[URY914]

Originally Posted by svandamme

Because a round manhole cover cannot fall into the manhole.
a square, or oval manhole cover could and obviously eventually would
An even triangle could work fine, but more manufacturing cost + more hassle since it needs aligning to manhole.. a round manhole cover is always aligned with the round manhole...

A round cover has only one cross section dimension, therefore it does not have not "weak" spot.