Enhancing Sensitivity of Vibration-based Metrics for Damage Detection in Smart Structures

OVERVIEW The efficacy of modal analysis in detecting and localizing damage is debated based on sensitivity concerns – modal frequencies are extremely insensitive to small changes in local parameters and geometry. Convincing evidence of sensitivity issues is vast, and here we cite a single field study of a 163 ft. long, 10 ft. deep experimental test girder from an I40 bridge in Bernalilo County New Mexico. In-situ modal analysis of the span for four substantial damage cases shows small changes in the first three modal frequencies. Three of the damage cases, which range from a 2 ft. long, 3/8 in. wide cut in girder web to a 6 ft. cut in the web plus cuts halfway through each side of the bottom flange, produce measured frequency shifts of less than 2% in the first three modes. Only when the structure is nearly cut through, are frequency shifts significant.