system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are bank vault doors round and not rectangular like normal doors?
gpt
Traditionally, they work by pushing big rods into holes in the door frame. This is easiest if the rods are all the same length, which is what a circular door gets you. The circle shape also means you can't make use of stress concentrations at the corner, since there isn't one, and you can't really use a jack to force the door frame open like you could with a rectangular door, since it will just squeeze the rest of the door even tighter.