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Gerwick's 585-ton floatting cofferdam for the repair of a bridge

News International-French

29 Jan 2014

The company provided final design of a float-in steel fabricated cofferdam for the repair of the new SR-520 floating bridge pontoons and provided on-site inspection of the extent of cracking, design of repairs utilizing a FRP strengthening system.

The SR-520 bridge crosses Lake Washington to link Seattle and its neighboring cities to the East. It is crossed by approximately 115,000 vehicles and more than 190,000 people everyday. The floating section of the SR 520 bridge is 1.5 miles long, making it the longest floating bridge in the world. After floating for nearly 50 years, the four-lane bridge is often clogged by traffic and is showing its age. The floating bridge pontoons are vulnerable to windstorms, and bridge support columns are vulnerable to earthquakes. Therefore the State of Washington is replacing the existing floating bridge with a new six lane floating bridge, which will have an option to add light rail in the future.

Inspection of the first phase of bridge pontoons constructed revealed cracking in the pontoon hulls needed to be repaired. The extent of cracking included cracks at the pontoon ends with cracks along the external bottom surface (keel) and external walls. Due to the massive size of the pontoons at 75 feet wide, 360 feet long and 28 feet tall a drydock facility was not available to prevent construction schedule delay. Therefore Gerwick was selected to design a steel fabricated float-in cofferdam to allow repair of the bridge pontoon ends, while afloat, at the bridge site. This resulted in a highly asymmetric design both in plan view and in its cross section. Despite this asymmetry, a 3D CAD model was used to show the cofferdam would float level due to a low wall buoyancy tank and ballast.

The cofferdam has an estimated dry weight of 585 tons, a length of 96 ft, a width of 44 ft and a wall height of 35.5 ft. Fabrication of the cofferdam was completed on a barge outfitted with a launch rail system. The completed cofferdam was towed on its fabrication barge to the Lake Washington bridge site where it was side launched off the barge. In the side launch process the cofferdam, resting on a rail system, was pushed by hydraulic jacks while the barge ballast was adjusted to keep the heel angle within 4 degrees for safety of operations. The rail system extended over the starboard side of the barge by rocker beams with a pinned connection to the side of the barge. As the cofferdam center of gravity passed the pin, the rocker beams rotated into the lake causing the cofferdam to slide off the barge into the lake.



After a successful launch, ballast water was added to the cofferdam tanks to allow it to float into position underneath the pontoon end. When in position under the pontoon, the cofferdam was raised vertically into contact with the pontoon by removing ballast water. In addition, a hydraulic lifting system reacting on flexifloat pontoons was used to pull the cofferdam into final contact with the pontoon keel (bottom) by compressing a rubber seal. After contact with the pontoon keel was obtained, two sliding side gates were closed to seal the cofferdam to the pontoon sides. Next the sealed region between the cofferdam and pontoon was dewatered. This water was pumped into either tanks on the pontoon top deck, into tanks within the pontoon, or into tanks within the cofferdam perimeter. With the sealed region between the cofferdam and pontoon dewatered, repair personnel have 7.5 ft of clearance underneath the pontoon end and access to the pontoon end walls for repairs.

In addition to design of the floating cofferdam, Gerwick provided on-site inspection of the extent of cracking, design of repairs utilizing a FRP strengthening system, and on-site review of repairs conducted. Opening of the new bridge is anticipated in 2016. This project is funded by a variety of state and federal sources, including SR-520 bridge tolling.

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