Experimental Validation of a Gap Damper To Control the Displacement Demands in a Seismically Isolated Building

Abstract eng:
Base isolation systems generally perform well under design-level ground motions to reduce both interstory drift and acceleration demands. During a larger than anticipated earthquake, however, large displacements in the base level may cause pounding between the structure and perimeter moat wall, which can lead to very high acceleration in the superstructure. A phased passive control system, or ‘gap damper’, was conceived to control base isolator displacement during extreme events while having no effect on the isolation system performance for earthquakes up to and including the design level. Providing passive supplemental energy dissipation at a specified gap greater than the design displacement allows the base isolation system to act traditionally in design level events while activating the secondary system in extreme events. Use of this gap damper device eliminates undesirable effects associated with large amounts of supplemental damping at lower intensity motions. This paper presents the experimental validation of a prototype gap damper developed for practical implementation in a seismically-isolated building. The gap damper device uses four viscous dampers, two in each direction oriented around and attached to a contact surface at one end and a fixed point at the other end. An isolation nub extends down from the base of the building to sit inside the contact surface. An initial gap is provided between the isolation nub outer surface and the inner contact surface. When subjected to earthquake loading, the isolation nub moves within the contact surface and the dampers are not activated until the displacement amplitude exceeds the system initial gap. When the gap is closed, the dampers are activated and impact forces transferred from the isolation nub to the contact system are absorbed. Shake table testing was performed at the University of Nevada, Reno to simulate the gap damper system functioning within a base isolated building during large motions. Two test configurations consisting of a base isolated building without and with a gap damper system were tested. To the extent possible, the same trials were carried out in both configurations to quantify the influence of the gap damper on isolator displacements and superstructure accelerations compared to the isolated building without a gap damper. The gap damper is found to be more effective in limiting isolator displacements during pulse-type motions compared to long duration cyclic motions, and in predominantly unidirectional motions compared to motions with a strong bidirectional component. In addition, several factors are identified that limited the effectiveness of the gap damper relative to predictions from numerical simulations, which are possible to overcome in the design process. Superstructure accelerations increased as a result of activation of the gap damper, but these high frequency acceleration spikes are not seen to be detrimental to the overall structural performance relative to a conventionally designed building without special seismic protection.

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