Nonstructural Components and Systems
Seismic isolation of specific equipment, or specific floors of buildings, can be an effective protective measure for acceleration sensitive equipment or equipment sets. However, satisfying performance requirements is difficult due to severe horizontal absolute floor accelerations and large isolator displacements. This is particularly challenging at upper floors of a building where displacement demands are higher and equipment-protective mechanisms will require significant free space to accommodate their larger displacements. Another challenge in isolating a piece of equipment is achieving the target mitigation responses using passive systems whose performance depends on the dynamic properties of the equipment, e.g. limits on isolator performance (low stiffness required) due to the relatively low mass supported by the isolation system. Several seismic isolation systems have been developed and studied to protect equipment within data centers, nuclear plants, hospitals, and emergency response centers. Although proven reliable for some applications, many of these systems have limited displacement capacity and exhibit complex behavior that can be difficult to predict. The ideal seismic isolation system is: a system whose performance is independent of the dynamic properties of the supported equipment; a system which offers stable cyclic properties; a system which can mitigate significant absolute floor accelerations while accommodating large displacement responses at higher floors; a system that provides energy dissipation capacity to control displacements.
In the present study, a novel variable stiffness system is proposed for seismic isolation of acceleration-sensitive equipment in an attempt to achieve the desired isolation platform performance. It will be shown that the variable stiffness system can be designed to have positive stiffness at small and large displacements, and zero stiffness in between. This results in a variable restoring force that ensures stability of the system under service loading, limits excessive displacements under extreme seismic loading, and allows for zero stiffness isolation at the design level earthquake. Zero stiffness isolation leads to smaller payload accelerations, smaller forces transmitted to the building floor, and provides effective isolation for a broad band of excitation frequencies. During the presentation of the research results, the concept for the variable stiffness system will be presented along with a model for predicting its force-displacement characteristics. Then, the results of laboratory testing on a small-scale prototype system will be provided for validation of the variable stiffness concept. It will be shown that the proposed variable stiffness system is capable of achieving the desired force-displacement behavior for effective isolation of acceleration-sensitive equipment.