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May 26, 2017
The superstructure of the state Route 520 floating bridge consists of three distinct structures supported on columns that connect to the pontoons along the bridge’s 1.5-mile length: two high-rise components one at each end of the bridge and a unique, 5,500-foot-long, low-rise component.
That long, low-rise superstructure that was specially designed to comply with maximum height requirements set by the bridge owner, the Washington State Department of Transportation. WSDOT had restricted the maximum height above water to avoid blocking nearby residents’ views of Lake Washington and Mount Rainier.
As constructed, the superstructure deviated significantly from the original concept design, which included significant steel bracing. The Kiewit/General/Manson team evaluated several potential alternatives, focusing on solutions that maximized off-site construction of bridge components to minimize work over water and expedite on-lake construction. The resulting low-rise superstructure uses segmental precast concrete deck panels to form both the structure and roadway surface.
Using these panels significantly shrunk the height of the structural section, providing superior vertical clearance between the pontoon deck and superstructure soffit. The Kiewit/General/Manson design provided 10-foot clearance, well above the 7-foot-6-inch minimum identified in RFP documents. This clearance improves maintenance access on the pontoon deck, allowing WSDOT bridge maintenance staff to maneuver and perform critical tasks more easily and safely below the roadway deck.
Another major benefit of this unique panel system is the elimination of exposed steel elements in the bridge superstructure. The Kiewit/General/Manson design provides an all-concrete superstructure that removed exposed steel envisioned in RFP conceptual documents. This provides a much more durable structure with greater performance in the difficult conditions of Lake Washington.