The Floating Bridges of Seattle

A bridge of concrete and steel that floats may seem highly unusual, if not impossible, but there are twenty such bridges around the world, five in the U.S. state of Washington alone, of which four are the longest floating bridges in the world.

Floating bridges, also known as pontoon bridges, are usually temporary structures built out of wood during times of emergencies such as war. Wooden floats and sometimes boats are lashed together and flat planks are laid over creating a roadway, allowing men and materials to cross bodies of water. Pontoon bridges have been used to great advantage in many battles throughout history, including the Second World War and during the Iran–Iraq War.

Homer Hadley Floating Bridge and Lacey V. Murrow Memorial Bridge. Photo credit: Atmoc Taco/Flickr

The longest permanent floating bridge, Evergreen Point, commonly called the SR 520 Bridge, lies across Lake Washington, in Seattle. It carries the traffic of State Route 520 and is 4,750 meters long. The Lacey V. Murrow Memorial Bridge, the second longest bridge in the world, lies across the same lake just a few miles to the south, and is 2,020 meters long.

But why floating bridges? The answer lies in Lake Washington’s tricky geographical location. The lake bed is too soft for piers of a conventional bridge. The other option, a suspension bridge, would require bridge towers the height of Seattle’s Space Needle, which would have been too expensive.

The idea of a floating bridge across Lake Washington was first proposed by engineer Homer Hadley in the 1930s. Hadley had worked for a firm designing concrete barges during World War I and he proposed the idea of connecting hollow concrete barges end-to-end to Lacey V. Murrow, the state's then director of highways. Hadley’s floating bridge was such a success that Washington State adopted the concept for future bridges. The second bridge, Evergreen Point, the longest in the world, was opened in 1963. In honor of the man who first spearheaded the idea, the third bridge was named after Homer Hadley. Today, Lake Washington is home to three, and all rank among the five longest floating bridges in the world.

The original Lake Washington floating bridge, opened on July 3, 1940. Photo credit: seattlepi.com

The Lacey V. Murrow bridge (right) is the world's third-longest floating bridge at 6,603 feet. The current bridge was built in 1993, replacing a structure that sank during a severe storm. Photo credit: Joshua Trujillo/seattlepi.com

Homer Hadley Bridge (left) and Lacey V. Murrow Memorial Bridge (right). Photo credit: Joshua Trujillo/seattlepi.com

Photo credit: Anna/Flickr

Evergreen Point Floating Bridge. Photo credit: Atomic Taco/Flickr

Aerial view of Evergreen Point Road, Eastside staging area and floating bridge construction barges. Photo credit: Washington State Dept of Transportation/Flickr

A photograph taken on June, 2012, shows the newly set girders with the SR 520 floating bridge. Photo credit: Washington State Dept of Transportation/Flickr

Evergreen Point Floating Bridge. Photo credit: SounderBruce/Flickr

Evergreen Point Floating Bridge. Photo credit: Ramanathan Kathiresan/Flickr

Evergreen Point Floating Bridge. Photo credit: Peter Svensk/Flickr

Sources: Seattle Pi / The Engines of Our Ingenuity / Wikipedia