One reason could be that they are better fixed in ground than in walls. Was this the only reason?
- Floors and ceilings are stronger than walls
- One can climb horizontal bars as one would a ladder
- One can use the body's strongest muscles to apply force to the bars
Regarding @SNag's quote from Quora- Namit Kothari must not be a welder. Usually in steel the weld is the strongest part of the structure.
I'll give you one more reason that hasn't been mentioned - it's easier to put a door into a vertical bar setup, since the vertical bars wouldn't require an additional frame to support the door.
This is kind of user experience, so...
- This is a wild guess, but if they were horizontal, prisoners can use the added support of gravity to bend them.
- They fit better in the ground.
- Usually the distance from ground to ceiling is shorter than the distance between side walls. More the distance, easier it is to bend a bar.
- Prisoners can use horizontal bars to hang themselves.
- Horizontal bars are easy to climb.
- If you have horizontal bars, you need a column in the middle to attach a door, if not the door is one of those weird cat-flap things.
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I believe the answer lies in Newton's third law of motion. The bars are made thick to prevent a prisoner to break out free.
Case 1 : Assume the bars are horizontal. In this case, the prisoner has a better chance to bend the bars as the ground gives the desired 'normal reaction' to the prisoner against the force applied by the prisoner to bend the bars. This makes it easy to achieve the result.
Case 2 : Assume the bars are vertical. The force required by the arms (way weaker than thighs) is same but the arms require better leverage than thighs to bend the bars which makes it way more difficult to break free.
There are two reasons for this - strength and cost - both of which are related to the welding.
The weakest point of a metal structure is the weld holding it together. A vertical door (ie one longer in height than width) will have less individual bars (and less weak points) than a horizonal one if the bars are the same thickness. Therefore it is a stronger door, which is its main purpose.
The other factor is that a weld must be applied by a welder, which takes time and effort. Less welds equal less time, which is generally cheaper.
...there might also be a psychological or affordance-based reason, namely that the horizontal bars would allow the prisoner to look from side to side over the horizon and have sense of "free range" from that. The vertical bars will obstruct this visual sense of freedom and "clip" the visual space into smaller bits. I am guessing that could be an "effect" subconsciously wanted by the imprisoner.
See all answers here QUORA: Why are jail bars vertical and not horizontal?