Date of Award

5-2007

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Eric N. Landis

Second Committee Member

William G. Davids

Third Committee Member

Habib J. Dagher

Abstract

The use of fiber-reinforced plastic (FRP) in the wood products industry has led to the reinforcement of glued-laminated (glulam) beams. Adhesively bonding the FRP to the tension face of the beam has been shown to significantly increase its flexural strength and, to some degree, its stiffness. These improvements make FRP-reinforced glulams a viable option for timber bridge applications. However, the long term performance of the wood-FRP bond in a field environment is not well understood. In bridge applications, the FRP-reinforced glulam is subjected to repeated stresses due to live load cycling as well as hygrothermal effects. Four FRP-reinforced glulam demonstration bridges were constructed in the state of Maine. In this study, the bond lines of three of these bridges were inspected for a second time. These included the East Dixfield Bridge over Seven-Mile Stream, the West Seboeis Stream Bridge, and the Medway Highway Bridge. The fourth bridge, the Fairfield Biotech Park Bridge, was inspected for the first time. In addition to the bond line inspections of FRP-reinforced glulam bridges, a fracture mechanics study was performed to determine the fracture toughness of the wood- FRP bond. Fifty-three fracture test specimens were constructed using similar materials and methods to those used in the Fairfield Biotech Park Bridge. The fracture specimens were tested under pure Mode I and pure Mode II loading as well as three ratios of mixed Mode I-Mode II loading. Pure Mode I loading was accomplished using the double-cantilever beam (DCB) specimen while pure Mode II loading was accomplished using the end-notched flexure (ENF) specimen loaded in three-point bending. The mixed mode bending (MMB) test fixture was used to achieve various ratios of Mode I and Mode II mixity. The three ratios of Gu to G chosen for this research were 25%, 50%, and 75%. By combining the results of the mixed mode testing with those from the pure Mode I and pure Mode II loading, the complete fracture toughness contour of the wood-FRP bond is defined. A finite element analysis was then performed on a single FRP-reinforced glulam girder to determine the strain energy release rate at a crack tip when live loads were applied. The upstream girder of the West Seboeis Stream Bridge was chosen to be analyzed because, while not a result of live loads or hygrothermal cycling, the delamination caused by ice impact did represent the worst delamination observed in any bond line inspection. The design truck described by AASHTO was used to load the FRP-reinforced glulam girder to maximize bending moment in the beam as well as shear at the crack tip. The finite element analysis showed that the strain energy release rates due to live loads were far less than the fracture toughness of the wood-FRP bond implying that crack growth would not occur for the particular loadings and wood-FRP system tested.

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