Spanning the Decades: Timber Bridges Provide Vital Back-Country Links Glulam beams offer long-term durability, safety for National Forest System bridges

The U.S. Forest Service has often used timber components to build bridges, many of which connect some of the most remote back-country roads in the nation. About 4,100 bridges using timber as a primary structural component remain in the National Forest System highway network. Many of these bridges are made of glued laminated (glulam) timber materials. Glulam beams are manufactured using dimension lumber and industrial strength waterproof adhesives to bond the many layers together into one solid piece. Though durable for decades of use, timber bridges are eventually subject to load restrictions like any other bridge structure.

Jenny Creek Bridge glulams loaded to move overland to Wisconsin.

Jenny Creek Bridge glulams loaded to move overland to Wisconsin.

A number of glulam girder bridges in the National Forest System are now facing restricted load capacities simply due to age even though they are in satisfactory condition otherwise. These bridges use timber girders that were manufactured prior to 1970, when the American Institute of Timber Construction first introduced a national standard for tension lamination quality. The “tension” laminate uses higher-grade lumber to create the bottom layer of a glulam beam, which is critical for its bending strength. Because safe load-carrying capacities must be assigned to all bridges, over 160 timber bridge structures throughout the National Forest System could be affected by new restrictions. Restricting weight limits for vehicles crossing over a remote national forest road bridge can have severe implications for fire and rescue operations critical to the health and safety of the forests and the people who use them.


In 2009, Forest Service engineers began the process of decommissioning the Jenny Creek Bridge, an older glulam structure near the village of Kake, on Kupreanof Island, in southeast Alaska. Located within the Tongass National Forest, America’s largest National Forest, the original bridge was installed in 1967 with four primary girders. In 1978 it was modified by adding two exterior glulam beams to replace and widen the deck. The four interior girders, however, are assumed to be manufactured without specially-graded tension laminations. The 2009 reconstruction project provided a unique opportunity for full-scale testing of salvaged glulam bridge beams in a laboratory setting to directly measure their ultimate load capacity using standardized laboratory test methods. FPL researchers jumped at the chance. Through a coordinated effort by Forest Service (FS) engineers from the Pacific Northwest region (FS-Region 6), Alaska (FS-Region 10), and the FPL, arrangements for laboratory strength tests were made.

Existing Jenny Creek bridge girders removal, near Kake, Alaska.

“Working on projects like this is very rewarding,” says Jim Wacker, FPL’s lead research general engineer for this project. “The testing done on these historic bridges can help field engineers and policy-makers make informed decisions about the current status and future use of these vital links in our National Forest transportation network.”

Bridges using glulam girders manufactured prior to 1970 are facing reduced load ratings, says Wacker. New restrictions would require that signs be installed to indicate load limits and many bridges would be slated for replacement. The average replacement cost per bridge is about $300,000. Several bridges would need immediate replacement due to their vital service towards forest fire suppression and other public safety responsibilities. Replacing all the pre-1970 glulam bridges would potentially cost taxpayers around $48 million dollars.

The cost of transporting the 50-foot-long glulam girders from the Jenny Creek Bridge was a barrier until Economic Recovery (Stimulus) funds became available through the U.S. Department of Transportation’s Federal Highway Administration (FHWA). After being carefully removed and marked to indicate locations in the original bridge structure, the girders were shipped to a nearby port where all six beams were barged to Seattle, Wash. From there they were loaded on a flatbed truck and delivered to the FPL in Madison, Wisc.

“This has truly been a group effort,” says Wacker, noting the extensive collaboration needed to coordinate such a project. “It wouldn’t have been possible without the combined work of my other Forest Service colleagues, the FHWA, and all the folks here at FPL.” Wacker has been collaborating on this project with Dave Strahl and Rod Dell’Andrea, FS engineers from the Pacific Northwest Engineering Structures unit and Alaska Engineering Structures unit, respectively, and Scott Groenier of the Forest Service’s Missoula Technology and Development Center.

Tests are currently being conducted in FPL’s Engineering Mechanics and Remote Sensing Laboratory which is one of the only facilities in the world with the capacity to perform bending strength tests for such large beams. Each girder is approximately 50 feet long, 40-5/8 inches deep, and 14-1/2 inches wide, weighing about 6,500 pounds apiece. The creosote-treated beams are made of Douglas fir lumber. A variety of nondestructive readings have been taken at several locations along each beam to assess their internal condition.

Research is needed to verify whether or not in-service glulam bridge girders manufactured prior to 1970 are still safe. A key focus during a preliminary series of live-load field tests, performed on a number of bridges in Oregon & Washington in 2009 and 2010, was gathering details about the glulam girder laminations in the tension zone. During these extensive tests many material properties of various glulam beams were assessed. Core samples were taken to determine the wood species via microscopic analysis and the locations of glued scarf joints and knots in the girder’s tension zones were recorded. Sound-wave testing was also performed to learn more about the internal integrity of the tension laminations. These results, along with laboratory research accomplished at the FPL, will form the basis for a new load-rating strategy and should provide engineers with increased confidence in assigning a safe load capacity to these historic first-generation glulam girder bridges.

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By James T. Spartz, FPL Public Affairs Specialist