Washington now holds the title for the world’s longest floating bridge. Once again. As with the 3 other longest and heaviest floating bridges on the planet, the colorfully named SR 520 Bridge can make its house in the Seattle place.
SR 520, which cost $4.five billion and replaces an aging bridge spanning the identical stretch of Lake Washington, not long ago opened to westbound visitors on Point out Route 520 (the other facet opens quickly). It is a lot more than 7,seven-hundred toes lengthy, which raises two queries: Why span a lake with glorified pool noodles? And how do you make all that concrete float reliably enough to carry cars and even trains?
Washington’s love of floating bridges can be traced to the topography all over Seattle. Lake Washington, east of the city, plunges to a depth of a lot more than 200 toes, and the base is, from an engineering standpoint, crap. It is delicate silt, which can make setting up a conventional suspension bridge with rooted towers quite challenging (read through: highly-priced). Every single tower would have to be about 630 toes tall, the point out DOT says—twice the top of the sentinels keeping up the Brooklyn Bridge.
Plunking down a large ol’ suspension bridge in the tony Seattle suburbs also would be quite challenging politically, says Dave Becher, a point out DOT engineer and building manager for the job. There would be “pushback from the inhabitants,” he says, not minimum mainly because all those lofty towers would hinder their beautiful (and useful) lake sights.
The challenges have built Washington a world-wide authority on floating bridges, one that attracts engineers from international locations mulling equivalent assignments. A Japanese delegation attended the SR 520 project’s grand opening previously this month, eager to see how it is completed.
Concrete Around H2o
Floating bridges have been all over a great deal extended than Washington. The plan dates to at minimum 480 BC, when King Xerxes used three hundred ships moored bow to stern to get his army throughout the Dardanelles. Washington’s latest bridge is a contact a lot more subtle. Alternatively of ships (or defunct plane carriers), these floating bridges use pontoons. In the situation of SR 520, that’s seventy seven giant packing containers of air, the major of them 28 toes tall, seventy five toes vast, and 360 toes lengthy.
If you believe producing a box with concrete is a rapid way to make it sink, you are not accounting for the power of buoyancy. As with a cup put facedown in a bathtub, the upward pressure exerted by the h2o is equal to the downward pressure of the pontoons, so that about 20 feet of every single pontoon settles down below the h2o, and 7 feet rests earlier mentioned the area. Washington’s engineers developed a specialised concrete combine and pouring method to hold the pontoons pretty much crack-absolutely free.
Inside of every single pontoon are more compact, watertight compartments. If one thing goes pretty completely wrong, the plan goes, a leak will be restricted to just one compartment, minimizing the chance of submersion. Need to sensors in every single pontoon detect an H2O invasion, they send out urgent alerts to the DOT’s nearby bridge maintenance facility, to its communications heart, and to unique transportation officers.
The bridge is constructed to continue to be unflappable in 89 mph winds, which would qualify as a 100-calendar year storm in the Puget Audio region. It is secured to the base of Lake Washington by fifty eight huge anchors, every single of which is affixed to 3-inch steel cables. Washington is not messing all over with “large” listed here. A single kind of anchor, developed to sink into delicate silt, weighs 107 tons. The other kind, which supplies security and retains the bridge from wobbling with the waves, clocks in at 450 tons—more than a totally loaded Boeing 777. They’re built not to go, even if the bridge itself goes: The anchors that secured SR 520’s predecessor are no extended necessary, but they are nevertheless hanging out at the base of the lake.
In situation you are thinking, these things do go down at the time in a though. In 1979, a part of the Hood Canal floating bridge foundered in a storm. In 1990, a maintenance crew performing on a shut, nineteen forties-period bridge connecting Mercer Island to Seattle still left pontoon hatches open over Thanksgiving getaway. The pontoons began to fill with h2o. Little cracks in the bridge’s compartments did not assist the circumstance, possibly. “The failure was reported to have proceeded in a falling-domino fashion,” engineer Henry Petroski wrote in American Scientist. Not a fun impression, but with any luck , one that will not reappear.
“Since 1940, we have an understanding of a great deal superior the mechanisms that you have on a floating bridge,” says Becher. The point out transportation company is so self-assured in its bridge that there are plans to install gentle rail on the SR 520 one day (though opening it to eastbound visitors, on April 25, will absolutely come first). But even if you believe in science and engineering, you can be forgiven for ducking out of that initially gentle rail exam push throughout a floating bridge.
Go Back to Prime. Skip To: Get started of Write-up.