Base-Isolation of a Rocking Object on a Rocking Pedestal: Response to Pulse-Type Ground Motion

The rocking response of rigid blocks, such as statues, storage systems, and slender structures in general, was documented both in the field and in the laboratory. These objects/structures must be protected from earthquakes, as their value may be inestimable in some cases. The use of base isolation devices for this structural typology was already studied and successfully applied to real cases. Previous research has focused on the isolation of a single rigid block system, although there are circumstances in which the use of a single block is insufficient to define the response. Configurations in which one block is stacked on top of another, such as ancient Greek and Roman structures composed of large heavy members or statues placed on a pedestal, are examples. In this paper the rocking response of two stacked rigid blocks placed on a seismic isolation device is investigated. The dynamics of this multi-degree of freedom system is briefly summarized, together with the conditions associated to an impact or a change in the rocking configuration. Many parameters govern the response of the system and to understand their influence on the dynamic behavior, analysis of the response to a pulse-type ground motion is carried out. In general, the seismic isolation device is shown to be effective for a wide range of scenarios. However, the study also highlights how, for large period pulses or short period devices, the response of the isolated system can be amplified compared to the scenario where the blocks are placed on a rigid foundation. This phenomenon is due to the amplification of the acceleration of the isolation device for specific ratios between the period of the pulse and that of the isolator; the need of a careful design of the device is evident.