Pontoon Float Out Ends Phase II

By Lucy Bodilly

The Hood Canal Bridge, one of only 11 floating bridges in the world and the longest over salt water, withstands natural forces that would strain most other structures past the breaking point.

In fact, the original bridge sank in 1979 after a storm with sustained winds of 85 mph blew through the Hood Canal, which separates the Kitsap and Olympic peninsulas, in northwest Washington State.

Daily pressures include a tidal variation, which changes the bridge elevation by as much as 16 ft every day. Other unusual The state and its contractors meet another milestone in the construction of a floating bridge that is part boat, part road. Stresses include waves hitting the bridge, the daily use by an average of 17,000 vehicles and possible earthquakes.

A $471 million repair and retrofitting project guarantees that the structure will continue to withstand the pressure. A joint venture of Kiewit Construction Inc. and General Construction Inc., both of Seattle, is in charge of the project.

When the last two pontoons that will support the bridge floated out of the graving dock in Tacoma in August, it marked the 80% completion point of the seven-year project.

“We borrowed a lot of technology from the oil industry,” says Becky Hixson, a Washington DOT spokesperson. The steel used in the trusses is round, as it is on oil derricks, instead of flat, so that water doesn’t collect on the surfaces and cause corrosion.

An inspector with expertise in the oil industry flies in from Dallas just to inspect the welds,” Hixson says.

The pontoons are essentially rectangular concrete boxes filled with empty cells, rebar, mechanical and electrical equipment. Post tensioning runs through each side of the pontoons and also ties the sides together. When the pontoons are floated into place, more post-tensioning will be used to attach them to the bridge and the anchors that secure the structure.

“None of the materials can be overweight,” Hixson adds. “They have to meet the specification exactly or the pontoon won’t float.”

The pontoons go from the graving dock in Tacoma to Todd Pacific Shipyard in Seattle. There the electrical and mechanical systems will be added. Columns will be attached and act as a connector between the pontoon and the bridge structure.

All the bridge parts have to be completed and floated into place by May to meet the timing for the six-week bridge closure. Travelers will either use a special ferry run arranged by the DOT, or bypass the Seattle area entirely by driving through Olympia, Wash.

When the pontoons were completed, they had a combined weight of 24 million lbs. The 17 pontoons together contain 29,000 cu yds of concrete. The last two pontoons were 18 ft tall, 60 ft wide and about 340 ft long.

Construction workers built the pontoons in a 150-ft-wide, 465-ft-long graving dock. The dock is located on the waterfront, but below tide level. A gate between the water and dock keep the work area dry during construction.

The day the pontoons were completed, workers from Foss Tug of Seattle opened a hole in the gate, allowing water to fill the dock and the pontoons to float. Then tug boats then took the pontoons to Seattle.