UC San Diego

Seismic isolation is an effective method to mitigate the damaging effects of horizontal ground motions. The flexible layer, typically placed at the base of the structure, reduces forces transmitted from ground shaking at the expense of concentrated displacements at the seismic isolation layer. Base isolation systems within a basement of a building require a free clearance to allow for such displacements to occur. A surrounding moat wall can be placed to constrain the isolation devices from exceeding their displacement capacity. However, impact to the moat wall can be damaging to the structure with recent studies concluding that the required clearance to stop (CS) specified by building design codes is insufficient for high consequence low-frequency ground motions.The Lead Rubber Bearing (LRB) is widely used in practice for implementation of seismic isolation. Current models for LRB are not able to capture the salient characteristics of bearing behaviors observed in experimental data, especially under large displacement demands. Therefore, the large strain lead rubber bearing (LSLRB) model is proposed to better predict the response of base isolated structures under extreme earthquake shaking considering the combined effects of lead core heating, and material strain hardening in the lead and rubber. The LSRLB model was implemented in a full-scale Nuclear Power Plant (NPP) numerical model under earthquake loading and demonstrated to reduce displacement demands and lower velocities in the case of impact to a moat wall. Consideration of extending the moat wall clearance and allowing the bearings to reach strain hardening at large displacements showed to be effective in improving the overall seismic response under large ground motions. In terms of the effect of bearing models on the critical internal contents of NPPs, the LSLRB model showed a reduction in floor spectral accelerations throughout the superstructure compared to current models utilized in practice. The results presented are also highly dependent on modeling of the moat wall impact. Current models are reviewed and extended to better predict the amount of moat wall deformation considering the concrete retaining wall, soil contribution and coefficient of restitution.