Materials

Materials should be selected, designed, fabricated, treated, installed, and maintained for durability as well as for strength, esthetics, economics, and environmental acceptability.
Timber should be treated with appropriate chemicals using environmentally sound treatment methods and cleaning after treatment to prevent future leaching of treatment chemicals and solvents. We strongly recommend requiring treatment in conformance with the Best Management Practices for the Use of Treated Wood in Aquatic Environments, available from the Western Wood Preservers Institute. Portions of trail bridges that trail users will touch frequently, such as rail systems, should be treated with waterborne treatments or with light solvent oil borne treatments. Untreated wood typically has a design life of one-twentieth to one-fifth the life of treated wood and is usually an unacceptable alternative. The U.S. Environmental Protection Agency (EPA) worked with pesticide manufacturers to voluntarily phase out chromated copper arsenate (CCA) use for wood products around the home and in children’s play areas. Effective December 31, 2003, no wood treater or manufacturer may treat wood with CCA for residential uses (with exceptions). Please refer to the report Preservative-Treated Wood and Alternative Products in the Forest Service (Username: t-d, Password: t-d).

Steel should be painted or galvanized, unless it is a corrosion-resistant weathering steel. Uncoated weathering steel should generally not be used in coastal areas or in areas with high rainfall, high humidity, or persistent fog. Galvanized steel is generally the least attractive surface treatment.

Concrete should have an air entrainment of 4 to 6 percent if the concrete will be exposed to freezing temperatures and should have a minimum design compressive strength of 3,000 pounds per square inch. Concrete can be texturized, colored, stained, or painted to better match esthetic constraints.

Fiberglass should have a waterproof, colored surface treatment (surface veil) to protect the fiberglass from ultraviolet radiation.