Replacing the world’s longest floating bridge was understood to be a technically challenging assignment, but also challenging was gaining public consensus on what, if anything, was to be built. In 1997, the owner started the long and difficult task of planning the replacement of this key part of the transportation system in Seattle.

Public discussion, debate, and planning continued for 14 years, looking at many alternative alignments and crossing types, concluding with a Final Environmental Impact Statement in 2011. Challenges included technical issues such as wind/wave analysis and wave interaction with a long floating structure, an active seismic area, a deep lake with unstable soils for anchor placement, a suitable pontoon-construction site, navigational requirements, maintenance of traffic during construction, and many others.

The new bridge consists of a floating bridge structure – at 7,710 feet, the longest in the world – with fixed approach and transition structures on the east and west ends connecting land based structures with the floating bridge. The owner furnished 33 of the floating structure’s pontoons; the design-builder constructed 44, along with bridge anchors and anchor cables. All 77 pontoons were towed to Lake Washington from casting basins in Grays Harbor (Pacific Ocean route) and Tacoma (Puget Sound route). Bridge assembly involved joining 21 longitudinal pontoons, 2 cross (end) pontoons, and 54 supplemental stability pontoons to form the floating bridge substructure. The superstructure that carries the roadway deck was installed on top of the pontoons. A new bridge maintenance facility, dock and crew access was constructed beneath the east approach structure. The old floating bridge and approach structures were removed from the lake.

To address the stringent requirements, a solution maximizing the versatility of segmental bridge technology was developed for two significant segments of the project. Precast segmental ribbed-slab segments were used for the 1-mile long low-rise section of the floating-bridge superstructure; and a three-span 630-foot cast-in-place segmental box-girder bridge was used for the two east approach structures to connect the bridge to land. Several noteworthy features of these structures included:

An Innovative Effort to Turn a “Marine” Job Into a “Land” Job Wherever Possible – Land-based construction and precast segmental technology provided a safer environment, mitigated potential negative environmental effects to the lake, simplified access for material and personnel, improved construction consistency and quality, allowed for standardization of equipment, shortened the project schedule, and greatly reduced costs.

Reduction of In-Water Work – Through the relocation of the east approach bridge footing to the shoreline. Originally planned to be placed within lake Washington, the proposal design moved this footing to shoreline. This eliminated in-water construction in a critical salmon-spawning lakeshore gravel bed.

Increased Maintenance Access on the Pontoon Deck – The thin structural section of the precast segmental ribbed-slab segments increased the vertical clearance to 10-feet, which exceeded the 7-foot-6-inch minimum and greatly enhanced maintenance access and mobility on the pontoon deck, key to bridge preservation activities.

Elimination of Exposed Steel Elements in the Bridge Superstructure – The final design provides an all-concrete superstructure; this eliminated the RFP concept of exposed steel, providing a much more durable structure with greater performance in the difficult conditions of Lake Washington.