Shake Tables & Terrorism

Things you learn in engineering school:

All systems have a resonance frequency. If you push on them at the right interval, the amount they displace skyrockets. This is why soldiers break step when crossing a bridge. This is why certain frequencies of light cause our cells damage. This is why you push someone on a swing when they reach the top of their arc.

Earthquakes are most damaging to a building not when they’re very strong, but when the frequency of their impulses match the resonance frequency of that building. This is huge, because it means you don’t have to push on a building with a lot of force to bring it down; you just have to push on it at the right rate.

Today, there are devices that do this very well: shake tables, found in many civil engineering schools. Here’s a video:

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http://youtube.com/watch?v=kzVvd4Dk6sw

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Crucial point: you don’t have to put the table under the building to cause it to shake. Take the table to the top of a building, bolt it to the floor, and turn it on. Adjust the frequency until you reach resonance. As you approach the correct value, motion will begin and then intensify. If the building is moving more than a couple inches, then the structure’s tolerance is probably being exceeded and its lateral system will start to fail (concrete will crack, steel will yield and elongate, timber will splinter).

The structure probably won’t collapse, because modern buildings are designed to eat up a lot of energy from earthquakes (cracks and yielding represent dissipated energy), but it will be effectively destroyed and need to be torn down. This strikes me as a big security vulnerability for any building, and I’m not sure how you prevent it, as shake table pieces are not particularly sinister looking.

Basically, there’s no way we can stop terrorism; there’s always ways to get around security and do messed-up shit. Many people have pointed out that despite all the heavy airline security, buses and trains (and bridges in Minnesota for that matter) are ridiculously vulnerable. Well, so is every tall building, regardless of whatever post-9/11 security has been implemented at them.

Hence the need for sane policy that prevents the causes of terrorism. A good start would be if the US wasn’t the biggest arms dealer in the world, but then I’m apparently a freedom-hating French commie. So don’t listen to me.

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