Development of Flexural Vibration Inspection Techniques to Rapidly Assess the Structural Health of Rural Bridge Systems - FY05 NATSRL

Principal Investigator:

Brian Brashaw, Program Director, UMD-NRRI

Project Summary:

Approximately 4,000 vehicle bridges in the State of Minnesota contain structural timber members. Research at the University of Minnesota Duluth Natural Resources Research Institute (UMD NRRI) has been conducted on vibration testing of timber bridges as a means of developing rapid in-place testing techniques for assessing the structural health of bridges. The technique involves measuring the frequency characteristics of the bridge superstructure under forced flexural vibration. The peak frequency of vibration was measured and compared to a set of load testing data for each of nine bridges. Each bridge was also inspected using commercially available advanced inspection equipment to identify any major structural problems with individual bridge components such as timber pilings, pile caps, and girders. Two bridges were identified that needed immediate maintenance attention.

The relationship between the load deflection data and the vibration characteristics showed a useful relationship and the results indicate that forced-vibration methods have potential for quickly assessing timber bridge superstructure stiffness. However, improvements must be made to the measurement system to correctly identify the first bending mode frequency of the field bridges. This global vibration technique has potential benefits for routine inspections and long-term health monitoring of timber bridge superstructures.

Key information will also be transferred to the UMD Mathematics and Statistics Department for use in understanding modeling issues associated with dynamic testing of timber bridges. Development and implementation of advanced testing techniques for timber, steel, and concrete bridges will allow public agency inspectors and engineers to better understand the integrity of our transportation infrastructure. It is believed that these techniques will offer a quantitative, ongoing measure of a bridge's stiffness, an alternative to infrequent load testing of bridges.

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