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From the perspective of "design the manhole cover first", the reason is that manhole covers aren't just round — they're (very subtle) domes. A dome is: 1. the
by derefr 1y ago
From the perspective of "design the manhole cover first", the reason is that manhole covers aren't just round — they're (very subtle) domes.
A dome is:
1. the best shape for taking stress from very heavy trucks putting all their weight on them without the manhole itself gradually bowing, and
2. is best at transferring that stress equally into the manhole wall (cast concrete cylinder) itself. (A square manhole + manhole cover would disperse force unfairly, potentially gradually cracking the manhole walls / requiring stronger walls. A flat circular manhole would disperse force upon the center of the manhole equally onto the manhole walls, allowing for lower-material-cost manhole walls. A domed manhole cover additionally disperses force from most points on the dome equally into the manhole walls — important, as vehicles won't necessarily be driving over the exact center of the manhole!)
...but really, this is the wrong direction to work in. The original reason manhole covers are round, is simply that the walls of a manhole are best made round, for the same reason drink cans and barrels are best made round: a closed cylinder is great at taking compressive force from a lid above; passing it through as soft, equal tensile force through its walls without buckling strain; and then turning that force back into an equal compressive force on the floor / subsurface.
Most manholes are generally small closed cylinders acting as maintenance areas for nearby pipes, with the pipes coming in through the sides of the manhole walls, and the concrete bottom floor of the manhole resting upon compacted earth.
In this situation, any shape for the manhole other than a cylinder — if driven over for years/decades by cars — would gradually pound the uneven force acting upon the manhole's floor into the earth below, unevenly accelerating soil subsidence. Eventually, you've created a sinkhole below the road, right outside the manhole wall on one side.
- adastra22 1y agoAs someone with a mechanical engineering background, all these clever answers to the question (e.g. "manhole covers are round to prevent the cover falling in") are strange. Covers are round because the hole is round, and the hole is round because a cylindrical shape is sturdy and prevents collapse. That's it.
- nomel 1y agoThat's not "it". A square cover would not be feasible, without a tether system to fish them out of the literal shit, after they fall in from a few inch misalignment.
- adastra22 1y agoWhy would you put a square cover on a round hole? The hole came first. They dig a hole, then they have a need to put a cover on it. Making a circular cover to fit over a circular hole is if anything cheaper and easier than making a square cover over an inscribed circular hole, at least when working with metal. The hole drove the design of the cover, not the other way around.
- derefr 1y agoIn this hypothetical, presuming you're putting the manhole (subsurface conduit/plumbing maintenance access point) in after you've already paved the road: it's because the tool you'd use to cut a hole into pavement (i.e. a concrete saw) cuts straight lines — and it's easier to make a square/rectangle out of straight lines than a circle. And sure enough, whenever you see workers hacking up the road, they generally are cutting square holes. Refer, after that, to the process of constructing a manhole (https://www.envirodesignproducts.com/blogs/news/how-are-manholes-made https://www.envirodesignproducts.com/blogs/news/how-are-manh...). At the end of this process, you have a square hole in the pavement, opening to a square excavation, bottoming out at a square concrete foundation, on which has been set a round concrete cylinder, which is then surrounded out to the edge of the square hole with packed earth. Given this, you could equally-well finish this job either: 1. by placing a square of metal to fill the entire square packed-earth space you've constructed (as when bridging a pothole with a temporary steel surface plate); 2. or by first paving over the exposed packed-earth part, and then placing a circle of metal to cover only the manhole entrance itself. ...which is why people do justifiably ask why, in practice, we seem to always favor option 2 over option 1.
- adastra22 1y ago
- rozab 1y agoYou seem very confident about all this. In the UK manhole covers are generally square.
- rpdillon 1y agoThis piqued my interest, and it seems you're completely correct, but also the metal covers seem much more complex than what I was imagining. https://www.randb-uk.com/product-category/ductile-iron-manhole-covers-frames/ https://www.randb-uk.com/product-category/ductile-iron-manho...
- Groxx 1y agoYep. You'll find square hole covers all over, they're not rare at all. The strength of the lid and its ability to be supported is not at all an issue on a single human sized hole - it's a thick, relatively cheap slab of metal over a small opening. They're the strongest surface of most roads, by a HUGE margin.
- deleted 1y ago[deleted]