SR 520 Pontoon Construction Project - Pontoon design

pontoon construction photo Bridge pontoons are the foundation of a floating bridge. These massive, hollow, concrete boxes are designed to support the weight of the road and the cars, trucks, buses, bicyclists and pedestrians that use the bridge every day. Pontoons are similar to a tanker ship or barge; though very heavy, they float because they are lighter than the weight of water they displace.

The new floating bridge’s largest pontoons, all built in Aberdeen, are each 360 feet long and weigh a little more than 11,000 tons. These “longitudinal pontoons” are placed end-to-end to form the backbone of the new bridge. All told, pontoon construction in Aberdeen required 112,000 cubic yards of concrete, 35,000 tons of steel rebar, and 2.7 million square feet of plywood formwork.

pontoon construction graphic

Fifty-four smaller supplemental stability pontoons are attached to the longitudinal pontoons to provide additional stability and buoyancy. These pontoons are 98 feet long and weigh between 2,600 and 3,000 tons. Forty-four of these smaller pontoons were built in Tacoma, the other 10 in Aberdeen.

The pontoons for the new floating bridge are larger than the pontoons that supported the old floating bridge. This is part of the design requirements to allow the new bridge to support a wider roadbed and withstand sustained winds of up to 89 mph.

You can read more about the pontoons in our online floating bridge booklet (pdf 6.8mb).

Pontoon design modifications
In May 2012, more-than-expected cracking was discovered in the first four large, concrete pontoons built for the new SR 520 floating bridge. The pontoons modification page in our Resource Library provides a chronological account of the discovery and the actions subsequently taken to modify the pontoons and ensure their 75-year design life.