Damage detection based on modal damping change in bridges

Abstract

Structural Health Monitoring has drawn more attention from bridge engineers since the tragic collapse of the I-35W Mississippi River Bridge. In health monitoring of bridges, visual inspection has been the principal technique, while the reliability of this subjective technique heavily depends on the skills and experiences of the inspector. Objective techniques for structural monitoring should be useful to assist the visual inspection. A possible objective technique that has been investigated worldwide is vibration-based health monitoring. This paper aims to discuss the feasibility of vibration-based health monitoring in bridges, focusing on the damage detection based on the identification of change in their modal damping, by reporting the results of three different studies. A possibility of the damping change based damage detection was first examined by using the measurement data acquired from load car running test in the truss bridge with the cracks in the diagonal member. The energy-based modal damping analysis was next carried out to authenticate the accuracy of experimentally identified modal damping ratios in the steel arch bridge. Furthermore, by paying attention to the fact that the corrosion of RC structures is invisible, the detection of the corrosion-induced damage in RC beams by modal damping identification was studied experimentally. The main conclusions derived in this study are as follow: 1) The damping ratio of diagonal member-coupled mode was definitely increased by coupling of damaged diagonal member, suggesting the possibility of structural member’s damage detection by identifying changes in modal damping ratio of steel truss bridges. 2) Possibility of finding equivalent loss factors of the steel arch bridge components using energy-based damping analysis was successfully presented, and the energy-based damping analysis can be a powerful tool for the damage detection of bridges. 3) The modal damping is very sensitive indictor against corrosion-induced damage in the RC beams, and the local corrosion level might be detectable by measuring the modal damping ratio of RC structures.