A new method for estimating the restoring force of an isolation layer in a base-isolated system is proposed. The hybrid motion equation involving the modal coordinates and the physical coordinates is derived by component mode synthesis. This significantly reduces the number of unknown parameters after the mode shape information of the superstructure is substituted and makes it possible to identify the isolation layer directly and locally. The effectiveness of this method was validated in a simulation example and in an application to an actual base-isolated building. The difficulty lies in how to describe the state of a nonlinear device of a real structure. In this paper, the amplitude-dependent equivalent stiffness and damping coefficient are adopted to describe the nonlinearity of the isolation layer. The identified results by our proposed method reconfirm the experimental observation of nonlinearity in the layer made up of isolators.
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