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With 77 concrete pontoons, the Evergreen Point Floating Bridge spans 2.35 km of Lake Washington without a series of deep pillars, using buoyancy and submerged anchoring to support and stabilize the highway on the water
The Evergreen Point Floating Bridge, located on SR 520, spans Lake Washington in the United States, featuring a floating span of approximately 2.35 km supported by 77 concrete pontoons. Instead of relying on a series of deep pillars, the structure utilizes buoyancy and submerged anchoring to keep the highway above the water.
The Evergreen Point Floating Bridge Avoids a Series of Pillars on the Lake Bottom
The solution implemented for the crossing is directly related to the characteristics of Lake Washington. The lake’s depth makes a conventional structure supported by a series of pillars connecting the deck to the bottom less attractive.
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In the main span, the highway rests on enormous watertight concrete boxes. The pontoons remain at the surface and function similarly to the hull of a vessel, using water displacement to generate buoyancy.
Although made of concrete, the pontoons contain large, sealed volumes. Therefore, they displace enough water to produce the necessary buoyancy to support the structure above them.
The principle depends on the relationship between mass and displaced volume. Thus, a structure can be heavy and still float as long as its configuration allows it to generate sufficient buoyancy to counteract its weight.
The 77 Pontoons Form a Continuous Platform for the Highway
The Evergreen Point Floating Bridge employs 77 interconnected concrete pontoons. Together, they distribute the highway’s loads across the span, preventing all the weight from being concentrated on just a few points anchored to the bottom.
These units serve different functions within the system. In addition to generating buoyancy, they help distribute the weight of the deck and traffic, forming a continuous platform for the bridge’s upper elements.
The configuration recorded in the SR 520 program shows how the pontoons work as parts of a single set. The buoyancy is primarily responsible for the vertical support of the structure.
This means that the weight of the highway acts downward while the buoyancy produced by the volume of water displaced by the pontoons acts in the opposite direction, keeping the platform on the surface.
The result is a roadway infrastructure mainly supported by the water displaced by the large concrete boxes, rather than solely relying on supports built directly on the lake bed.
Anchoring system prevents the bridge from floating freely
The ability to float only solves part of the challenge. A bridge of this scale must also remain aligned despite the effects of wind, waves, and variations in water conditions.
Therefore, the structure does not function like a free-floating vessel. A submerged anchoring system restricts horizontal movement and keeps the pontoons positioned within the predetermined limits for crossing.
The anchoring lines are attached to stable points on the bottom. While the pontoons support most of the vertical weight, the anchors help prevent excessive lateral displacement caused by environmental forces.
Some elements involved in the anchoring have masses of hundreds of tons. The scale indicates that a structure can float while simultaneously relying on extremely heavy components to control its position.
Weight, buoyancy, wind, waves, and anchoring must work together. The system allows the bridge to respond to lake conditions without losing the necessary alignment to permit vehicle passage.
Floating span reaches 7,710 feet, about 2.35 kilometers
The floating section of the Evergreen Point Floating Bridge measures 7,710 feet, approximately 2.35 km. Over this distance, pontoons, connections, joints, decking, and anchoring systems must function as parts of a single infrastructure.
The length also increases the importance of load distribution. The 77 pontoons provide a base over kilometers and help prevent the concentration of weight on a few support points.
The use of concrete further demonstrates that floating is not synonymous with lightness. The ability to remain on the surface depends on the combination of shape, internal volume, mass, water displacement, and control provided by anchoring.
In practice, pontoons transform the physical principle of buoyancy into a foundation for a multi-kilometer roadway.
The platform rests on the water while the submerged anchoring system restricts movement and keeps the crossing positioned.
This article was developed based on the information provided about the Evergreen Point Floating Bridge and the SR 520 program, with data, numbers, and technical descriptions preserved as per the consulted content.
