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Innovative Design and Construction Make Sauk River Crossing a Success

Summer 2009

Spanning a federally designated "wild and scenic" river, the Sauk River Bridge fords one of the most spectacular white-water rafting and fishing stretches in the country. Salmon swim to spawning grounds and bald eagles soar overhead. Built in 1930, the existing two-truss steel bridge served as the only access to Darrington, Washington, and its main employer, the Hampton Logging Mill, from the Sauk Prairie area east of the river. But the bridge was extremely narrow and dangerous, especially for truck traffic. The bridge was determined to be both functionally obsolete and structurally deficient, with overhead clearance, bridge curb-to-curb width, and structural load-carrying capacity that did not meet current standards. In addition to these concerns, the west pier of the bridge was scour critical and was considered extremely vulnerable to one of the most energetic hydraulic environments in Washington.

Carrying two lanes of traffic and providing a wide pedestrian shoulder, the new two-span steel truss bridge is the Snohomish County's longest at nearly 479 feet. Built on a new alignment just downstream from the existing bridge, the new structure features drilled-shaft, scour-proof foundations up to 125 feet deep that will endure extreme floods. The bridge now handles an average daily traffic flow of 750 vehicles, 25 percent of which are heavy logging trucks. It also provides a dramatic stopping point for tourists on the Mountain Loop Highway, viewed against a backdrop of the snowcapped Whitehorse Mountain.

Engineers often use precast concrete as a bridge building material to keep initial capital costs low and minimize future maintenance. In this case, however, a conventional concrete bridge, with a relatively deep superstructure, would have required the roadway to be raised to maintain clearance above the 100-year flood elevation. This also would have required the roadway approaches to be raised, encroaching on the adjacent lumber mill and making access and drainage features much more costly.

By keeping the structure light during erection, the design team could minimize the profile of the roadway, keep the bridge surface relatively low, and provide adequate clearance during high water flows.

Historically, bridge engineers have built temporary structures to provide local traffic with unimpeded and continuous travel during construction. They could also erect falsework in the water to support new bridge while under construction. This was not permitted in a pristine, untamed river like the Sauk, where construction noise could harm endangered salmon species and disturb their habitat.

The new bridge was originally designed to be assembled onshore in two pieces and placed in position with a special heavy lift crane. Connection of the two pieces into a single continuous truss would be the only erection activities to occur over water. Environmental permits were secured to clear the land required for this, and the contract included provisions for preparing the river bank for the crane weight.

However, the contractor for the project, Mowat Construction, took advantage of the continuous truss design and worked with the County to implement an alternative erection method. They decided to assemble the structure onshore, in one piece, and then "launch" the assembled structure out over the river. An innovative cantilever/roller system was used to achieve this.

The trusses and framing elements were first constructed on the east approach roadway where pile-supported falsework was permitted to erect in the east span. After assembling the nearly complete truss superstructure, launching it over the river required intricate and precise control. Winches on either side of the trusses, designed for 25,000 pounds of line pull, guided the bridge over high-capacity, low-profile Hillman Roller dollies. Tiny deviations or irregularities in the tracks would have produced dangerous internal friction in the rollers. Careful planning and monitoring, however, limited the line pulls to only 5,000 pounds, and the bridge moved smoothly forward into position without a hitch.

To limit deflections and forces in the trusses while the bridge was being launched, the launching method employed also required a nose piece add-on to successfully reach the west abutment. The project team designed the nose piece, something like the prow of a ship, which was then removed after final positioning of the truss.

The original Sauk River Bridge was one of a select type of steel bridge, and very few of its age remain in the state. When replacement became inevitable, the County completed initial Historic American Engineering Record documentation on the bridge, in accordance with National Historic Preservation Act Section 106 requirements.