Structural time-varying modal parameter identification based on its strong motion records

To study changes of structural dynamic characteristics and structural seismic damage process during earthquakes for the purpose of structural seismic damage assessment, time-varying modal parameters were identified based on the strong motion records by using the on-line system identification method-RARX model. As an example, the natural frequencies, damping ratios and equivalent stiffnesses of a 6-story reinforcement concrete frame structure were identified from its strong motion records during two earthquakes; one is small earthquake, and the other is great earthquake. The results showed that the natural frequencies do not change during small earthquake; in great earthquake, the natural frequencies decrease gradually until they reach the lowest point values, and then increase until the end of the earthquake but they can not be recovered to the initial values, those mean the performance of structures can be recovered to a certain extent after the earthquake; the equivalent stiffnesses are changed with the same trend as natural frequencies, but decrease much greater than that of natural frequencies; however, the structural damping ratios increase after damage under the excitation of great earthquake. The results provided a reference for the damage detection and prognosis.