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Floating Bridge Gets Engineering Accolade

Tuesday, May 9, 2017

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The Washington State Department of Transportation’s SR 520 Floating Bridge in Seattle, the centerpiece of a project that has had its share of missteps, has received validation from the engineering community.

The bridge won this year’s Grand Conceptor Award, which signifies the year’s most outstanding engineering achievement, at the 50th Anniversary Engineering Excellence Awards on April 25 in Washington, D.C.

The awards are a national juried competition sponsored by the American Council of Engineering.

“This is one of the great engineering feats of our time,” Washington State Secretary of Transportation Roger Millar said at the ceremony. “The new bridge is an example of how our state is working to build a resilient, world-class, multimodal transportation system that will serve generations to come.”

Sizing Up the Bridge

The span, which was designed by engineering firm HDR (Omaha, Nebraska), is the world’s largest, longest floating bridge. At 7,710 feet, it surpasses by 132 feet the Albert D. Rosellini Bridge in Seattle, built in 1963.

It is supported by 21 longitudinal pontoons, which each weigh 11,000 pounds. The bridge is stabilized by 54 supplemental pontoons, as well as cables and anchors.

The superstructure was engineered to resist winds of up to 98 mph and 6.3-foot wave heights.

Kiewit/General/Manson was the design builder for the floating bridge, and KPFF served as the bridge’s lead design consultant.

Ongoing Construction

The bridge over Lake Washington opened to two-way traffic inn April 2016 as part of the $4.6 billion SR 520 Bridge Replacement Project, which began in 2011 and will continue for another decade.

A floating bridge is needed across Lake Washington because of the body of water’s depth and its soft lakebed. Engineers designed the new bridge to accommodate bus/carpool lanes and a bicycle-pedestrian path. It is capable of being retrofitted for light rail, if officials decide to take that step.

Work continues on the West Approach Bridge North Project, which extends SR 520’s three westbound lanes from the floating bridge to the Montlake area in Seattle. That span is scheduled to open to traffic this summer.

Problems arise

Accolades for the bridge came two months after state transportation officials acknowledged the bridge was "structurally deficient" months before it opened to traffic. 

The determination came from an inspection given to the Federal Highway Administration. It was included in the National Bridge Inventory, published by the American Road and Transportation Builders Association.

Photo courtesy of Washington State Department of Transportation

The SR 520 Bridge in Seattle is the world’s largest, longest floating bridge. Despite numerous missteps and construction delays, it opened to traffic in April 2016.

After installation, kinks or anomalies were found in 46 of the 58 cables that connect the bridge span to anchors at the bottom of the lake. Washington's Department of Transportation decided to have them all replaced.

"They already passed the strength test," WSDOT spokesperson Steve Peer told King 5 News in Seattle. "But, we want (the cables) to pass our longevity test as well.

"It's really a difficult thing to determine whether it was in the spooling, how we installed them, in the design, if it was a manufacture default," said Peer, "The main thing is it was there."

Troubled History

The project has experienced other setbacks:

  • In May 2012, crews found cracking and spalling after completing post-tensioning on a longitudinal pontoon that was designed to keep the bridge afloat.
  • In 2015 a pipe that was being transported hit a traffic sign, causing it to fall into traffic. Eight people on a bus were injured. Flatiron West Inc., the general contractor for that part of the project, faced $5,500 in fines from the Occupational Safety and Health Administration related to that incident; that figure was later reduced.
  • Also in 2015, a worker on another section of the project, managed by Kiewit-General-Manson, was killed in a fall. K-G-M was fined $14,400 by OSHA over that incident.
  • A floating construction crane broke away from its moorings during a burst of wind on March 1. The crane ran aground in shallow water but didn’t cause any damage. In 2013, during an earlier phase of the project, a similar crane floated away.

Other Honorees

A number of other infrastructure-related projects were honored at the Engineering Excellence Awards. Projects recieving Grand Awards included:

  • The Pearl Harbor Memorial Bridge, New Haven (AECOM);
  • The foundation design for the Tappan Zee Bridge replacement, New York City (GZA Geoenvironmental);
  • The U.S. 84 Mississippi Bridge pin-and-link replacement, Natchez, Mississippi (HNTB);
  • Croton Water Filtration Plant, New York City (AECOM/Hazen and Sawyer JV);
  • Claude "Bud" Lewis Desalination Plant, Carlsbad, California (Arcadis & Kleinfelder);
  • Palmetto Expressway and Dolphin Expressway Interchange Improvements, Miami (BCC Engineering);
  • University Link Extension, Seattle (McMillen Jacobs Associates); and
  • Elizabeth River Tunnels, Norfolk and Portsmouth, Virginia (WSP | Parsons Brinckerhoff).


Tagged categories: Awards and honors; Bridges; Construction; Engineers; Health and safety; North America; Program/Project Management

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