Performance study of in-service FRP-reinforced glulam bridge girders
Publication Name: ASCE Structures Congress 2003: Engineering Smarter
Publication URL: https://digitalcommons.library.umaine.edu/etd/2621/
The use of fiber-reinforced plastic to reinforce glue-laminated timber may allow glulam members to be a more viable consideration for longer-span timber bridge designs. University of Maine researchers have constructed FRP-glulam girder bridges to serve as demonstrations, but with a design that does not fully utilize the FRP properties. One reason for this reluctance is the lack of historical performance data on the bond line between the wood and FRP. The demonstrations bridges have been constructed with the intent of monitoring and inspecting the bond line to give researchers a better understanding of the effects of in-service conditions.
Three bridges in the state of Maine were inspected, with close attention to the FRP-wood bond line. These bridges included the Medway Highway Bridge, the East Dixfield Bridge over Seven-Mile Stream, and the West Seboeis Stream Bridge. The West Seboeis Stream Bridge has a low vertical clearance, and because of this, the first up-stream girder was damaged due to ice impact. There was extensive delamination between the FRP and wood, as well as between the FRP layers. The remaining girders did not see such damage, and therefore had a minimal amount of delamination.
The East Dixfield Bridge girders were reinforced using the hand wet-preg method. During the curing time, the FRP did not adhere to the wood properly, and had to be re-bonded. Because of this, extensive delamination was found between FRP layers and the FRP-wood bond line. Since the girders were designed without taking the FRP reinforcement into consideration, the delaminations do not affect the structural integrity of the bridge. The Medway Highway Bridge perhaps exhibits what would be considered normal wear for such a bridge design. A minimal amount of delaminations were found throughout the girders, with a majority (96.25%) of the total length having delaminations less than 1/8-inch.
Finite element models were created to characterize the delaminations found in a bridge. Specifically, the energy release rate, J, was found for various lengths of delaminations, at various locations throughout the length of the girder. In comparing the results to laboratory results obtained by other researchers, it was found that the models results were below the critical energy release rate for an FW-glulam composite. However, delamination growth toward the load was found to be unstable.
In addition, the stresses that are induced due to differential hygro-thermal shrinkage between the FW and wood were also examined. A finite element model of a cross section was created and subjected to a moisture gradient. Delaminations of various lengths were also modeled, and the stresses at the tip of the delaminations were investigated. It was found that for an FRP-wood girder, the stresses decrease with delaminations less than one-half inch. For delaminations greater than one-half inch, the stresses steadily increased. It was concluded that the trend of the stresses are accurate, and therefore, delaminations found that are greater than one-half inch should be further investigated to determine their effect on the energy release rate.