Strengthening timber bridge beams using carbon fiber

This research project demonstrates how advanced composite materials can be used to strengthen existing timber bridge beams in order to increase the load capacity of the bridge. Many times, the timber bridges were not designed to withstand the heavy truck traffic that they are currently carrying, and are therefore replaced in favor of modern concrete or steel bridges. Current methods of strengthening timber bridges are not always practical or economical and so these bridges are simply replaced at a high cost to the public. This project investigated whether applying composites in the form of either a fabric wrap or laminate strips to timber beams would increase the load capacity of the beams. Bidirectional carbon fabric was the primary strengthening material used. A total of 10 solid-sawn Douglas Fir timber beams were taken from a timber stringer bridge in Yuma, Ariz. that was replaced in 1999. Seven of the 10 creosote-treated beams were reinforced with carbon fiber and then tested for bending strength, shear strength, and stiffness. Three of the beams were tested as unreinforced control specimens. The results show that applying carbon fabric to the timber beams provides significant increases in the bending and shear capacity, and nominal increases in the stiffness of the beams. Allowable stress modification factors are conceptually discussed that could potentially be used by engineers to determine the safe load-carrying capacity of beams reinforced with carbon fiber. However, a statistically significant number of timber beams strengthened with carbon fiber need to be tested to arrive at definitive stress modification factors.