAMAGE TO FOUNDATIONS CAUSED BY EARTHQUAKES

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The studies carried out in the latest three years are summarized as follows.When studying the mitigation of damage to rigid-body foundations by earthquakes, it is very important to know the dynamic properties of the foundations. Thus, dynamic spring constants and damping coefficients were preliminarily investigated by subjecting rigid-body foundations to direct and external vibration tests. Also, in consideration of earthquake motion inputs from different directions, rigid-body foundations were subjected to torsional and vertical vibrations simultaneously. After carrying out these tests, experiments concerning the mitigation of damage to foundations by earthquakes were conducted as the main part of this study. Vibration tests of foundations incorporating base-isolation designs are detailed in this report.Circular base-isolation structures (0.2-m diameter) were installed near a rectangular rigid-body foundation, and the reduction of the earthquake motion acting on the foundation was eva … More luated. Before installing the base-isolation structures, the response of the foundation to earthquake motion was measured. Earthquake ground motion was generated by burying a vertical wall at a distance of 6 m from the foundation center and impacting a 1.0-t concrete block against the wall in a pendulum-like motion. The intensity of the ground motion was varied by changing the swinging height of the block. The response of the foundation was evaluated by measuring acceleration at a point 1.0m from the ground motion source and calculating the ratio of the measured acceleration to the foundation response. Tests were carried out with both one line and two lines of base-isolation structures and their effect of ground motion damping was evaluated.The results indicate that, when ground motion is small, the foundation without base isolation exhibits smaller response than the foundation with base isolation. However, the damping effect of base isolation becomes significant with increasing ground motion. No significant difference is found between the results obtained by the one-line and the two-line base isolation structures ; this indicates one line of base-isolation structures is sufficient to bring about the effect of ground motion damping. Less