Health Monitoring of Ironton-Russell: Bridge for Rating Purposes

The Ironton-Russell truss bridge spanning the Ohio River between the cities of Ironton, Ohio, and Russell, Kentucky, consists of a three-span cantilever through truss with a suspended truss in the center span and a fourth span that is essentially a simple-span through truss. Visual inspections of this bridge revealed that recent welded repairs and modifications to various truss members reduced their fatigue classification as well as modified original design and construction details. When the visual inspection team identified these details along with section loss at certain truss verticals and diagonals, it was their recommendation to reduce the allowable load on the bridge from 208 260 to 26 700 N (23.4 to 3 tons). However, a 26 700-N (3-ton) load limit would hinder the transportation of heavy materials across the bridge and therefore have a severe impact on the local economy. Consequently, a series of nondestructive field tests were organized and performed on the bridge to provide better assessments of load capacity and in situ structural health. Controlled truck load tests involving resistance-based strain gauges were conducted, and an instrumented monitor utilizing low-speed (vibrating-wire) strain gauges was installed at the site. The respective data recorded during these field efforts were used not only to evaluate environmental effects on the bridge but also to compute the load rating for each of the instrumented members. The resulting objective-based load ratings and their corresponding allowable live loads greatly exceeded the recommended 26 700-N (3-ton) load limit. Essentially, objective data identified or characterized in situ structural mechanisms and therefore provided the basis for more sound and realistic load ratings.