The Portage Bay Viaduct (Skanska) 

The Washington State Department of Transportation (WSDOT) has awarded the design-build contract to Skanska with Aecom as lead designer for the project.

The existing Portage Bay Viaduct is the final stretch of state highway SR 520 that connects Montlake to Interstate 5 over Portage Bay in Seattle. According to WSDOT, the bridge built in the 1960s is structurally vulnerable and nearing the end of its functional life. The existing four-lane bridge is supported by hollow concrete columns, which puts it at risk of collapse in the case of a severe earthquake.

As part of the project, the existing bridge will be replaced with two new parallel segmental box girder bridges that meet current seismic resiliency standards. Additionally, the eastbound bridge span will extend the regional bicycle and pedestrian SR 520 Trail across Portage Bay. The project also includes road updates, dedicated lanes for carpool and public transport, and the creation of a landscaped open space lid over the highway.

The replacement is the last in a series of bridge projects intended to provide seismically resilient connections between the communities east of Lake Washington and the city of Seattle. 

“Our work to replace the Portage Bay Bridge is an important piece of broader improvements to the SR 520 corridor, one of the busiest across the Puget Sound region. The project will work to ensure safer travel across the bay, improve mobility and add more connections to Seattle’s non-motorized trail network,” said James Bailey, executive vice president of Skanska USA Civil’s West Coast operations.

Construction is expected to begin in August 2024 and finish in 2031.