Shake table testing of a base isolated model

The seismic characteristics of a five-storey benchmark model isolated with rubber bearings have been studied using a shake table. Experimental modal analyses using impact excitation, a HP-VXI data acquisition system and LMS CADA-X software were conducted to identify dynamic parameters of the benchmark model with and without rubber bearings. The natural frequencies, damping ratios and mode shapes as well as structural properties, namely, the mass, stiffness and damping matrices, were obtained. In order to evaluate the efficiency of the base isolation system and the effect of earthquake ground motions, both numerical analysis and shake table testing of the model with and without the isolation system were carried out under four different strong ground motions, namely Northridge 1994, Hachinohe 1968, El-Centro 1940, and Kobe 1995 earthquakes. It was found, from both numerical analysis and shake table testing, that the isolation effectiveness offered by the rubber bearings to earthquake inputs is strongly dependent on the type of earthquake motion. The maximum acceleration for all floors was significantly reduced with the addition of a rubber isolation system, regardless of ground motion input. However, the relative displacements of the rubber-isolated model are strongly dependent on the type of earthquake excitation. The seismic responses of the isolated model were improved when subjected to the Northridge 1994, Hachinohe 1968, and Kobe 1995 earthquakes, with best results obtained when Hachinohe earthquake was applied. The maximum accelerations, displacements, inter-storey drifts and base shear force as well as over-turning moment for the base isolated model were significantly reduced, and the vibration amplitude of the accelerations, and inter-storey drifts decreased much more quickly with time than those without rubber bearings. The present base isolation system was not effective in combating the El-Centro earthquake. A reasonable agreement between results obtained from numerical analysis and those of shake table testing was also observed.