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Jalopnik: Shocking Video! Manhattan Bridge Twisting And Turning [Infrastructure]

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504x_Manhattan_Bridge.JPGIf you understand anything about materials properties, try not to roll you eyes at this breathless NBC Nightly News video showing time lapse photography of the Manhattan Bridge flexing under the load of a train.

To the layman, the idea of a bridge flexing is certainly terrifying. Bridges are supposed to be strong, symbols of rigidity, unwavering under the highest of loads. Well, yes, but in order to do that, they have to be flexible. There's an interesting correlation between durability and flexibility. The more rigid a structure, the more susceptible it is to stress fractures, and over repeated duty cycling (like running a train over a bridge) a rigid structure will eventually fail catastrophically because of the inherently brittle nature of rigid structure.

Conversely, flexible structures are much more able to deal with repeated loading, and over the life of a structure, the likelihood of catastrophic failure is significantly lower than a rigid structures, instead, a predictable creep will stretch the structure over time. Think of it this way, take a cheap-o pen made out of plastic and an expensive fancy one made out of rigid lacquer, bend the lacquer pen and it's going to break on the first try, bend the plastic one and it'll give, then return to shape, no worse for the wear. The same holds true for iron girders and steel cables, only we're not used to seeing those kinds of things bend and flex.

So for you New Yorkers apparently scared of taking the Manhattan bridge, relax. The video of the bridge flexing under the load of a train is proof it's working properly. If it wasn't bending like you see in the video, you'd have to worry about it randomly shattering instead of sagging under a load. So look at this video as a tribute to engineering rather than the latest sensationalistic lunacy to be irrationally afraid of.

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I watched the video first and thought, "that's what it's supposed to do, are they saying it flexes more than it should?" Then I read the article. Talk about non-news. Only good reason for this to be in the news is that it sounds like the video hit the internet & scared people, so putting it on the news gives a chance to relieve some fears. The way it was presented in the news video, though, seems like it would just add to them.

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Actually, this isn't surprising. The Manhattan bridge was built before the subway and was not really designed to have trains on it... ESPECIALLY on the outside edges, which are somewhat cantilevered out over the main supports.

In 1986, it was discovered that the bridge WAS bending well beyond its limits, in the form of sagging at the edges, and the subway B and D trains were rerouted... it took 20 years of reconstruction before they were rerouted back a few years ago.

Being an infrastructure geek of sorts, I'm not afraid of any of the NYC bridges or tunnels... but they have been out there in the elements for a long time and during NYC's darker days suffered from a lot of "deferred maintenance". Once that steel rusts away, it doesn't come back. Sure, a lot gets fixed now, replacing or reinforcing beams, but one needs to be realistic and know that this stuff is somewhat compromised.

Just driving over the Brooklyn Bridge, you can clearly see how the lanes sag and rise over the supports. BFD, I drive over it during off-peak... if it was to go, it work happen during peak when its a parking lot.

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