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The seismic performance and parameter identification of the base isolated Christchurch Women’s Hospital (CWH) building are investigated using the recorded seismic accelerations during the two large earthquakes in Christchurch. A four degrees of freedom shear model is applied to characterize the dynamic behaviour of the CWH building during these earthquakes. A modified Gauss-Newton method is employed to identify the equivalent stiffness and Rayleigh damping coefficients of the building. The identification method is first validated using a simulated example structure and finally applied to the CWH building using recorded measurements from the Mw 6.0 and Mw 5.8 Christchurch earthquakes on December 23, 2011. The estimated response and recorded response for both earthquakes are compared with the cross correlation coefficients and the mean absolute percentage errors reported. The results indicate that the dynamic behaviour of the superstructure and base isolator was essentially within elastic range and the proposed shear linear model is sufficient for the prediction of the structural response of the CWH Hospital during these events. INTRODUCTION The isolation of structures from the ground motion is an effective way to protect the structure from damage in a strong earthquake. The basic concept of base isolation is to provide a low lateral stiffness between the structure and the foundation to lengthen the natural period of the building from its fixedbase value and the dominant periods of the seismic ground motion. Thus, the transmission of earthquake motion and force to the superstructure of the isolated building can be significantly reduced. Successful field performance of base-isolated buildings was first recorded and validated in the University of Southern California (USC) Hospital building during the 1994 Northridge earthquake. Due to the lead rubber bearings (LRBs) isolation system, peak roof accelerations were reduced to 50% of the foundation acceleration, and the peak drift of the superstructure was less than 30% of the code specification [1]. The measured response of two high damping rubber bearing isolated buildings in Miyagi and Chiba, Japan were investigated during the 2011 Tohoku earthquake [2]. The study confirmed acceleration reductions in both buildings, and the maximum accelerations at the floor above the isolated layer were 41-83% of those in the basement. The strong motion seismic records of Fukushima Dai-Ichi Nuclear Power Plant were also reported in the Great East Japan Earthquake in 2011 [3]. This base-isolated structure performed well in horizontal motion with the response reduced by 30% from the basement pit. The Christchurch Women’s Hospital (CWH) building was opened in 2005 and is the only base-isolated structure in the South Island of New Zealand. The base isolation system of the CWH building consists of 41 LRBs and four pot bearings founded on a concrete raft. The design specifications of the LRBs and pot bearings are listed in Tables 1 and 2, respectively [4]. Table 1: Design parameters for LRBs bearings. Designation Total design displacement (mm) Total maximum displacement (mm) Compression stiffness (kN/mm) Design shear force at total design displacement (kN) Average (dead load(DL)+serviceabilit y live load (SLL)) (kN) Maximum ( DL+ live load (LL)) (kN) LRBs 265 42

Performance evaluation of CWH base isolated building during two major earthquakes in Christchurch