Nonlinear Structural Identification of a Three-Story Infilled Frame Using Instantaneous Modal Parameters

Abstract

Even though modal analysis theory is not applicable to nonlinear dynamic structural systems, such systems can be characterized by their time-varying amplitude-dependent instantaneous modal parameters. In this study, the nonlinear behavior of a large-scale test structure is characterized based on its time-varying instantaneous modal parameters identified during an earthquake. The test structure is a 2/3-scale, three-story, two bay, masonry-infilled reinforced concrete frame, tested on the University of California, San Diego (UCSD) outdoor shake table. Deterministic stochastic subspace identification (DSI) method is used for estimation of instantaneous modal parameters of the structure based on sliding time windows of input-output data during a seismic base excitation. These identified time-variant modal parameters are used to estimate the effective stiffness of different components of the test structure corresponding to its tangent stiffness matrix through a linear finite element (FE) model updating strategy. Variation of the identified stiffness as a function of maximum displacement can be used to characterize the hysteretic behavior at element/substructure level.