base isolation system

One of the methods to prevent the direct transmission of earthquake forces from the ground to a structure is seismic isolation. The main goal of seismic isolation is to reduce the amount of acceleration and forces transferred to the structure during an earthquake. This goal is achieved by installing the structure on separating layers that possess high horizontal flexibility. In this way, when the ground beneath the structure vibrates strongly during an earthquake, minimal movement is induced in the structure. In this research, a new index has been introduced to compare the performance of base isolation systems, which can lead to the design of base isolation systems with improved performance for the structure. By utilizing the proposed index, known as the Weighted Relative Performance Index (WRPI), the performance of both the structure and the base isolation system can be evaluated. This evaluation considers the amount of strain energy absorbed in the structure and the isolator, as well as the maximum acceleration value and the displacement created in both the structure and the isolator. To evaluate the effectiveness of the proposed index and compare it to previous indices, a 6-story steel moment frame with and without base isolation system was studied. Overall, the WRPI index demonstrates better efficiency and performance than previously used indices, particularly in structures sensitive to acceleration and inter-story drifts.

Seismic isolation research was largely predicated on the observation that most strong-motion records recorded up to that time had very low spectral acceleration values (~2 sec.) in the long-period range. In recent years, however, a number of records from near-source sites have been obtained, raising serious questions about the viability of seismic isolation in near-fault locations. In this study, various analytical models of base-isolated structures have been simulated in a nonlinear platform. The used base isolation systems have been the Lead Rubber Bearing (LRB) and the Friction Pendulum (FP). Arrays of far-field and near-fault strong ground motions have been considered for performance evaluation. The main issue is that increasing the bearing damping in isolators beyond a certain value, may decrease the bearing displacement; however, it may transmit higher accelerations into the superstructure. Considering all these, the seismic performance of the investigated structure and the BI systems have been assessed considering a probabilistic approach utilizing fragility curves. The results indicated that the BI systems have very adequate seismic performance in far-field regions; however, their efficiency may significantly decrease in near-fault regions.