Manhole covers are back in the news again… sadly, it’s because they’re being stolen and sold at metal scrap yards for cash. But while we’re on the subject:
When asked “Why are manhole covers round?”
many smart people would reply that,
if the covers were square or rectangular,
they could fall in to the manhole.While this is certainly true, there’s at least one
other valid (and similarly logical) reason…What is it?
HERE’S one answer.
Comments (24)
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The stand pipe that it covers is round.
posted by Ed on 8-4-2008 at 6:59 am
Good point, Ed!
posted by Sandy on 8-4-2008 at 7:22 am
The covers are round because the holes are round!
Also, it’s easier to cover a round hole with a round cover: you wouldn’t have to worry about lining up edges or having the cover fall. The round cover lines up in any orientation.
posted by TomH on 8-4-2008 at 7:27 am
i had the same smart a$$ answer as tom. nice work!
posted by grant on 8-4-2008 at 7:32 am
Is a circle hole easier to cut than a square one would be?
posted by mark on 8-4-2008 at 7:42 am
a round cover is structurally stronger and it distributes loads from above more evenly
posted by The Other Brian on 8-4-2008 at 8:05 am
Like Tom said, round covers can’t fall through the hole, but a square one could (supposing you moved it to a diagonal position)
posted by Mikey on 8-4-2008 at 8:11 am
Having worked in the construction industry for many years I would like to give you the real answer. They are round so they can not fall into the drain that they cover therefore making them safe and simple to use.
posted by Jimbo on 8-4-2008 at 8:14 am
With a nod to Ed for the point; I must agree with Sandy’s answer.
I must disagree with TomH that a circle or square would have about the same degree of difficulty in alignment. Edges are edges & must be fit appropriately – 1 degree off is still off. Plus the advantage of a multi-sided cover is if it is bigger than the hole it will have natural stops at each tip & not requiring the entire cover to have a lip.
Mark, I will also have to disagree with your comment. Circles are more difficult to cut than a square & produce much more waste.
Therefore, ease of movement upon removal is the a very logical answer.
posted by Joey on 8-4-2008 at 8:15 am
A round cover has no specific weak points / flaws like a square or other-shaped cover.
posted by Chris on 8-4-2008 at 8:20 am
Who says that manhole covers are cut? Isn’t it more likely that they are formed or poured?
posted by Johnie on 8-4-2008 at 8:29 am
I could be not thinking clearly, but wouldn’t it be less metal to make a circle than a square thus saving on cost? If the area of a circle is pie time radius squared. And an area of square is two side multiplied. Then… a two foot hole would be 3.14 sq for a circle and 4.0 sq for a square.
posted by marty on 8-4-2008 at 8:44 am
I have to say… the mental work you folks can do amazes me. We should form a brain trust!
posted by Sandy on 8-4-2008 at 8:47 am
Another shape that would work is an equilateral triangle. You’re right about the rolling part, though. In addition to being easier to move from place to place theyr’e also easier to roll in and out of position when removing or putting it down.
posted by Joe on 8-4-2008 at 8:52 am
I meant the manhole, not the cover. If the hole itself was easier to cut as a circle, then the cover would need to be a circle, too.
posted by mark on 8-4-2008 at 8:52 am
An equilateral triangle wouldn’t work necessarily. It’s height is less than the length of a side, so it is possible to make a triangle cover fall in.
posted by JM on 8-4-2008 at 9:24 am
It would be less metal, but more wastage unless they melt down the metal in the end and recycle it.
posted by Ash on 8-4-2008 at 9:25 am
Yes Ash, I wonder if it is cut like a bisquit out of dough, in that case waste, or if it is forged metal thus no waste. Hopefully, there is a mental flosser out there who works for a local metropolitan sewer district and can answer all these pressing questions!
posted by marty on 8-4-2008 at 10:27 am
Also, beacuse they’re round, you don’t have to worry about the particular orientation, making it quicker to close. A square cover would only have 4 proper orientations, and a one shaped as an equilateral triangle would only have 3 proper orientations.
posted by gmsc on 8-4-2008 at 10:51 am
coz given any volume of metal a circle has the maximum circumference so the hole can be made large :)
posted by Jason on 8-4-2008 at 11:55 am
Nashua, NH uses triangle manhole covers. As far as I know it is the only place to do so.
posted by Mark on 8-4-2008 at 12:54 pm
Mark, the Nashua are not actually traingles, per say, as the sides are not straight. The are called Rouleaux triangles and have sides that on an arc from the opposing vertex. This allows them the safety of not falling into the manhole as the width is equal in all directions.
posted by Jeffrey on 8-4-2008 at 5:57 pm
why would you want to roll a manhole cover around all day? why not just set it in it’s place, and fugedaboudit???
posted by not.so.fast on 8-5-2008 at 1:52 am
Answer to big question of the day, “How do they get there in the first place?”
I don’t suppose any of you have driven down a soon-to-be-resurfaced street recently and seen the signs saying “RAISED STRUCTURES AHEAD,” have you? If you haven’t, go have a look at one of these streets and then come back–you can see how these things are installed.
I’ll wait.
Go on, do it now.
OK, you’re back now? Good.
Undoubtedly, you noticed that the manhole cover is a two-piece affair consisting of a collar and a cover. The collar is a flange of sorts with a lip on the inside of the circumference and a flange on the outside of the circumference. Since manholes aren’t just dirt holes in the ground (reminding me of the famous joke, “How much dirt is in a hole that’s 3′x3′x3′”?”) but are actually structures built in the ground, complete with roofs and whatnot, the flange sits on top of the hole in the roof of the structure. I’d guess it’s held down by bolts so that it doesn’t wander around as the street surface is formed around it.
Then the paving guys come along and either place concrete or tarmac around the collar. It’s usually several layers thick on heavily-travelled roads, so when resurfacing time comes along, you might not be able to see the whole depth of the collar.
As usual, Wikipedia has a pretty complete article on manhole covers, and there are a bunch of good reasons they’re round other than these listed here.
posted by Bill Eccles on 8-5-2008 at 11:29 am