With all the discussions about why manhole covers are round ( http://www.straightdope.com/classics/a1_247a.html ), I’m surprised noone has mentioned the obvious: They’re round because the manholes are round.
Then why are manholes round? Because the covers are?
I believe that what perry62 meant to say is that the pipes that the manhole covers cover are round (tubular).
Seems to me, that rather than the shape being the result of some kind of engineering plan, the fact that round manhole covers can’t fall into the holes is merely a lucky accident.
It doesn’t matter what shape they are. They don’t fall in because there’s a lip under them that prevents them from falling in. That is, the size of the actual hole is smaller than the size of the cover.
It does matter. If the holes/covers were square, for instance, you could lift the cover, turn it vertical and diagonal to the hole and it would fall in. Granted, if the lip were big enough, most shapes would work; but would be impractical.
But if the covers are any other shape the lip would have to be bigger. Tho I have seen sqaure covers that are hinged like a trap door.
The vault or whatever is often not round*, there can be a small tube between the chamber and the surface.
Brian
*I work for comapny that makes mapping software for utilities. I’ve seen 100s of manhole reciords many of which are not round.
dantheman, I suspect you did not read the article about manhole covers.
Anyway, I decided to test your theory just now. I took the lid from a square metal box, which fits over the body of the box, and successfully dropped it into the box by placing the lid on edge between diagonal corners of the open box. The sides of the lid measure 19.3mm, while the opening of the box measures 18.8mm between parallel sides.
Shape matters.
Ask Britney Spears’ manager.
"(3) it would look stupid. Regarding point #3, someone has written me claiming the English have triangular manhole covers. 'Nuff said. "
>>Is this some kind of dig at the English? Are manhole cover shapes an indication of national intelligence?
Here are some pictures of the vaults that N9IWP is talking about. They’re definitely not round - the common sizes are listed. They have a round access (manhole) port on top that isn’t clearly visible.
http://www.hartfordconcrete.com/manhole.asp
And here’s some that don’t even have round tops. There are sometimes called “handholes” because they aren’t very deep.
http://www.hartfordconcrete.com/qazite.asp
I think this whole issue (hole issue?) may boil down to the usual engineering tradeoff on material strength. Depending on the material of the roadway or the hole, if the opening were square or had discernible corners it would provide a location for stress concentration. This is, by the way, the same reason that the windows on ships and airliners don’t have square corners (well, the Comet did, but they tended to fly apart in mid-air because of it).
A concrete roadway with non-round manhole covers would be more susceptible to cracking at the stress concentrations around the corners. Likewise the hole itself would not be nearly as strong if weren’t round, and a square hole would certainly be subject to failures at the corners. The subsequent caving-in of the city infrastructure would be considered not desirable and probably make the mayor lose the next election. For this reason I submit that manhole covers are round for political reasons.
Cheers.
In an earlier thread on this column, Chas.E also opined that manhole covers are round because the manholes are. He referenced and old Martin Gardner column in Scientific American. However, assuming that’s true, that still leaves the question of why the manholes are round.
My WAG would be that early manholes were brick, and building them in a circular shape resisted deformation from the surrounding soil better than other shapes. Thus, standard manholes were round, as were their covers, and the standard persists to this day.
Nashua, NH has triangular manhole covers… not only are the unique, they’re superior (see link)!
http://www.nhcentury.com/nashua/nasleg/thedonsha/index.shtml
From a pragmatic point of view, a circular manhole is also easier to open and close. With a round one, all you have to do is loosen it, jam a crowbar into the centre hole, lever it open with your foot on the pivot point and swinging it around. Spent a summer doing this, and ideally it takes about 20-30 seconds - unless it’s rusted shut and/or buried 1 foot deep in a vege garden.
A rectangular one has to be manually pulled out, which entails bending down and causing back problems. Takes load more time, as you can’t roll it around like the circular ones.
Just wanted to say I have been a reader of this site for a couple years now and love to come here to get the straight dope. However you mentioned in your article that manholes were for sewage workers. What about people who work in communications? But I don’t want to nit-pick.
I believe perry62’s quip is probably the correct answer. Any geometrical advantages to round manholes were probably just unintended benefits. I work for a civil engineering firm that has designed quite a number of sanitary sewer (there’s an oxymoron) systems, and SS manholes (i.e. the vault itself) are round 99.999% of the time. They typically come in 4’, 5’, or 6’ diameters.
