The Washington State Department of Transportation's (WSDOT) SR-520 Bridge Replacement and HOV Program


The Washington State Department of Transportation’s (WSDOT) SR-520 Bridge Replacement and HOV Program has reached a major milestone this April with the opening of the new SR-520 floating bridge, connecting the cities of Seattle and Bellevue, across Lake Washington. The bridge supports WSDOT’s efforts to improve safety, reliability, and travel times, and, at nearly 1.5 miles long, will replace its predecessor as the world’s longest floating bridge.

The bridge incorporates concrete pontoons, balanced cantilever cast-in-place segmental approaches, concrete and steel girder construction, and a precast segmental superstructure supported from columns founded on the floating pontoons.

Kiewit General Manson, A Joint Venture (KGM) began construction in 2012 on the land-based balanced cantilever segmental work, which was designed by BergerABAM under the direction of KPFF. The work then progressed onto the floating structure with the construction and setting of the precast roadway sections designed by International Bridge Technologies (IBT), also under the direction of KPFF. KGM completed the cast-in-place balanced cantilever segmental work in early 2015 and the precast segmental erection in August 2015.

Incorporating precast segmental construction increased the vertical clearance between the pontoon deck and the underside of the roadway to 9-10 ft., creating access under the superstructure for bridge maintenance equipment and crews. In addition, the precasting full width deck segments in parallel with casting of the floating pontoons, saved valuable time when it came to constructing the elevated roadway.

The floating bridge provides two general purpose lanes, an HOV, and transit lane in each direction, with a 14-ft.-wide bicycle and pedestrian path. The lightweight precast superstructure accommodates future capacity changes such as widening and the addition of light rail, easily meeting the freeboard requirements to absorb the additional weight. The upgrades, as well as the higher elevation of the new bridge, will eliminate closures during winter storms when the bridge is vulnerable to high winds and storm waves.

Project Details





Design Build Team:

Kiewit General Manson, A Joint Venture


Kiewit General Manson, A Joint Venture

Contractor Engineering Services:


Constructability Review / Estimating Services:

Kiewit General Manson, A Joint Venture

Construction Engineering Inspection:


Form Travelers for Cast-in-Place Segments:

Parkin Engineering

Formwork for Precast Segments:


Precast Producer:

Kiewit General Manson, A Joint Venture

Post-Tensioning Materials:



Scougal Rubber

Expansion Joints:

mageba USA

Epoxy Supplier:

Euclid Chemical Company

Prepackaged Grout: