Forest Service Timber Bridge Specifications

Approximately one half of the 12,000 Forest Service road and trail bridges are constructed of wood or have wood as a major component. The number constructed each year fluctuates with the cost of timber and the complexity of the individual bridges. The typical Forest Service road bridge is a pressure treated, single span, single lane, 40 ft. long structure with timber abutments, glu-lam or solid sawn stringers, laminated deck, timber curbs and rail posts, and a steel deep beam railing. Design, with minor exceptions such as truck loadings and railing, is in accordance with the AASHTO Standard Specifications for Highway Bridges. Installation is controlled by Forest Service Standard Specifications for Construction of Roads and Bridges. The specifications are quite similar to the FP-79 specifications developed by the Federal Highway Administration. There are regional differences depending on availability of construction materials and on the environmental conditions.