ABERDEEN — A second batch of six pontoons for the new Highway 520 floating bridge in Seattle is floating at Aberdeen.
They were floated out of the casting basin early Monday, taking advantage of high tide, KXRO reported.
The six include three longitudinal pontoons, two stability pontoons and one cross pontoon. The longitudinal pontoons are the biggest built for the project; each is 360 feet long.
Design and construction changes were made to prevent cracking problems that showed up a year ago in the first batch of Aberdeen pontoons, the Transportation Department said. They required millions of dollars in repairs.
The newest pontoons will remain at the Port of Grays Harbor for inspections and then be towed to Lake Washington when needed.
The Transportation Department said 24 of the 77 pontoons for the new bridge have been built. A total of 33 will be built at Aberdeen and 44 at Tacoma. In addition, 55 anchors were built in Kenmore, and the last was sunk into place in March.
The new six-lane toll bridge is the centerpiece of a 13-mile project costing more than $4 billion to improve Highway 520 between Seattle and its eastern suburbs. The existing four-lane bridge is 50 years old.
The new bridge, scheduled to open in 2015, is designed to last 75 years, withstand winds up to 89 mph and survive a 1,000-year earthquake event.
The Highway 520 floating bridge is over a mile long, making it one of the longest floating bridges in the world. Another is the I-90 floating bridge, also on Lake Washington. The freshwater lake is more than 200 feet deep — too deep for traditional bridge supports.
A third floating bridge in Washington carries Highway 104 across the saltwater of the Hood Canal.
In 1990, a previous Interstate 90 floating bridge sank in a storm because some hatches had been left uncovered during renovation. In 1979, half of the Hood Canal bridge sank in a windstorm.