Durability and Wood Protection for Historic Covered Bridges in the United States

Covered bridges are iconic structures in the United States. Over half of the 16,000 covered bridges found across the globe are located in the US. Most were built in the mid- 19th century and found mostly east of the Mississippi River. The unique cultural and architectural qualities of covered bridges drive efforts to protect them from biological and physical deterioration as well as structural damage by vandalism and arsonists.

Portland Mills Bridge, Parke County, Indiana

Portland Mills Bridge, Parke County, Indiana

The National Historic Covered Bridge Preservation Program, sponsored by the Federal Highway Administration, was established to preserve these unique historic structures through research to restore, rehabilitate, and protect them. Vina Yang and Carol Clausen of the Forest Products Laboratory’s Durability and Wood Protection research group, presented a poster paper for the International Research Group on Wood Protection. Their poster, Durability and Wood Protection for Historic Covered Bridges in the United States, was presented at a spring conference in St. George, Utah.

The Portland Mills bridge was constructed using a Burr Arch truss.

The Portland Mills bridge was constructed using a Burr Arch truss.

Protecting covered bridges from decay and insect damage is a top goal and typically done through in-place remedial treatments. Naturally-durable locally-sourced wood species for above-ground replacement components are suitable alternatives to treated wood during bridge rehabilitation. Likewise, guidance for selection of replacement fasteners is available.

Fire is a leading cause of loss and damage for covered bridges, sometimes accidental but also commonly by arsonists and vandals. Traditional fire prevention measures such as sprinklers, alarms, and fire retardant treatments have been evaluated along with the development of new technologies based on flame detectors, fiber optic sensors, and infra-red camera systems that could be used to alert authorities to possible acts of vandalism.

Three-dimensional laser scanning is being used to document as-built design details to authenticate restoration efforts. A variety of new remote sensing technologies are also under development, focusing on continuous remote monitoring of biological and physical conditions in bridges.