At more than 1,000 ft long, pontoons B, C and D alone prove impressive as a floating bridge structure. But with a new 7,700-ft-long floating bridge under construction across Lake Washington near Seattle, swinging these three pontoons into place caps a beyond-impressive span of floating concrete.

Constructed on the lake north of the rest of the floating bridge, crews for general contractor Kiewit-General-Manson built the three pontoons together to keep a section of the lake open to boaters longer. When finished, they swung them into place in early July, leaving just Pontoon A, a cross-pontoon, to finish out the expanse of the new bridge from Medina on the east end of the lake to Seattle on the west end.

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“With the final three longitudinal pontoons now in their permanent location, you can actually see this great new bridge stretching end to end across the lake,” Julie Meredith, Washington State Dept of Transportation’s administrator of the SR 520 Bridge Replacement and HOV Program. “It’s an exciting day for everyone involved in the project.”

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The longitudinal pontoons form the structural spine of the world’s longest floating bridge. Within a few weeks, crews will tow Pontoon A back into position and connect it to Pontoon B. The bridge consists of 21 longitudinal pontoons, each 11,000 tons and 360 ft long. There are two cross-pontoons that bookend the floating highway. A total of 54 smaller, supplemental pontoons flank the large versions to give the bridge added support and stability.

The “raft” of B, C and D already has a majority of its rising superstructure built on it, including columns and roadway deck, allowing it to connect to the new west approach bridge and form the majority of the “west high-rise.”

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The bridge remains on track to open to traffic in Spring 2016 with two general-purpose lanes and one transit-carpool lane in each direction. The roadway is elevated 20 ft to withstand windstorms and wave action while allowing for maintenance below the roadway without disrupting traffic. A 14-ft-wide bicycle and pedestrian path is also included.


Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for Popular MechanicsSports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.