Institute for Transportation

Covered timber bridges are historic structures with unique aesthetic value. To preserve this value and maintain bridges in service, robust evaluation of their performance and safety is necessary. The strength of the timber found in covered bridges can vary considerably, not only because of age and condition, but also because of species and grade. For the practicing engineer or inspector evaluating a covered bridge, design values for existing wood members must be determined with as much accuracy as possible by accounting for bridge-specific conditions. The current practice is to assign wood values for strength and moduli from existing specifications without specific reference to timber species, condition, or grade, which can result in an inaccurate assessment of the structure. This manual provides a walk-through for basic wood identification, inspection, condition assessment, field grading, and design value estimation for wood in covered bridges.