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Posted 7 February 2013
Built in 1962, the 240th Street SE Bridge over North Creek in Bothell, Washington, had a high propensity for flooding and displayed many signs of wear. Part way through the process of designing a replacement bridge, further inspection revealed advanced deterioration of the bridge’s timber foundation. Along with the ongoing scour action of the creek water further compromising the integrity of the structure, the City of Bothell was forced to immediately close the bridge to traffic.
As thousands of commuters use the city’s roadway systems that connect with the 240th Street SE Bridge, the City needed to act quickly with a replacement plan. During the emergency closure, the City and the design team mobilized to develop a new design strategy for the replacement bridge that could be fast-tracked: designed, permitted, and constructed a year ahead of schedule.
The resulting unique bridge design placed the girders/main structural elements above the roadway surface, adding clearance below for the 100-year flood level. This innovative solution achieved numerous cost-saving benefits by limiting work on the approaches and provided a wider opening under the bridge to meet current FEMA flood-level requirements. The design also included a sheet pile wall system with dead-man anchor tiebacks to enable the contractor to separate bridge construction from in-water work. This type of construction provided a barrier to the stream, enabling construction to continue near the stream while outside of the in-water fish work window. This solution allowed the team to deliver the project within the accelerated schedule.
An adjacent pedestrian bridge that carries the North Creek Trail was also replaced. A practical, prefabricated bridge design was employed that provides a safe path for residents and visitors and serves commuters traveling from the many residential neighborhoods to major employment and activity centers, including the University of Washington Bothell campus and Cascadia Community College. The longer span of the pedestrian bridge also helped to minimize the hydraulic and environmental impacts of replacing the vehicular bridge.
During recent heavy rains in November and December of 2012, the replacement bridge performed as expected with significant improvements. Though the design team did not expect to eliminate the historical flooding issue with the project, City inspector and staff reported approximately 1 inch of clearance under the bridge with a minimal closure time during one of the heaviest rainfalls of the season.