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Innovative floating cofferdam helps replace the world's longest floating bridge

Photo: COWI

​Washington State is home to the world's longest floating bridge. To sustain the many commuters crossing every day and secure their safety, the existing bridge is being replaced. COWI has designed the floating cofferdam that will make the new bridge possible.

​The SR-520 Bridge crossing Lake Washington links Seattle and its neighboring cities to the east. Every day, around 115,000 vehicles cross the bridge, but the bridge can no longer accommodate the vast amount of commuters and is often clogged by traffic.

To handle the growing traffic, the State of Washington is replacing the bridge with a six-lane floating bridge, thereby keeping its record as the world's longest floating bridge. The new bridge will enhance safety by replacing the aging floating bridge and keep the region moving thanks to vital transit and roadway improvements throughout the corridor.

COWI cofferdam enables the repair works

The first phase of the pre-casting resulted in a structural cracking at the ends of four concrete pontoons due to a design error. Following the discovery of the cracking, Kiewit-General-Manson JV (KGM), the contractor, hired COWI's North American company COWI MARINE North America to develop a repair plan to ensure that the bridge will meet the performance requirements.

COWI's repair plan consists of multiple repair measures including crack injection with epoxy, transverse post-tensioning, waterproofing membrane and carbon-fiber reinforced plastic wrap.

However, all of the repair work needs to be done in a dry environment, and since the bridge is situated in a lake, that is hard to come by. Therefore, COWI MARINE North Americahas used a floating cofferdam of own design to facilitate the repair work.

An innovation wonder

The floating cofferdam weighs 600 tons and is 96 feet wide, 44 feet long and 35.5 feet tall. Besides being a massive structure, the cofferdam also includes many innovative features in order to meet project requirements in terms of safety, operability, water tightness and adequate contingency.

As an example of the innovative features, an elaborate seal system has been developed, which ensures a complete watertight seal of the cofferdam, creating a dry work environment 26 feet below the lake surface.

Another example is the integration of the ballast tanks and floating tanks into the structural framings of the cofferdam. This ensures structural efficiency and realises substantial cost savings.

Once installed, the cofferdam is subjected to buoyancy of 6.4 million pounds and must be able to withstand a five-year storm.

 Published 01.10.2014

The world's longest floating bridge

  • Governor Albert D. Rossellini Bridge is the official name of the bridge.
  • The current floating bridge is 60 feet wide.
  • The floating section of the SR 520 Bridge is 7,497 feet long, making it the world's longest floating bridge.
  • The bridge is supported by 33 bridge pontoons.
  • The SR 520 bridge pontoons standard anchor weighs 77 tons
  • The existing bridge was designed to withstand 50-70-mph winds, while the new bridge will hold up to 92-mph winds.
  • Pre-stressed concrete pontoons were part of the original bridge design, each pontoon measuring approximately 360 feet in length, 60 feet in width and 14.9 feet in depth.

LAST UPDATED: 16.09.2016