Physical parameter identification of nonlinear base-isolated buildings using seismic response data

Base isolation is an increasingly applied earthquake-resistant design technique in highly seismic areas. Examination of the actual performance of isolated structures in real earthquake has become a critical issue. In this paper, a new computational method for system identification is proposed for obtaining insight into the linear and nonlinear structural properties of based-isolated buildings. A bilinear hysteresis model is used to model the isolation system and the superstructure is assumed linear. The method is based on linear and nonlinear regression analysis techniques. Response time histories are divided into different loading or unloading segments. A one-step multiple linear regression is implemented to simultaneously estimate storey stiffness and damping parameters of the superstructure. A two-step regression-based procedure is proposed to identify the nonlinear physical parameters of the isolation system. First, standard multiple linear regression is implemented to deduce equivalent linear system parameters. Analysis of the varying equivalent linear system parameters with displacement is used to distinguish linear and nonlinear segments. Second, nonlinear regression is applied for the nonlinear segments to obtain nonlinear physical parameters. A 3-storey base-isolated building was simulated to real earthquake ground motions and recorded responses were used to demonstrate the feasibility of the proposed method. Superstructure and isolation bearing properties were identified to within 6% those of actual model value even with a SNR 30 dB signal noise level. The overall method allows the simple, effective analysis of nonlinear base isolated structures. The approach to multi-degree of freedom nonlinear structures could be readily generalised to nonlinear, fixed-base, multi-storey structures.