Construction of one of the world's longest floating bridges, connecting Seattle with Medina, Washington on State Route (SR) 520, has been completed and will open for traffic in mid-April. The 7,710-foot bridge replaces the Evergreen Point Floating Bridge, which stood for 52 years as the longest floating highway.

Work on the new bridge began in 2011 after years of patchwork maintenance to the existing bridge, including repair of cracks in the concrete of the pontoons that anchor the bridge to the bottom of Lake Washington. Engineers also deemed that the bridge, built to lesser standards in the early 1960s, was vulnerable to failure during severe windstorms and that the highway's fixed-column bridge approaches could fail in a strong earthquake.

The new span is one of only a handful of floating bridges in the world and solves the design challenge of building a safe and sturdy roadway over a deep body of water with a soft bottom. According to the Washington State Department of Transportation (WSDOT), the deepest point in Lake Washington is 214 feet, and the support towers required of a conventional suspension bridge would have to have been approximately 630 feet in height—nearly that of the Space Needle—to anchor it.

Additionally, WSDOT says, suspension bridges need to travel in a fairly straight line. Because SR 520 is a curved corridor, building a suspension bridge was infeasible.

Floating bridges are made of large, water-tight concrete pontoons connected rigidly end to end, upon which the roadway is built. Despite their heavy concrete composition, the weight of the water displaced by the pontoons is equal to the weight of the structure (including all traffic), which allows the bridge to float.

A tugboat pushes a pontoon into place on Lake Washington. Image credit: Washington State DOT. The SR 520 bridge's pontoons were built on dry land and then floated and towed like barges to the bridge site. Crews connected them to grounded approach structures on each end, starting at the edge and then piecing them together, eventually joining at the bridge’s center. The pontoons are held in place by enormous steel cables connected to anchors buried deep in the lakebed.

All 776 of the new bridge’s roadway deck sections were constructed at a lakeside facility. Barges delivered them to the bridge construction site on Lake Washington. From the top down, the bridge's main structures include:

· 331 concrete and steel girders that support the roadway deck;

· 771 concrete columns on which the girders lie;

· 77 concrete pontoons that keep the entire structure afloat (the 21 largest ones are 360 feet long and weigh 11,000 tons each);

· 58 anchors, some weighing 450 tons, that hold the floating bridge in place.

According to WSDOT, wind and wave forces are typically the controlling forces in the design of floating bridges. A major factor in wind and wave effects on floating bridges is called the fetch, which is the unobstructed clear distance over the water that wind can travel to the bridge. The longer the fetch, the higher the wind and wave forces will be.

In Lake Washington the critical fetch is to the southwest of the bridge, since the largest storms historically come from the southwest. Wind and wave forces cause the pontoons to bend, heave and twist, creating large stresses in the pontoons and anchor system. If a 100-year-storm event were to occur, the pontoons are designed to prevent large cracks from developing that would allow water to leak in and sink the bridge.

In the case of earthquakes—which are fairly common in the region—the floating section of the SR 520 bridge will not be affected directly by ground shaking because of its pontoon construction. However, very deep, low-frequency earthquakes can cause a seiche, or a surface wave similar to a tsunami, which WSDOT says could cause the floating bridge to bend and heave at the lake surface, adding large loads of pressure to the pontoons and anchor systems. A seiche in Lake Washington could also create an underwater landslide that could cause the pontoon anchors to slip or break.

WSDOT says the bridge's pontoons, bridge deck and anchor cables are designed to withstand storms with sustained winds of up to 89 mph—a 100-year storm. Its expected service life is 75 years.