The Chief Joseph Dam Timber Truss Bridge built in the late 1950s in Douglas County, Wash., was one of the last remaining wood truss designs in the region. The original 309 ft- long timber-bridge consisted of a single, 130-ft long x 20-ft deep Howe Truss, with five timber girder approach spans. It was registered as a National Historic Place because of its rarity, structural form, size, and age.
“But despite being rehabilitated in the early 2000s the bridge had major structural deficiencies and needed to be replaced”, says Jason Pang, project engineer for KPFF Consulting Engineers in Seattle.
After evaluating numerous alternatives, the designers selected a single span precast concrete spliced girder bridge as the preferred solution to address the client’s specific project goals for span, construction time line, and future maintenance. “The precast concrete composition of the girders provided the material performances necessary to achieve an excessive span length and facilitated an acceleration of the schedule during this phase of the project,” says Aaron Simmons, P.E., assistant county engineer for Douglas County Transportation and Land Services in Douglas County, Wash.
“It also addressed the challenging composition of the bridge site”, Pang says. “Foster Creek is the last creek along the Columbia River that naturally supports the spawning of wild salmon and steelhead because no fish ladder is equipped at Chief Joseph Dam.” Due to the environmentally fragile nature of Foster Creek, no structure or construction activities were allowed in the creek between the two existing truss piers. Native American archeological sites were also located in the ravine near the existing bridge, which meant the bridge needed to be built on the same alignment. “Using precast spliced girders allowed designers to extend the span range to eliminate costly intermediate piers on the steep ravine, and avoid impacting the environmentally sensitive areas all while satisfying the owner’s requirement for an all-concrete bridge”, Pang says.
The new two-lane single span bridge is 32 ft wide and 309 ft long comprised of a single post-tensioned 240-ft long span and a 69 ft earth-filled approach. It is framed by five girder lines, each consisting of three precast, pre-tensioned segments erected on false work, and post-tensioned together. “At the time of construction, this bridge was the longest precast concrete girder single-span bridge in Washington State,” Pang reports.
The girder segments feature a 100 in. deep cross-section and are constructed with high-performance concrete with a compressive strength of 10.8-ksi. The superstructure is supported on concrete and structural earth wall abutments founded on H-piles, and the deck is constructed of cast-in-place concrete.
Along with addressing the constraints of the project site, the project team had to address several unique obstacles in the planning and implementation of the project. Because the truss is registered as a National Historic Place, the project plan had to include a strategy to salvage the truss. Extensive analysis were performed and several construction sequences were developed to either dismantle the truss in place without entering the creek or to lift it out whole. “The innovation in the ultimate construction sequence was moving the existing truss whole to a temporary trestle and using it as the girder launching truss,” Pang says. After the new bridge superstructure was completed the existing truss was lifted onto the new bridge to be dismantled.
The girder segments also faced a long journey to the job site. The girders were cast by Concrete Technology Corporation at a plant 230 miles from the bridge site. To clear the lowest bridge along the hauling route, the hauler required girder stirrups that were pre-bent with only five-inch extensions. “A special stirrup detail was created to accommodate the variable haunch dimension,” Pang says.
This helped deliver a cost-effective and durable solution that addressed the unique challenges of the site and met the goals of the project, Pang says. “This project is an innovative example where precast concrete was the solution to minimize disruption to the natural environment, while promoting constructability and facilitating salvaging a historic structure.”