Identification of dynamic properties from ambient vibration measurements

To better understand the dynamic behavior of structures under normal dynamic loads as well as extreme loads such as those caused by seismic events or high winds, it is desirable to measure the dynamic properties (resonant frequencies, mode shapes and modal damping) of these structures. The cross-correlation function between two response measurements made on an ambiently excited structure is shown to have the same form as the system`s impulse response function. Therefore, standard time-domain curve-fitting procedures such as the complex exponential method, which are typically applied to impulse response functions, can now be applied to the cross-correlation functions to estimate the resonant frequencies and modal damping of the structure. A direct comparison of resonant frequencies identified by curve-fitting the cross-correlation functions, using traffic excitation as the ambient vibration source, and modal properties identified by standard forced vibration testing of a highway bridge, after traffic was removed, showed a maximum discrepancy of 3.63%. Similar comparisons for the average modal damping values identified by the two methods showed a 9.82% difference. This experimental verification implies that the proposed method of analyzing ambient vibration data has the potential to accurately assess the dynamic properties of large structures subjected to seismic excitations and small structures that are tested on a shake table.