Abstract
Building Research Institute (BRI) is conducting strong motion observation of a base isolated building in Kushiro City, Hokkaido. A number of valuable strong motion records including the record from the 2003 Off-Tokachi Earthquake have been obtained since the building was completed in 2000.
From the analysis of the strong motion records from the Off-Tokachi Earthquake, the non-linear behavior of the surface geology and the base isolation device was recognized. The performance of the base isolation device, which consists of natural rubber bearings, lead dampers and steel dampers, was verified through the seismic response analysis using a multi-degree-of-freedom system model.
The result of the analysis estimating the natural frequency and the damping ratio of the upper structure showed amplitude-dependency of the stiffness. The natural frequency decreased as the maximum displacement of the upper structure increased. In addition, the difference in the natural frequencies between the cases before and after the Off-Tokachi Earthquake was found out.
The estimation of the stiffness and the damping ratio were made using a model with the base isolated storey. The result showed good agreement with the equivalent stiffness and the equivalent damping ratio evaluated from the hysteresis characteristics of the base isolation devices in large displacement range.
