US Geological Survey


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Struct. Control Health Monit. 2014; 21:649–674 DOI: 10.1002/stc.1593


This article is a U.S. government work, and is not subject to copyright in the United States.


Response-only identification of civil structures has attracted much attention during recent years, as input excitations are rarelymeasurable for ambient vibrations. Although various techniques have been developed by which identification can be carried out using ambient responses, these techniques are generally not applicable to non-stationary excitations that structures experience during moderate-to-severe earthquakes. Recently, the authors proposed a new response-only modal identification method that is applicable to strong shaking data. This new method is highly attractive for cases in which the true input motions are unavailable. For example, when soil–structure interaction effects are non-negligible, neither the free-field motions nor the recorded foundation responses may be assumed as input. Even in the absence of soil–structure interaction, inmany instances, the foundation responses are not recorded (or are recorded with low signal-to-noise ratios). Thus far, the said method has been only applicable to fully instrumented systems wherein the number of sensors is equal to or greater than the number of active modes. In this study, we offer various improvements, including an extension that enables the treatment of sparsely instrumented systems. Specifically, a cluster-based underdetermined time–frequency method is employed at judiciously selected auto-source points to determine the mode shapes. The mode shape matrix identified in this manner is not square, which precludes the use of simple matrix inversion to extract the modal coordinates. As such, natural frequencies and damping ratios are identified from the recovered modal coordinates' time–frequency distributions using a subspace method. Simulated data are used for verifying the proposed identification method.