An increasing number of new bridges are drawing on the early history of our nation as designers produce graceful, nostalgic spans that recall the days when timber bridges spanned many of America’s streams.
Because of its strength and aesthetic appeal, timber is being revived as a popular bridge framing material, much as it was for our ancestors. This is especially true for new bridges built of glued-laminated timbers.
Glulam bridges won 12 of the 15 national awards last year in the National Timber Bridge Award program, which has been renewed for 2005.
A unique vehicular timber bridge was just completed in the Blue Ridge Mountains near Cashiers, N.C. It is the Norton Creek Bridge, a four-span structure made with glued-laminated timber girders and deck panels. This bridge is owned by Trillium Links and Village, a golf course residential development near Cashiers. The owners selected a timber bridge because the aesthetics of the wood span blended in with the natural beauty of the wooded site. Actual construction took only two months.
Chocolate bowstring
The bridge is a two-lane span 28 ft wide. Overall length is 150 ft. It is made up of four individual spans of 25 ft, 55 ft, 35 ft and 35 ft. In order to maintain constant girder sizes, the first and third spans are cantilevered, reducing the length of the girders in the second span to 40 ft. The bridge is designed to carry HS20-44 loading in conformance with the AASHTO Standard Specification for Highway Bridges.
A unique feature of this bridge is the decorative heavy timber bowstring truss at the 55-ft span. This truss has the appearance of carrying this longer section. The bowstring truss is made of 83?4-in. x 9-in. top and bottom chords and 83?4-in. x 6-in. webs. The connections are made with steel side plates and machine bolts.
The bridge section consists of six glulam girders spaced at 4 ft 93?4 in. on center. The exterior girders are 63?4 in. wide x 39 in. deep. The four interior girders are 51?8-in. x 39-in. reinforced glulam girders.
A 51?8-in. glulam deck panel system is used to form the structural deck. These deck panels are attached to the girders with cast aluminum Weyco deck clips.
A sawn lumber timber running plank wearing surface is made up of 3 x 12 boards laid side by side across the full width of the roadway and running the length of the bridge.
The bridge utilizes a combination rail system with two 51?8-in. x 71?2-in. traffic rails and one 21?8-in. x 6-in. pedestrian rails. The rails are attached to 83?4-in. x 9-in. glulam posts. The rail system conforms to the static loading and geometry requirements of AASHTO.
The bridge was designed and supplied by Western Wood Structures Inc., Tualatin, Ore. Western Wood Structures has been designing and supplying glulam bridges since the early 1970s. They also were responsible for designing the concrete foundations. After the bridge components were completely fabricated they were shipped across the country by truck.
The glulam members were completely fabricated before pressure treating. This insures that drilling and routing of the timber during the erection process do not violate the preservative envelope. All of the steel assemblies and connecting hardware is hot-dip galvanized after fabrication. A modern timber bridge fabricated and pressure treated in this manner will have a life expectancy of at least 50 years, according to Western Wood Structures.
The glulam members on this project are pressure treated with pentachlorophenal in a heavy oil carrier. This treatment gives the wood a chocolate brown color. The running plank was treated with pentachlorophenal in light solvent. This lighter treatment was chosen for the wearing surface to provide greater slip resistance.
One constraint on this project was a very tight construction schedule. This bridge was ordered in April 2004. The first shipments of bridge material were sent to the site in early July of the same year. The bridge was completed in August 2004. This demonstrates the speed at which a glulam vehicle bridge can be designed, supplied and installed.
The wood products industry and the U.S. Forest Service have renewed their sponsorship of the National Timber Bridge Awards program. Roads & Bridges magazine also is a sponsor.
The 2005 timber bridge competition is the fourth such event sponsored by the American Institute of Timber Construction, APA, the Engineered Wood Association and the U.S. Forest Service.
Bridges must be open before Dec. 31, 2004, to be eligible. The awards deadline is April 30, 2005.
For more information, contact AITC at 303/792-9559 or fax 303/792-0669. You also may click on the bridge award entry blank at www.aitc-glulam.org.
