An Experimental Study on the Seismic Response of Base-Isolated Secondary Systems

The paper provides an experimental study on the feasibility of base isolation for seismic protection of nonstructural secondary system such as sensitive instrumentation, computer equipment, communication network, HVAC facilities, and power transmission systems housed in nonisolated primary structures. Damages to these secondary systems may result in significant social chaos and costly economic loss. A one-sixth-scaled three-story building model with a single-degree-of-freedom secondary system placed on its third floor is employed in this study. The secondary system is base-isolated by a laminated rubber bearing (LRB) base isolation system from the supporting floor. The ground motion input is simulated by a shaking table which generates three different types of signal including sweeping harmonic sinusoidal, the S00E component of the 1940 E1 Centro earthquake, and the simulated white noise. The experimental results demonstrate significant reduction in both displacement and acceleration responses of the secondary system by using the base isolation. Effects of parameters of the base isolation such as damping ratio and mass on its performance are also investigated. This study provides a valuable guideline for future work in this area and also verifies some previous analytical work by the authors (Hou et al., 2001). Secondary systems in a structure can be divided into two categories: structural and nonstructural. A thorough review of these types of systems and their response behavior under seismic loading has been presented by (Chen and Soong, 1988). In this paper, only nonstructural secondary systems are the subject of the study. These systems include sensitive instrumentation, computer equipment, communication network, HVAC facilities, and power transmission system that reside within a primary structure system. The huge investment made, the mission, and the critical role played by secondary systems in modern day warrant an effective protection strategy against earthquake-induced vibrations. Studies on employing base isolation techniques to protect primary structures have shown the ability of such techniques to limit dynamic response of structures in seismic disturbance (Kelley, 1986; and Ahmadi, 1988); this paper proposes the adoption of a base isolation mechanism for seismic protection of secondary systems and presents the experimental results achieved with this strategy in a scaled-down system designed without primary structural isolation. This study provides a valuable guideline for future work in this area and also verifies some previous analytical work by the authors (Ghantous, 1991; and Samaha, 1992).