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One would actually think round manhole covers have several disadvantages to offset their one purported strength. For example, it would be more difficult to atta
by sho 18y ago
One would actually think round manhole covers have several disadvantages to offset their one purported strength. For example, it would be more difficult to attach a hinge or locking mechanism. Also, the grip hole would not be guaranteed to be in the same alignment every time - if you were using a machine to raise the cover, you might have to manouevre the machine into incovenient angles to properly access the grip, according to the whim of the previous team who replaced the cover. With a square lid, the alignment could be standardised to the most convenient position for machine access. Also, assuming the bars in the ladder are straight, which AFAIK they always are, placing a ladder in a round hole wastes space.
Feynmann's answer about the cylinder being the strongest structure against compression is true but one wouldn't expect this to be a critical factor at typical street-access depths. Maybe if you were going down 50m or more, but even so, dirt is hardly as effective at exerting pressure as a free-to-move liquid.
- swombat 18y agoWith a square lid, the alignment could be standardised to the most convenient position for machine access. Small quibble, but most of those manhole covers and such are placed where they are because someone built some kind of tunnel there a hundred years ago (for whatever reason, from sewage to water to anything else). The layout of the streets then often evolves largely independently of the position of the manhole covers, thus resulting in some "interesting" placements of manhole covers, sometimes not all that accessible (though I guess it depends on how strict the local regulators are on the architects who design the new layout).
- sho 18y agoAh yes, like so many things it is probably for "legacy reasons". And needless to say really old manhole tunnels would definitely tend to be round, especially if they were constructed out of shaped stones, where the shape of the structure is an important contributor to its integrity.
- something 18y ago>Also, the grip hole would not be guaranteed to be in the same alignment every time it is if it's in the center.
- biotech 18y agoIt is easier to replace a circular cover, since you don't have to worry about it's orientation. I suspect that this is the main reason to use a circular manhole cover over some other shape.
- sho 18y agoBut then it would have to be lifted straight up, which would require a lot of force. Are they really like this? Now you've piqued my curiousity, I'm going to look on the street!
- zvikara 18y agoI had a car accident because of a rounded manhole cover. My wife had to emergency stop because a rounded cover in the middle of the road was half open, sitting vertically on top of the hole. Thankfully no one got hurt.
- jasonkester 18y agoFor what it's worth, I spent a summer pulling off manhole covers for a living. There's no machine required, just a bent rod with a handle, and they're easy enough to open. And for the record, yes, it is quite easy to drop the rectangular covers into the hole.