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Bridge Structural Health Monitoring and Diagnostics in Vermont
- By Jody Ciano
Using innovation to work smarter, more effectively and more economically, VTrans initiated a collaborative research project with The University of Vermont's Structural Monitoring and Diagnostics Lab to monitor the structural performance of bridges 58 N&S along I-89 in the town of Richmond. This effort seeks to determine the actual live load characteristics of the bridge structures. Dr. Eric Hernandez, Assistant Professor, College of Engineering and Mathematical Sciences; graduate student, Geoff May; and members from VTrans staff strategically installed 28 strain sensors and 10 accelerometers along the stringers that support the two bridge decks in order to monitor the structures' dynamic response to vehicular traffic. Vanasse Hangen Brustlin, a local engineering firm, generated the instrumentation plan under Dr. Hernandez's guidance. The data collected with the sensors will be processed and analyzed using recently developed structural health monitoring (SHM) algorithms by Prof. Hernandez. The results obtained will be used to determine the distribution factors and overall structural health of the bridges, update their load ratings and improve our understanding of the bridges' capacities to withstand overweight traffic loads.
Although there is no concern about the overall structural safety of the bridges under normal traffic loads, VTrans reroutes overloaded vehicles along other highways. Those overloaded vehicles that are permitted to cross the structures must do so by traveling down the centerline at a maximum speed of 5 mph. With the current SHM system in place in conjunction with nearby weigh in motion (WIM) stations, more accurate understanding of the bridges response to loading will be obtained, enabling the agency to maximize the safe use of the transportation infrastructure. Under U.S. law, trucks can weigh a maximum of 80,000 pounds on interstate highways. However, Maine and Vermont allow 100,000 pound trucks on their interstates as trucks are the only transportation mode that logging and paper companies can use to haul felled trees and other forest materials. With truck miles virtually doubling during the last 20 years, big brand companies are now pressing more states for heavier load capacities with the argument that higher weight limits nationwide will allow trucks to drive fewer miles, thereby consuming less fuel in a time of rapidly rising diesel fuel prices.
With the average age of our nation's bridges at 43 years, and the average bridge built to have a service life of 50 years, the added stress to our nation's bridges with vehicles bearing heavier loads presents a significant challenge. By taking this real-time bridge health monitoring initiative, VTrans will be able to better understand the levels of live loads and the reliability of similar bridges. Lessons learned from the project can be applied to other bridges in Vermont and nationwide by allowing structural design engineers to better predict a bridge's response to future loads, ultimately increasing safety and improving the reliability in the flow of commerce.
This project was co-funded by the Transportation Research Center UTC grant.