BRIDGE CONSTRUCTION: First pontoon in place on west end of new S.R. 520 floating bridge

Event marks the beginning of assembly of the new bridge

Bridge Construction News WSDOT April 21, 2014
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Construction of the new S.R. 520 floating bridge across Lake Washington has reached another milestone with the first permanent placement of a bridge pontoon on the lake’s west side.


The 240-ft-long, 10,000-ton Pontoon A, with large roadway-support columns mounted atop its deck, will be the new bridge’s western-most pontoon. Pontoon A and Pontoon W, now anchored near the lake’s eastern shoreline, will bookend the floating bridge’s 21 longitudinal pontoons. The latter structures, each 360 ft long, are the floating bridge’s primary support.


“This pontoon placement essentially represents the start of bridge assembly,” said Julie Meredith, director of the S.R. 520 Bridge Replacement and HOV Program for the Washington State Department of Transportation. “In coming months, the public will start to see a new floating bridge take shape as we align and connect other pontoons in between A and W.”


Crews have towed Pontoon A from its moorage site near Medina to its permanent location about 50 ft north of the existing floating bridge’s west approach, near Madison Park. The pontoon has been secured in place with large steel cables attached to anchors secured to the lakebed.


Fifty-two of the new bridge’s 77 pontoons have been constructed so far, with 34 now on Lake Washington. The new, six-lane bridge is scheduled to open to traffic in spring 2016.


Pontoon A’s permanent location will obstruct the west navigation channel running beneath the existing S.R. 520 floating bridge. As a result, the 206-ft-wide boating channel will reduce in width to 120 ft. The narrower channel will be marked by traditional red and green buoy markers.


This temporary channel configuration, approved by the U.S. Coast Guard, will be in place until additional pontoons are joined with Pontoon A. The west navigation channel’s vertical clearance of 45 ft will not change.


In addition, once Pontoon A is secured to its permanent anchors, a new exclusion zone for water craft will be activated. This off-limits area will be marked by six white buoys located north and south of the pontoon and with a sign on each end of the pontoon. The white buoys represent the locations of anchor lines, and it is dangerous for boats to pass between the sign and the buoys.

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