Signal versus noise in damage detection by experimental modal analysis

Remote bridge-monitoring systems based on measured structural vibration have been perceived to be able to assist in bridge inspection for future. Sensitivity of measured modal properties to potential damage is very critical for their practical application, which is examined here. Modal tests were conducted on a one-sixth scale multiple steel-girder model bridge and a fracture critical field bridge, including both intact and simulated damage states. Sensitivity of modal parameters to changes of structural condition was studied using statistical methods (for frequencies, damping ratios, mode shapes, and their derivatives). Results show that, even though modal frequencies and mode shapes may be used to identify the existence of commonly observed bridge damage with certain sensitivity, it is difficult to identify their locations.