Trail bridges (figure 1) not only provide convenient access to the national forests for hikers or packstock, they can protect fragile riparian ecosystems. Trail bridges can be difficult—in some cases, dangerous—to build. The bridges may be miles from a trailhead. Hauling the bridge materials on packstock through steep, rugged country and relying only on human power for assembly and installation makes the work challenging. Helicopters can't be used in wilderness areas without permission of the forest supervisor, and they may be too expensive for some projects even where they can be used.

Figure 1—A fiber-reinforced polymer trail
bridge in the Santa Fe National Forest.

Historically, trail bridges were built from native logs cut on the site. Most areas do not have logs that are strong enough to span longer crossings. In addition, trail bridges made from native logs may have a life expectancy of no more than 5 to 15 years. At some sites, repeated replacement of bridges made from native logs has left small clear cuts around the bridge site. Increasing recreational use and tightened budgets also contribute to the need for lightweight, low-maintenance bridges that are easy to construct.

Fiber-reinforced polymer (FRP) bridges, commonly called fiberglass bridges, offer a potential solution. FRP trail bridge members are fabricated from reinforcing resins (commonly referred to as polymers or plastics) and strands of materials (usually fiberglass) with tensile and bending strengths comparable to those of steel or concrete.

FRP materials are lightweight and durable. Common shapes match those of the rolled steel materials used for trail bridge components, such as tubes, channels, W shapes, and angles. The lightweight FRP structural members are easier to transport to remote locations than common bridge materials, such as steel or timber. In addition, their light weight makes them simpler and safer to assemble.

During the 1990s, several national forests and national parks installed FRP trail bridges, but very little was known about their design or long-term durability. An evaluation was needed to verify that FRP trail bridges were acceptable, safe, and economical.

In 1997, the Fiber-Reinforced Polymer Trail Bridge Project at the Missoula Technology and Development Center (MTDC) began evaluating the feasibility of FRP materials for trail bridges used by the U.S. Department of Agriculture (USDA) Forest Service and the U.S. Department of the Interior National Park Service. One of the project's first accomplishments was to arrange a partnership with the U.S. Department of Transportation's Federal Highway Administration (FHWA), Eastern Federal Lands Highway Division, Bridge Design Group to jointly design, fund, test, and install prototype trail bridges.

A 44-foot bridge was funded and designed by the FHWA. A 22-foot bridge was purchased by the Forest Service as an "off-the-shelf" bridge designed by E.T. Techtonics, Inc., a major supplier of FRP trail bridges.

A second partnership with FHWA's Recreational Trails Program helped to fund this project and disseminate the results.The plan for these prototype bridges was to:

1. Have an experienced bridge-design group review the available design information and develop a design and drawings.
   
   
2. Install the bridges at a test facility and monitor bridge behavior under design loading and severe environmental conditions.
   
   
3. Install the bridges at field locations to determine installation strategies and techniques.
   
   
4. Monitor the field installations to determine unique maintenance requirements.
   
   
5. Publish a guide explaining FRP technology and presenting design methodologies, performance-based specifications for purchasing materials, and recommendations for installation and maintenance.