SR 520 Evergreen Point Floating Bridge and Landings Project – Central Puget Sound, Washington

The State Route 520 Floating Bridge and Landings Project is one of several large construction projects along the SR 520 corridor in Washington’s Central Puget Sound region. The bridge project enhances traffic safety, improves travel reliability, relieves congestion and expands the public’s transportation options by:

  • Replacing an old, structurally vulnerable four-lane floating bridge with a stronger, six-lane bridge.
  • Adding dedicated HOV/transit lanes in each direction.
  • Adding a separate bicycle and pedestrian path on the bridge to give the public a new option for commuting between major employment centers on either side of Lake Washington.
  • Creating a bridge design that can accommodate future light rail.
  • Elevating the bridge superstructure’s roadway so wind-whipped waves no longer crash over vehicles and the roadway, and bridge maintenance crews have access to the pontoons below the roadway deck without disrupting traffic.
  • Adding shoulders on the roadway to minimize traffic disruptions from disabled vehicles.
  • Eliminating the need for a drawspan due to higher, 70-foot clearance for boat passage under the new bridge, eliminating traffic delays on the bridge for boater drawspan openings.

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.

The use of segmental bridge technology allowed the project to be delivered within 56-months.

The $750 million project was delivered at a cost 10% lower than the next highest bid with the use of segmental bridge technology.

The new bridge was constructed adjacent to the existing floating bridge allowing it to remain in full service throughout construction. Only two weekend closures were required at the conclusion of the project to transfer more than 200,000 daily commuters to the new bridge.

Both traditional balanced cantilever and innovative thin ribbed-slab segmental applications were used by this project to address the unique challenges for this world record-setting floating bridge. The innovative segmental ribbed-slab allows for a very slender superstructure, supported by innovative floating pontoons. Clean use of cast-in-place segmental for approach structure.


2017 ASBI Bridge Award of Excellence
Category: Bridges Over Water


Washington State Department of Transportation

Owner’s Engineers:
HDR, Inc.

KPFF, BergerABAM, and International Bridge Technologies, Inc.

Design-Build Team:
Kiewit/General/Manson, A Joint Venture (KGM), KPFF, BergerABAM, and International Bridge Technologies, Inc.

Kiewit/General/Manson, A Joint Venture (KGM)

Construction Engineering Services:
KPFF, BergerABAM, and International Bridge Technologies, Inc.

Constructability Review/ Estimating Services:
Kiewit/General/Manson, A Joint Venture (KGM)

Construction Engineering Inspection:
Kiewit/General/Manson, A Joint Venture (KGM)

Precast Producer:
Kiewit/General/Manson, A Joint Venture (KGM)

Formwork for Precast Segments:
Helser Industries

Form Travelers for Cast-in-Place Segments:
Structural Technologies VSL

Erection Equipment:
Derrick Crane M24

Schwager Davis, Inc.

Scougal Rubber Corporation

Expansion Joints:
mageba USA

Epoxy Supplier:
The Euclid Chemical Company

Prepackaged Grout:

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