May 9, 2017

Grand Award
Public works: Bridges

Photo courtesy of WACA

Precast concrete deck panels do double duty as the bridge structure and driving surface, eliminating the need for 3.2 million pounds of steel.

State Route 520 floating bridge
and landings Public works Bridges

Location: Medina

To maximize durability and reliability, the state Route 520 floating bridge uses concrete and steel in four distinct structure types: approaches, transitions, high-rise spans and low-rise spans.

For the approaches, more than 27,000 cubic yards of concrete were placed to erect twin three-span, 630-foot-long, cast-in-place segmental bridges founded on spread footings. These structures connect the bridge to land, as well as support the four 190-foot-long steel-girder transition spans with bridge decks cast in place from 1,300 cubic yards of concrete.

Ready-mix concrete allowed the use of cast-in-place balanced cantilever bridges, eliminating more than 27,000 square feet of trestle and falsework. Using spread footings reduced the number of piles required to build the bridge by 75 percent and preserved 95 percent — or 7,860 square feet — of permanent lake bed habitat.

This design also allowed the team to move one of the piers out of the lake. The sleek bridge produced from this type of concrete reduced shading of the shallow-water habitat by 79 percent.

For the next segment, the 2,130-foot-long floating high-rise structure, 12,000 cubic yards of concrete form the columns and crossbeams that support the 90-foot-long precast girder spans.

From the high-rise, vehicles move to the mile-long low-rise structure that crosses the lake. Here, concrete elements replaced the steel bracing and framing suggested in the original concept design.

Instead, the design-build team used segmental precast concrete deck panels to form both the structure and driving surface, eliminating 3.2 million pounds of steel. The 776 panels were formed by the team in a nearby casting yard using more than 45,200 cubic yards of 6,500-psi concrete.

To provide additional flotation and stability for the large, longitudinal pontoons supplied by the project’s owner, the team installed supplemental pontoons, similar to outriggers on a canoe.

The team constructed 44 supplemental pontoons in a casting basin in Tacoma using 44,000 cubic yards of 7,500-psi concrete. These pontoons are 100 feet long, 50 to 60 feet wide, and 28 feet tall.

The pontoons were towed to Lake Washington, where they were joined with the longitudinal pontoons to form the foundation for the floating bridge.

The pontoon concrete needed to flow up to 30 linear feet to meet the tight schedule, so the team generated a 28-inch-spread mix composed of fly ash, microsilica, a high-range water reducer and a shrinkage reducer. Careful thermal control, external vibrators, watertight forms and the high-tech concrete were used to meet the goal of casting concrete with no cracking greater than 0.006 inches.

These high-quality, state-of-the-art mix designs, combined with the 3.5 million man hours spent designing and constructing the project, will ensure the bridge meets the demands of its 75-year design life.

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