The final eight “supplemental stability pontoons” for the new State Route 520 floating bridge floated out of their casting basin this week, marking the completion of the 44 bridge pontoons built in Tacoma.

Just three more pontoons for the new bridge remain to be built. Now under construction in Grays Harbor County, the last pontoons will be completed next spring.

“Pontoon construction began in Tacoma in January 2012,” said Julie Meredith, SR 520 program administrator for the Washington State Department of Transportation. “Since then, 44 of the bridge’s smaller, supplemental pontoons have been completed here. These pontoons will help connect communities on both sides of Lake Washington and support an economic link that’s vital to the entire state.”

Supplemental pontoons give the bridge added stability and flotation. The first of the eight basketball court-size pontoons was towed out the casting basin by tugboats Wednesday, Dec. 3. These smaller pontoons, which flank both sides of the bridge, are 98 feet long and 60 feet wide. The concrete structures now will be towed to Lake Washington.

With the final cycle of Tacoma-built pontoons complete, 74 of the new bridge’s 77 pontoons are now constructed. The 33 built in Aberdeen include 21 of the massive longitudinal pontoons; they are 360 feet long and weigh 11,000 tons.

Meanwhile, crews continue to bolt together and anchor pontoons on Lake Washington to form the world’s longest floating bridge. The new bridge, with six-lanes and a bicycle/pedestrian path, is designed to resist stronger wind storms than the current floating bridge. The new bridge is scheduled to open in spring 2016.