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Comment by Retric | Hacker News Reader

Elevators use simple physical safety systems.

Which is why you see headlines like: "Skyscraper elevator plunges 84 floors — and everyone survives" https://www.cbc.ca/news/world/elevator-plunge-chicago-1.4913...

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Every link i’ve seen to this story annoys the hell out of me because they don’t explain HOW the elevator eventually stopped. That’s literally all I want to know.

It reached the bottom. Probably hit one of these: http://elevation.wikia.com/wiki/Buffers

Note that it wasn't descending particularly quickly; one cable snapped but other safety systems were still operable.

The article says it went from the 95th to the 11th floor. It appears it stopped before reaching the bottom buffer.

The best book ever for understanding how Elevators work is a wonderful late 20th century work of fiction called The Intuitionist by Colson Whitehead, which was described by its author as what you would expect to happen if you let a child grow up reading elevator manuals.

Entering “elevator safety mechanism” into google will bring up several links with explainations. The first link for me references this story.

There was an elevator hacking talk that went into the buffers at the bottom of the elevator. You may have seen them in elevators that have glass on the outside of the shaft like in malls and hotels. The elevator never rests on those. Those are to take the impact of an elevator that crashes. Inspectors will often test the buffers as well as part of routine maintenance:

https://www.youtube.com/watch?v=ZUvGfuLlZus

The basic theory is that between the car and the wire there is a spring connected to brakes. When the car hangs in the wire this spring is loaded and the brakes disengage, if the wire breaks, the spring returns and brakes engage. All this is mechanical.

What I never understood is how elevators can travel downwards so quickly without such a system engaging. Can they block it somehow or are they never accelerating fast enough down?

I’d also like to know what they meant by “plunges”. How fast did it descend? Was it in free fall?

You can easily tell that it wasn't in free fall, because if it fell 84 floors in free fall and then stopped, the passengers would have died.

It could have slowed down gradually and stopped. The articles I’ve read don’t do a good job describing anything.

If it slows down gradually, it's not in free fall.

Guess again. The cable could break at floor 84, and the elevator car could travel through the next 50 floors, and not slow it’s descent until the 34th floor, at which point, given ten vertical feet per floor, it would still have a total stopping distance of nearly 200 feet, or 2/3 (66%) of a football field.

So, 500 feet of freefall is still substantial. That puts the situation on par with a bungee jump.

What's deadly is the other type of failure, where the counter weight causes the elevator to shoot upwards, like what happened in that elevator in Chile. There's a video of the guy confused as the doors continued to open/close at each floor. He had severe injuries.

Apparently if that ever happens, your best bet is to lay flat on the floor with your hands covering your head, as the impact will launch you upward.

Thats how I usually approach stressful situations, so I should be fine.

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