America's Historic Covered Timber Bridges

Preservation for Future Generations

Synopsis:

• Covered wooden bridges proliferated in the United States in the mid-nineteenth century.
• Today an estimated 800 covered bridge structures remain as cherished links to the technological heritage of the United States.
• The through-truss designs vary from the Kingpost trusses built in the craft tradition to the engineered Burr arch and Paddleford trusses.
• Covered bridges are links to the past and the pioneering bridge-building efforts and, as such, are considered important early American transportation structures to preserve for posterity.

Overview: Partnering with the Federal Highway Administration and National Park Service, the Forest Products Laboratory (FPL) has taken the lead in conducting research efforts aimed at preserving and/or restoring historic covered bridges. Several collaborative studies have been initiated to improve engineering evaluation methods, increase the durability of the bridge components, and develop new historic structure documentation techniques. Technology transfer efforts have included K-12 educational modules, historical videos, and technical conferences.

• Combining hygrothermal and corrosion models to predict corrosion of metal fasteners embedded in wood
• Corrosion of metals in wood: Comparing the results of a rapid test method with long-term exposure tests across six wood treatments
• Development of a smart timber bridge girder with fiber optic sensors
• Exposure testing of fasteners in preservative treated wood: Gravimetric corrosion rates and corrosion product analyses
• Guide for In-Place Preservative Treatment of Covered Bridges
• Improved Analytical Techniques for Historic Covered Bridges
• Naturally Durable Wood Species for Repair and Rehabilitation of Aboveground Components in Covered Bridges
• Overview of the National Historic Covered Bridge Preservation (NHCBP) Program
• Species Identification and Field-Grading of Woods in Covered Bridges
• Synthesis of Published and Unpublished Corrosion Data From Long Term Tests of Fasteners Embedded in Wood: Calculation of Corrosion Rates and the Effect of Corrosion on Lateral Joint Strength
• The effect of tannins and pH on the corrosion of steel in wood extracts
• Use of Laser Scanning Technology to Obtain As-Built Records of Historic Covered Bridges