Project DescriptionDesigned and fabricated by Western Wood Structures, the Placer River pedestrian bridge is a 280-foot camelback truss bridge believed to be the longest clear-span glulam timber truss bridge in North America. Funded by the American Recovery and Reinvestment Act, the bridge is the latest addition to the U.S. Forest Service's Whistle Stop Railroad project, a unique summertime travel experience created in partnership with the Alaska Railroad that takes visitors into the heart of the Chugach National Forest.
Due to its proximity to Spencer Glacier, the bridge needed to be constructed high enough to eliminate potential collisions with icebergs; additionally, ice floes in the river would make protection of interior piers difficult. These requirements resulted in a height of 25 feet above the water line with a clear span of 280 feet.
In addition to a 90-psf pedestrian load, the bridge will face numerous other stresses due to its location—most notably wind gusts up to 120 mph, 200-psf ground snow load, flooding potential, and high seismic events. Lateral loads due to wind and seismic events are resisted by both vertical portal frames at each truss panel point and horizontal chevron bracing in the planes of the top and bottom chords. The bridge was analyzed as a three-dimensional structure using SAP2000, and the design conforms to the current AASHTO LRFD Bridge Design Specification.
Douglas fir glulam treated with pentachlorophenol preservative was used for the truss members, floor beams, purlins, and all bracing members. The treating specifications required adherence to "Best Management Practices for the Use of Preservative Treated Wood in Aquatic Environments," which minimizes the amount of preservative chemical available to leach into the environment, while providing a clean surface on the timber members and an anticipated 75-year service life.
The Forest Service in Alaska promotes the commercial use of Alaskan Yellow Cedar (AYC) as a structural material. AYC was chosen for the deck, posts, and rails since it is naturally decay-resistant and does not require preservative treatment. This allowed for no preservative use in the pedestrian contact areas.
The 15-foot-wide structure features a 6-foot walkway, ample enough to accommodate administrative vehicles. The 280-foot trusses are 15-feet high at the ends and more than 27-feet high at midspan. The structure rests on steel H-piles driven about 40 feet into the ground at the east and west ends. ASTM A588 weathering steel, which blends nicely with the color of the treated wood, was used for all truss member connections.