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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
(Source: Namit Kothari's answer to Why are jail bars vertical and not horizontal?)
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
(Source: Tom LeGrice's comment on Namit Kothari's answer)
...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.
(Source: Martin Kofod Ludvigsen's answer to Why are jail bars vertical and not horizontal?)
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