Where Is The Longest Floating Bridge In The World? (And What Is Its Total Length?)

One of the earliest floating bridges was constructed by Persian Engineers by parking 676 ships in two parallel rows to help Xerxes's army cross the Hellespont. Since then, floating bridges have evolved a lot from their makeshift nature to a sturdy transportation solution. Floating bridges made of wood or inflated tubes are typically used for military purposes, as they are quick to assemble and work well for lighter loads. These bridges are rare for nonmilitary use, as they are much more complex to build than conventional bridges over water.

Currently, there are only a handful of floating bridges that are open for nonmilitary use, with the Evergreen Point bridge in Seattle being the largest of the lot. The bridge is close to 7,710 feet in length and spans across Lake Washington. The current bridge is a replacement of the previous Evergreen Point bridge built in 1963, which was also a floating bridge made from concrete pontoons. In fact, the Lacey V. Murrow bridge on Lake Washington built in 1940 was the first ever bridge to use reinforced concrete pontoons.

The current Evergreen Point bridge is an engineering marvel, with a construction cost of close to $4.5 billion, though it is quite shorter than some of the biggest bridges in the U.S. Given the high cost of construction, which beats a conventional bridge of this length by a long shot, you may question why floating bridges are necessary on Lake Washington?

Lake Washington is the second-largest lake in Washington, spanning over 21,500 acres. The deepest point in the lake is close to 214 feet and is about 200 feet deep on average. The lakebed is made of soft silt, which makes constructing suspension bridges or bridge pilings extremely difficult and expensive. While constructing a suspension bridge was theoretically possible, it would have required massive pillars (around 600 feet) on both sides of the lake. Constructing such massive infrastructure would have been costly and probably disliked by the locals. Hence, a more feasible and community-friendly floating bridge was chosen for the purpose.

The Evergreen Point bridge (or the SR 520 bridge) was constructed by placing seventy-seven concrete pontoons over Lake Washington, to keep the entire structure afloat. The pontoons vary in size, with the largest ones being 28 feet tall, 75 feet wide, and 360 feet long. The bridge can sustain fast winds of over 89 mph and even has earthquake resistance, which is a huge upgrade over the previous bridge that had to be frequently closed on windy days. The water damage and cracks were a few of the primary reasons the previous Evergreen point bridge was decommissioned in 2016, preventing a disaster from the bridge's collapse.

Remember Archimedes' principle from high school science? It states that an object submerged underwater is pushed upwards by the water's buoyant force, equal to the weight of water displaced by the object.

In the case of the SR 520 bridge, the pontoons are the submerged objects that need to displace enough water weight to support the weight of the structure and vehicles crossing the bridge.

However, a concrete block drowns in water due to its high density, so how do concrete pontoons on the SR 520 not only float but also support such gargantuan weight? It is because of a clever engineering technique– the pontoons on the SR 520 are fairly hollow concrete blocks and displace much more water weight than their own, allowing them to float on water.

That said, simply getting pontoons to float is only half the work. Engineers also needed to keep the pontoons stable to keep the structure safe from water currents, tides, storms, and even earthquakes. This is done by anchoring the pontoons to the lake bed using 3-inch steel cables. These anchors need to be extremely steady to prevent unwanted movements and wobbling of the bridge. Some of the anchors on the SR 520 weigh around 520 tons and are securely fixed to the lake's bottom.