SYSTEM-IDENTIFICATION FROM AMBIENT VIBRATION MEASUREMENTS ON A BRIDGE

The cross-correlation function between two response measurements made on an ambiently excited structure is shown to have the same form as th e system's impulse response function. Therefore, standard time domain curve-fitting procedures, which are typically applied to impulse respo nse functions, can now be applied to the cross-correlation functions t o estimate the resonant frequencies and modal damping of the structure . This derivation is based on the assumption that the ambient vibratio n source is a white noise random process. Curve-fitting cross-correlat ion functions to obtain modal properties offers advantages over standa rd procedures that identify resonant frequencies from peaks in the pow er spectrum and damping from the width of the power spectrum. The prim ary advantage is the ability to identify closely spaced modes and thei r associated damping. The resonant frequencies of a highway bridge tha t were identified by curve-fitting the cross-correlation functions, us ing traffic excitation as the ambient vibration source, are compared t o modal properties identified by standard forced vibration testing met hods. Results of this comparison showed a maximum discrepancy of 3.63 percent. Similar comparisons for the average modal damping values iden tified by the two methods showed a 9.82 percent difference. This exper imental verification implies that the proposed method of analyzing amb ient vibration data can be used to accurately assess the dynamic prope rties of structures in a non-intrusive manner. (C) 1997 Academic Press Limited.