Institute for Transportation

Historic covered bridges and current timber bridges can be vulnerable to damage from biodeterioration or fire. This guide describes procedures for selecting and applying in-place treatments to prevent or arrest these forms of degradation. Vulnerable areas for biodeterioration in covered bridges include members contacting abutments, members near the ends of bridges subject to wetting from splashing and members below windows or other openings that allow entry of wind-blown precipitation. Pressure-treated timber bridge members can be vulnerable when untreated wood is exposed by field fabrication or by the development of drying checks. The objective of an in-place preservative treatment is to distribute preservative into areas of a structure that are vulnerable to moisture accumulation and/or not protected by the original pressure treatment. Types of field treatments range from finishes, to boron rods or pastes, to fumigants. A limitation of in-place treatments is that they cannot be forced deeply into the wood as is done in pressure-treatment processes. However, some can be applied into the center of large members via treatment holes. These preservatives may be available as liquids, rods or pastes. Bridge members can be treated with fire retardants to delay ignition, reduce heat release, and slow the spread of flames. In-place coating products are available to reduce surface flammability, but these coatings may need to be reapplied on a regular basis if exposed to weathering. For more integrated protection, fire retardant treatment of bridge members may be combined with other forms of protection such as lights, alarms, sprinklers and monitoring systems.