Christchurch Women’s Hospital: Performance Analysis of the Base-Isolation System during the Series of Canterbury Earthquakes 2011–2012

Abstract

Live monitoring data and simple dynamic reduced-order models of the Christchurch Women’s Hospital (CWH) help explain the performance of the base-isolation (BI) system of the hospital during the series of Canterbury earthquakes in 2011–2012. A Park-Wen-Ang hysteresis model is employed to simulate the performance of the BI system and results are compared to measured data recorded above the isolation layer and on the sixth story. Simplified single, two, and three degree-of-freedom models (SDOF, 2DOF, and 3DOF) show that the CWH structure did not behave as an isolated but as a fixed-base structure. Comparisons of accelerations and deflections between simulated and monitored data show a good match for isolation stiffness values of approximately two times of the value documented in the design specification and test protocol. Furthermore, an analysis of purely measured data revealed very little to no relative motion across the isolators for large events of moment magnitude scale (Mw) 5.8 and 6.0 that occurred within 3 h of each other on December 23, 2011. One of the major findings is that the BI system during the seismic events on December 23, 2011, did not yield and that the superstructure performed as a fixed-base building, indicating a need to reevaluate the analysis, design, and implementation of these structures.