Isolating layer in foundation-structure dynamic behavior

We investigate the effect of inserting an isolating layer between a built structure and its base. Such a device is often used in order to reduce the damaging effects of the earthquakes. In this paper the structure and the base are viewed as coupled oscillators, with a base excitation due to the seismic ground motion. We give results for the realistic case of a structure-layer eigenfrequency much lower than the ground-base eigenfrequency. We show that the resonance frequency of the rigid structure may be appreciably shifted by such a device, although an additional, low, resonance frequency may appear. In addition, the oscillation amplitude of the structure is appreciably reduced in comparison with the rigid structure-base coupling.