The numbers keep adding up. Already we’ve seen 60 of the 77 pontoons needed for the new State Route 520 floating bridge get built, with 48 of those arriving on Lake Washington.

Add two to that mix, as longitudinal pontoons D and E traveled from Grays Harbor, Wash., to Seattle’s Lake Washington, finishing the voyage on Saturday after passing through the Hiram M. Chittenden Locks with the help of tugboats Nancy M and Mudcat.

Pontoons D and E are part of 21 longitudinal pontoons needed to reconstruct the world’s longest floating bridge, connecting Seattle to points east. At approximately 360 ft long and weighing about 11,100 tons, the longitudinal pontoons form the backbone of the bridge and support the roadway superstructure.

The bridge will also need two cross pontoons to help with the transition from the floating bridge section and the east and west approach structures and 54 supplemental stability pontoons, ranging from 2,500 to 2,820 tons, to help stabilize and support the weight of the bridge by joining to the larger longitudinal pontoons.

With the arrival of pontoons D and E, pontoon joining can continue on the east side of Lake Washington, where six longitudinal pontoons have already been connected. Later this month, crews plan to tow eight supplemental stability pontoons from Tacoma.

To get to Lake Washington, the pontoons move through the Puget Sound from Aberdeen, Wash., or Tacoma and must navigate the locks in Ballard before a short trip to Lake Washington finishes the excursion.

The new six-lane bridge eats up $2.72 billion of a larger $4.3 billion project to reconstruct the SR 520 corridor. At 7,710 feet, the new bridge will add over 200 feet to what was already the longest floating bridge in the world when it opens in 2016, a process getting completed one pontoon at a time.

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