Seismic protection of vibration-isolated building nonstructural components
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Motivated by the repeated damage to vibration-isolated nonstructural components in buildings during almost all recent earthquakes worldwide, numerical and experimental studies are conducted to assess and improve the state-of-practice of seismic protection of this type of nonstructural components. Despite their inconsistent seismic performance, elastomeric snubbers (bumpers or seismic stops) have been predominantly used for the combined vibration isolation and seismic protection of nonstructural components for more than four decades. To obtain a preliminary understanding of the seismic response of a vibration-isolated nonstructural component protected by elastomeric snubbers, transient dynamic analysis is conducted on a simplified Single Degree-of-Freedom (SDOF) system under seismic loading. For a wide range of properties of the SDOF system and seismic loading, the best geometric and material properties of elastomeric snubber to minimize the seismic forces are established by implementing an evolutionary-based optimization methodology. To verify the results of the preliminary numerical study, and to assess the current code-based design procedure with equivalent static forces, triaxial earthquake-simulation experiments are conducted on two full-scale nonstructural components mounted on integrated isolation/restraint systems with built-in elastomeric snubbers. The experimental results confirm that controlling the displacement responses of vibration-isolated nonstructural components by elastomeric snubbers is achieved at the expense of amplified acceleration responses and dynamic loads, which are often much larger than those assumed by the code-based static method. The differences between the predictions of the static method and actual dynamic responses are attributed to four important aspects ignored by the static method: the rotational responses of the nonstructural component, the nonlinear relationship between the snubber gap size and the seismic demands, the effect of geometry and material properties of the snubbers, and the effect of flexibility of the nonstructural component. A numerical framework is developed and validated for modal and transient dynamic analysis of vibration-isolated nonstructural components, which can be modeled as rigid bodies. The numerical framework features models for geometric and material nonlinearity associated with the engagement of elastomeric snubbers. Finally, a new alternative is proposed for the combined vibration isolation and seismic protection of nonstructural components, which implements displacement-dependent energy dissipation by hysteretic softening. Through numerical studies, it is shown that with the proposed alternative it is feasible to improve the seismic performance of vibration-isolated nonstructural components significantly without compromising on their required performance under normal condition.