The longest permanent floating bridge, the Evergreen Point Bridge, is built across Lake Washington in Seattle, USA. The Washington State Highway SR 520 runs along it, a section of which stretches for 4.7 thousand meters. The Lacey V. Murrow Memorial Bridge, the second longest in the world, runs through the same lake two miles to the south, and its length is 2 thousand m.
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| The longest floating bridge in the World |
But why exactly floating bridges? The answer lies in the difficult geographic location of Lake Washington. The lake bottom is too soft for a conventional bridge to support. Another version of the bridge, suspended, would have required building pylons as high as the Space Needle tower in Seattle, which would have been too expensive.
The idea to build a floating bridge across Lake Washington was first proposed by engineer Homer Hadley in the 1930s. Hadley worked for a company that designed concrete barges during World War I and proposed the idea of joining hollow concrete barges back to back to Lacey V. Murrow, then director of the state's highways department. The Hadley Floating Bridge was such a success that Washington State adopted the concept for future bridges. The second bridge, Evergreen Point Bridge, the longest in the world, was opened in 1963. The third bridge was named after Homer Hadley himself, who first voiced this idea. Today, Lake Washington is home to three bridges, all of which are among the five longest floating bridges in the world.
Longest pontoon bridge across Lake Washington, built in 1963. for more than half a century, it occupied the first place among such structures. But time takes its toll, and it has exhausted its safety margin, and therefore it was decided to build a similar new bridge next to the old one.
Evergreen Point opened in early April 2016. During the celebrations, it was announced that the bridge entered the main book of world records, beating the achievement of its predecessor by almost 40 meters.Its length is 2.3 thousand meters, and its width is 35 meters.
On the muddy bottom, 58 anchors are fixed, which hold the pontoons in a static position on thick metal cables more than 7.5 centimeters thick each. As in the last time, a floating structure for crossing the lake proved to be the most expedient. In the middle, Lake Washington has a depth of about 70 m to the bottom line, and below there is an equally thick layer of soft silt and volcanic ash, on which it is almost impossible to place the supports of a conventional bridge.
On the surface of the water, 77 pontoons made of aerated concrete drift. The largest of them has a size of 23x110 m. On top of the pontoons, a flooring is laid, which consists of 776 prefabricated sections. Each of them rises 6 m higher than the pontoons. This decision made it possible to create an additional "corridor" under the roadway, provided for the maintenance of the facility.
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| The longest floating bridge in the World |
The project was not cheap. But it cannot be called unprofitable, since 74 thousand motorists use the paid commercial ferry every day. Considering the built-in safety factor for the next 75 years, the construction of the new bridge was considered a wise investment.
This bridge was designed to withstand the so-called 100-year storm (157 km / h).
The construction required accurate calculations and good organization of the process to coordinate several parallel operations along the entire length of the bridge. Installation of the structure was carried out at once from both ends and finished in the middle. The last of the pontoons was installed in a permanent place in the summer of 2015, one year before the start of operation.
The new bridge allows vehicles to move in two general-purpose lanes and use another additional lane in two directions. For unscheduled technical stops, wider shoulders are provided, there are dedicated paths for pedestrians and cyclists. As an additional option, it is planned to lay a high-speed tram line.
And the very first pontoon bridge appeared on Lake Washington in the 1940s. The eponymous predecessor Evergreen Point was built in 1963 due to the increased number of vehicles. It consisted of 33 pontoon units with four traffic lanes. In the center there was a drawbridge for the passage of barges and ships.
The bridge could withstand wind speeds of 123 km / h, if the gusts were stronger by only 5-6 kilometers, then the crossing had to be closed. In addition, the structure of the bridge was not sufficiently earthquake resistant and could suffer from strong tremors. By the beginning of the 90s, the pontoons had lost their former strength, and the bridge became vulnerable to the water element of the lake. Another reason for the closure of the old bridge is its insufficient capacity.
Planning for the new facility began in 1997, and construction only 15 years later, in 2012. The Washington State Department allocated $ 586 million for the project, but it ended up costing $ 930 million.