The floating in of the final three pontoons to Seattle’s Lake Washington for the State Route 520 floating bridge rebuild brings all the needed pontoons to the lake.

The final three pontoons of the 77 needed to rebuild the world’s longest floating bridge arrived on Lake Washington last week after a three-day journey from Grays Harbor near Aberdeen, Washington, and a trip through the ship canal in Seattle.

During the process, the football-field-sized pontoons arrived in Shilshole Bay and were moved through Seattle’s Hiram M. Chittenden Locks—also known as Ballard Locks for the Seattle neighborhood that separates the salt water from Puget Sound and the freshwater of Lake Union and Lake Washington.

It took four years to construct and transport all 77 pontoons from Aberdeen and Tacoma to Lake Washington. In all, 33 of the pontoons were constructed in Grays Harbor, including 21 of the longitudinal pontoons that run 360 ft long, three stories high and weigh 11,000 tons. Those pontoons form the backbone of the new floating bridge.

The new bridge, featuring six lanes, a shoulder and a bicycle/pedestrian path, was engineered to resist stronger windstorms than the existing bridge.

So far, more than 60 percent of the pontoons have been bolted in to their final locations and installation continues of bridge elements, such as high-rise bridge deck and low-rise roadway deck sections.

As crews continue to secure the pontoons needed to complete the connection from Seattle to points east, work continues on the West Approach Bridge North, which will link the new floating bridge to the Montlake interchange, also extending the regional bicycle/pedestrian path to Seattle.

The floating bridge is scheduled for a spring 2016 opening, while the new approach bridge should follow in summer 2017.

Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for Popular MechanicsSports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.