Many bridge designers are turning back the clock to produce graceful reminders of the past, when timber bridges were often the main crossings over America's streams. City, county, and state officials are specifying more wood structures because of their aesthetic appeal, competitive cost, and the value of using a material that is in harmony with the environment.
The Wilson River Bridge, a suspension bridge near the Oregon coast in Tillamook, spans a river widely known for its steelhead and salmon fishing. Completed in March by the Portland-based Precision Construction Co., it is a major landmark in the new Tillamook Forest Center. The bridge connects the visitor's center with the Jones Creek Campground directly across the river and the Wilson River Trail, a 30-mile-long network of interpretive trails and sites in the Tillamook State Forest.
The Oregon State Department of Forestry owns the bridge, built as a joint project with the non-profit Tillamook Forest Heritage Trust. The project received donations and support from more than 350 individuals, foundations, and businesses. The TrusJoist division of Weyerhaeuser Co., Federal Way, Wash., furnished the bridge design and engineering, all of the glued-laminated (glulam) timbers and other wood products to build it, and construction funds—a total gift valued at $750,000.
The bridge deck is 13 feet wide, suspended with 1 1/8-inch steel cables from 55-foot glulam towers. The deck is framed with 6¾x42-inch glulam outside girders and 5 1/8x 12-inch glulam stringers. The surface decking is 4x 12-inch solid sawn timbers. Each tower is constructed with 8 ¾x 27-inch glulam sections bolted together. The bridge is 250 feet long with a center span of 150 feet. The pressure treatment of railing posts, walking deck, and main bridge columns is Copper 8-Quinolinolate, an oil-based chemical. The girders, purlins, joists, and bracing under the deck were treated with water-based Pentacholorophenol Type A. Both of these are chemical treatments forced into the wood fiber under heat and pressure to protect from decay and damage by moisture and insects. Copper 8 was used on timbers too large to fit in the Pentacholorophenol treatment cylinders.
“The bridge is a keystone component of the overall project,” said Doug Decker, project leader for the Tillamook Forest Center. “The building has two major ‘book-ends.' On the north end, we have the Wilson River Bridge, and on the south end in the entry plaza is a full-size replica forest lookout tower. When visitors come away from their visit, they are going to remember this bridge and the incredible forest and river views it will offer.”
— Jaenicke is a Tacoma, Wash.-based business writer.