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January 22, 2013

ACEC Best in State: Gold award
Future value to engineering profession

Otak

Project: Spring Creek Pedestrian Bridge

Client: Mowat Construction for the town of Winthrop



Photo courtesy of Otak [enlarge]
Otak led a team that used modern design techniques to build a historic-style pedestrian bridge.

The town of Winthrop, located on the Methow River in north-central Washington, is famous for its 1880s Western look that was cultivated to coincide with the opening of the North Cascades Highway in 1972. This Western image has drawn thousands of visitors since.

But until recently residents and visitors in the South End business district had to walk along a dangerous narrow shoulder of Highway 20 to reach the historic downtown district. When the city decided to construct a safer pedestrian bridge across the Methow to connect the two business districts, it hired Otak to lead the design-build team.

The big challenge was to design a modern bridge with a historic feel. This vintage visual style meant applying modern, efficient design along with construction detailing rarely seen in new construction.

For instance, the somewhat antiquated woven lattice bracing in the bridge tower that was common 100 years ago had to be analyzed and designed using modern techniques, including stronger, solid sections placed in less obvious locations. The anchorage conditions on either side of the river required some of the backstay cables to be set into shallow bedrock while others had to be set into deep alluvial formations. Piles were added to provide redundancy in the case of scour due to currents in the Methow River.

The bridge also needed to be built to last, and built quickly enough to meet grant funding deadlines. Welding much of the built-up sections in the shop (in lieu of traditional riveting or bolting) helped keep costs down and speed up construction.

Through the use of modern materials and construction methods, combined with historic design and detailing practices, Otak was able to create a sleek, economical design that echoes the period style of the rest of the town. The new bridge helps the community better serve alternative modes of transportation, including bikes and pedestrians, and serves as an instant landmark that the community can enjoy for many years to come.


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