An improved equivalent linear model of seismic isolation system with bilinear behavior

•Requirements of code-recommended equivalent linear model are investigated.•An improved equivalent linear model is proposed for bilinear SDOF system.•The proposed model takes into account the effects of several parameters.•The proposed model yields more accurate results than other models.

Equivalent linearization of seismic isolation systems is often recommended in modern structural specifications for the purpose of simplification. In this paper, limited conditions specified in equivalent linearization of seismic isolation system are investigated when subjected to seismic loads. A large number of numerical simulations, including both approximate linear and exact nonlinear analyses, are performed using a program specially developed by MATLAB in conjunction with OpenSees. Approximate to exact maximum displacement ratio averaged over seven earthquake ground motions is selected as the evaluation indicator. Results reveal that, although satisfactory estimates can be obtained if the limited conditions are met, the application scope of equivalent linearization of seismic isolation system is significantly restricted. To improve the prediction accuracy of equivalent linear analysis in a wider parameter space, equivalent linear model recommended in codes is modified through introducing a factor, which is related to ductility ratio, post-to-pre yield stiffness ratio and initial period of seismic isolation system. It is demonstrated that the newly proposed equivalent linear model yields more accurate results when compared with other equivalent linear models.