The manholes that UncleBeer linked to and that N9IWP refered to from utility records are utility manholes, not SS manholes. Phone lines, fiber optic cables, etc. all use access manholes as well, but they don’t need to be round. So why do SS manholes need to be round? Simply because sewer lines shoot out from manholes at all kinds of arbitrary directions. Sewer lines have a tendancy to not run particularly parallel or perpendicular to roadways like other utilities. This is probably due to the design problems involved with gravity lines. Sewer lines can’t just go wherever you want. They always have to be running down hill since they aren’t pumped. If the ground general runs downhill in the direction you’re trying to go, you have some “slack” to play around with, but if you need to cross uphill or flat terrain, the line just keeps getting deeper, which is more expensive and dangerous to build. Remember, they have to dig a trench to build these things. Deeper trenches are more likely to collapse, and are more deadly when they do. To avoid extra depth, you want to keep line length as short as possible (less distance = less depth due to the downward slope). That means going in a straight line between two points, regardless of whether that line is parallel to anything. Additionally, if you’re trying to connect to an existing manhole, that sets a limitation on how deep you can be by the time you get to it. So if you take to long of a path, you may be too deep to make the connection. Hence straight paths again.
So back to the manholes themselves. If you can’t predict which way an intersecting line is going to hit the mahole, a round shape is the obvious choice. Sewer lines can’t have sharp bends in them like a water line, because they’ll get clogged if you do. So you can’t just bend it perpendicular to a square vault at the last minute. Cast iron pipe can only be bent a few feet in a hundred, and while PVC is more flexible, it doesn’t help that much. So if you had a square vault, you’d have to punch through them with oddly shaped holes that would be difficult to get exactly right, or it might hit directly on a corner. With a round manhole, the lines are guaranteed to hit it perpendicularly as long as they’re aimed reasonably close to the center of the manhole. The top portion of the manhole tapers inward to a narrower entry 5 or 6 feet below the surface. A typical manhole is going to be 10-20 feet deep.
Could you make a round manhole with a square access shaft? Yeah, but why would you?
I found this a while back and while it has a lot of the same arguments already posted, it’s funnier.
For those of you that don’t know:
- Microsoft is famous for asking seemingly odd questions at interviews to find out how you think
- Richard Feynman was a brilliant physicist who was also blessed with wit - he died quite a few years ago
http://www.sellsbrothers.com/fun/msiview/#Feynman
If Richard Feynman applied for a job at Microsoft
Interviewer: Now comes the part of the interview where we ask a question to test your creative thinking ability. Don’t think too hard about it, just apply everyday common sense, and describe your reasoning process.
Here’s the question: Why are manhole covers round?
Feynman: They’re not. Some manhole covers are square. It’s true that there are SOME round ones, but I’ve seen square ones, and rectangular ones.
Interviewer: But just considering the round ones, why are they round?
Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology.
Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones?
Feynman: Yes. Round covers are used when the hole they are covering up is also round. It’s simplest to cover a round hole with a round cover.
Interviewer: Can you think of a property of round covers that gives them an advantage over square ones?
Feynman: We have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it. Also, the term “manhole” implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section. So a cylindrical pipe is the natural shape for manholes. The covers are simply the shape needed to cover up a cylinder.
Interviewer: Do you believe there is a safety issue? I mean, couldn’t square covers fall into the hole and hurt someone?
Feynman: Not likely. Square covers are sometimes used on prefabricated vaults where the access passage is also square. The cover is larger than the passage, and sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. Let’s assume a two-foot square opening and a ledge width of 1-1/2 inches. In order to get it to fall in, you would have to lift one side of the cover, then rotate it 30 degrees so that the cover would clear the ledge, and then tilt the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. Yes, it’s possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely. Applying common engineering sense, the shape of a manhole cover is entirely determined by the shape of the opening it is intended to cover.
Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.)
(Interviewer returns after 10 minutes)
Interviewer: We are going to recommend you for immediate hiring into the marketing department.
Keith Michaels
Well, I guess that about sums it up. Thanks for playing!
Just to verify what I had read, I cut out an equilateral triangle, and indeed, it went through its corresponding hole fairly easily. (mild surprise)
http://www.nhcentury.com/nashua/nasleg/thedonsha/index.shtml
I thought it was interesting what the Nashua article said: “Square covers, used in some cities, have the added disadvantage that they can be easily dropped (accidentally, we hope) through the hole.”
So presumably, the manipulation of the triangular manhole cover – to cause it to fall through – would not fall under “easily dropped”.
Thinking more about the Nashua article, another “benefit” is that round covers can rattle, mentioning the addition of road tar. But what if road tar was added at the midpoint of one of the sides of the triangular cover? Seems it too would rattle.
Sounds like a challenge for those situated near Nashua! 
That old quote from the Book of Holes wouldn’t seem appropriate here. These folks know their holes.
The quote, anyway: “…for they knew not their holes from an ass on the ground…” (Firesign Theater, possibly an imperfect quote)
Re: Nashua
Well, we don’t know how big the lip is around the hole. Also, we don’t know what shape the hole is. It could be a round hole and have a triangular recess cut into the concrete/asphalt.