Why are manhole covers round? …and other trivia about the pesky drains covers

"Why are man­hole cov­ers round?" May of us know this as the by-now passe, declasse inter­view ques­tion. How would you han­dle this question?Here is where I will add and con­serve all the answers I can find.

"Why are man­hole cov­ers round?" May of us know this as the by-now cliche, declasse inter­view ques­tion. In fact, any per­son who has inter­viewed with a pass­ingly intel­li­gent firm would be famil­iar with it. Fol­low­ing are the usual bro­midic Big4 "solutions" –

1. The com­monly told story, which may even be true, is that man­hole cov­ers are round so some moron from the civil depart­ment of you munic­i­pal­ity won't be able to drop them acci­den­tally into the manhole.

2. Sec­ondly, the man­u­fac­ture of cir­cu­lar man­hole cov­ers is eas­ier and more accu­rate than the man­u­fac­ture of cov­ers of any other shape.

3. Thirdly, you can roll round man­hole cov­ers, which makes them easy to lift and carry around.

4. Because holes are round! ;-)

But is this all? For one thing, not all man­holes are around. It's true that MOST are round but I've also seen square, rec­tan­gu­lar, and in Tokyo, also some tri­an­gu­lar ones. Hop on to any man­hole pic­ture col­lec­tion on the net (e.g., Dan Heller's pic set, the designer set of Japan­ese man­holes, or the very com­pre­hen­sive and funny Drain Spot­ting.)

Now, assum­ing that drain cov­ers are in fact avail­able in all shapes and sizes (even though typ­i­cally in cir­cles) and if this ques­tion was posed ONLY for the ones that round, how would I answer it? I could say that we are just con­sid­er­ing the round ones, then they are round by def­i­n­i­tion. So that ques­tion is a tau­to­log­i­cal one.

Is there some par­tic­u­lar value to hav­ing a round man­hole cover?

Well, 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. Isn't it?

Can we think of a prop­erty of round cov­ers that gives them an advan­tage over square ones?

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.

Do we believe there is a safety issue? I mean, couldn't square cov­ers fall into the hole and hurt someone?

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.

How­ever, why only cir­cu­lar? Can you think of any coun­try that has non-circular coins? Coins that pain­lessly fall into the cir­cu­lar slots of vend­ing machines? What makes the round man­hole cov­ers any different?

The point is, if a con­sis­tent diam­e­ter is the only require­ment, any equi-diameter shape will do? This pre­cludes any reg­u­lar poly­gon because those are not equi-diameter shapes. [For exam­ple, a square man­hole cover can fall through the square hole along its diag­o­nal, and an equi­lat­eral tri­an­gu­lar man­hole cover can fall through the eqi­lat­eral tri­an­gu­lar hole along its altitude.]

How­ever, imag­ine a reg­u­lar poly­gon with an odd num­ber of sides. Place a com­pass point on one of its ver­tices. Set the radius of the com­pass to be the dis­tance to either of the two far­thest ver­tices (i.e., the ver­tices at the ends of side oppo­site the com­pass point). Draw an arc con­nect­ing those two far­thest ver­tices. Replace the straight side between those two far­thest ver­tices with the newly drawn arc. Repeat this process for each ver­tex of the orig­i­nal polygon.

You now have an object with an odd num­ber of sides that looks sort of like a reg­u­lar poly­gon but with curved sides instead of straight sides. By con­struc­tion, this object has a con­stant diam­e­ter when viewed edge-on from any angle. A man­hole cover with this shape will not fall through the hole. [And it will roll fairly well.]

What my civil engi­neer­ing friend (from Michi­gan, USA) has to say:

It's like this. San­i­tary man­hole cov­ers are usu­ally round (and solid, i.e. VERY heavy) but Storm sewer man­hole and Inlet cov­ers are usu­ally square or rec­tan­gu­lar grates which let water in. The decid­ing fac­tor is where they are placed rel­a­tive to the curb line. All cov­ers are actu­ally installed on a con­crete generic 'Mex­i­can Hat' struc­ture which can be cen­tered or off­set to one side. The struc­ture fits on the round con­crete cast­ing (5' — 8" diam­e­ter) and can be made to accept any solid or grated cast­ing. If this part of the world, where river flood­ing is fairly com­mon, San­i­tary man­holes and lift sta­tions are either raised above poten­tial flood lim­its if pos­si­ble or sealed and bolted shut, mak­ing them much more dif­fi­cult to open. It's also a very good idea to vent San­i­tary man­holes and lift sta­tion with a portable fan to avoid being over­come by methane and other gasses trapped in them.

Makes sense. The most inter­est­ing answer though came from one of my clearly eman­ci­pated female friends:

"Why is a man­hole cover round? What­ever the rea­son, let's not call it a man­hole cover shall we? 'Per­son­hole', please."

Given that the ques­tion is sup­posed to test how you think on your feet (or your ass, if you're sit­ting in an inter­view) I guess that could qual­ify as the spiffi­est answer I have come across. :)

  • rachelle

    it has to do with a math­mat­i­cal thing the other answers are wrong!!!!!!!!!!!!!!!!!!

  • http://sniptools.com snip­tools

    Thanks for drop­ping by Rachelle. What math­e­mat­i­cal thing? Btw, I dis­agree that all the other answers are 'wrong.'

  • April

    They are made like that so I can throw them like fris­bies at assholes.

  • http://sniptools.com snip­tools

    April, you sound like you speak from expe­ri­ence, so I'll let your post linger. In cer­tain cases, a retort like that may even get you the job :)

  • spooky

    to the first com­menter: if it was only about a math­e­mat­i­cal for­mula we wouldn't have other shapes of the cov­ers, and i have seen other shapes too.

  • b

    "Place a com­pass point on one of its ver­tices. Set the radius of the com­pass to be the dis­tance to either of the two far­thest ver­tices (i.e., the ver­tices at the ends of side oppo­site the com­pass point). Draw an arc con­nect­ing those two far­thest ver­tices. