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CCEER-15-04: Analytical and Experimental Study of Gap Damper System to Limit Seismic Isolator Displacements in Extreme Earthquakes
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Base isolation systems generally perform well under design-level ground motions to reduce both interstory drift and acceleration demands. During a maximum considered 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’, has been conceived to control base isolator displacement during extreme events while having no effect on the isolation system performance for earthquakes up to design level. It is by introducing an appropriate initial gap that the gap damper system triggers additional energy dissipation (various combinations of hysteretic and viscous damping mechanisms) during large earthquakes to limit displacements. This research will present development, experimentation and implementation of gap damper system. Numerical studies suggest that gap damper models incorporating a viscous dashpot are very effective in controlling displacement. However due to sudden engagement of a viscous damping device, the roof level acceleration will increase. In order to verify gap damper system function and effectiveness in reducing isolator displacements during extreme events, two test series were designed: the gap damper component and gap damper system tests. The first test which is designed and executed at Auburn University’s Structural Research Lab confirms successful development of simple, feasible, economic and reliable gap damper prototypes for practical implementation. The gap damper system test which is designed and executed at NEES facility at University of Nevada, Reno is conducted to simulate the gap damper system functioning within a base isolated building during large motions. Two test configurations consisting base isolated building with and without gap damper system are considered. The results of this test show gap damper system success in limiting isolator displacements during pulse-type motions in comparison to cyclic motions. Also gap damper system provides more energy dissipation and thus larger displacement reduction when the applied motions cause isolated structure move in the directions parallel to dampers alignment. Large high frequency acceleration spikes are developed due to gap damper activation in various floors of test specimen. Further analytical studies are designed as part of developing the gap damper system design procedure. In these studies, gap damper system is considered as backup system to limit displacement demands of case study isolated structure. The results of this case study also suggest the gap damper efficiency in limiting displacement demands in the expense of higher acceleration demands in superstructure. However in order to comment on acceleration demands amplification due to gap damper system activation as backup system, it is necessary to consider and study the acceleration demands will produce in absence of gap damper system and due to base isolated structure impact with moat wall.
Report No. CCEER-15-04