Deconstruction Monitoring of a Steel Truss Bridge

A research study was performed on a steel through-truss bridge more than 80 years of age that was being taken out of service. The main objective of the project was to learn as much as possible about the bridge and to further the state of knowledge about the behavior of steel truss bridges. The study began with a review of the existing condition and load testing of the structure. The main experiment was to monitor the truss members during deconstruction to identify the intrinsic forces present when the bridge was still in service. Those measurements were then compared with simulation results from a finite element model and the original design drawings. This study produced two main conclusions about steel through-truss behavior. First, it was found that the fracture-critical lower chord was redundant because of an adequate alternate load path provided by the floor system. This redundancy was identified through load testing of the structure as well as from numerical simulation results. The floor system was found to take up to 34% of the tensile horizontal forces from vehicular loading. The second main finding was the uncertainty over intrinsic forces within the steel truss members. Comparison of measured intrinsic forces and the finite element model indicated differences of roughly 9% to 34%. These differences were found to be greater than those from the load test (average of 12% difference). In addition, a comparison of the measured intrinsic forces with the original design plans illustrated substantial differences. The findings indicate that significant uncertainty about the intrinsic force distribution within constructed bridges can be present.