Pine bark beetles have killed large areas of lodgepole pine in the Northwest, leaving standing dead trees that are largely going to waste. But there is a tremendous opportunity to develop this natural resource for constructing buildings and bridges. Small-diameter roundwood (six to nine inches in diameter), or smallwood, is well-suited for joists and intermediate members. Friends of Missoula Parks, a Missoula nonprofit group received a grant from the United States Department of Agriculture Forest Service (Forest Service) to promote use of smallwood for construction of a pedestrian bridge over nearby Rattlesnake Creek.
Two basic alternatives were considered during preliminary design, a prefabricated steel bridge and a cable suspension bridge. Either could be used to meet the 90-foot span requirement to keep bridge abutments out of the 100-year floodplain. Friends of Missoula Parks chose the suspension bridge alternate for aesthetic reasons and to qualify for the grant funds.
The final design uses details from existing Forest Service suspension trail bridges, improving on these where possible and incorporating smallwood as a primary construction material. The objective to use smallwood fits well with the history of other cable suspension pack trail bridges in the Northwest. Ranging from 75 feet to almost 400 feet between towers, these remote bridges were constructed with local materials, whenever possible, because additional materials had to be brought in by pack mule train. The main steel cables were too heavy for one pack animal, so loops of cable were attached to several mules, with straight portions of cable in between. The mules had to walk more or less in step for up to 25 miles. This went against their independent nature and delicate temperament, and made for some interesting pack trips. The reasoning behind use of local smallwood obviously differs today, but it still makes effective use of an abundant local resource.
The Rattlesnake Creek Bridge design employs lattice stiffening trusses for aesthetics, constructability and the ease of replacing individual members as the bridge ages. These trusses consist of 6-inch half rounds with steel structural tees top and bottom to facilitate connections and eliminate the chord splice weakness inherent in earlier designs.
Spacing between floor beams was set at six feet to facilitate longitudinal decking in lieu of stringers and transverse decking. The Forest Service proposed a wood-plastic composite decking developed for the U.S. Navy by Washington State University and fabricated by McFarland Cascade for use in docks. The material is a 3 1/8-inch glued laminated decking, lightly re-sawn for traction. A new extrusion die was made to manufacture 4-inch by 12-inch members especially for this project, allowing the bridge to accommodate the 1,000-pound concentrated load required for horse traffic. These were fastened to the floor beams with special tee-clips.
Steel cones and plates were added to the tops of the towers to keep people from standing on them, since this bridge is under high voltage transmission lines. Vinyl-coated chain link fence placed inside the stiffening trusses satisfies minimum opening requirements, and heavy rubber matting from recycled tires was installed on the deck for horse traffic. Construction began in late 2005 and the bridge opened April 21, 2006.
Benefits
Environmental
Social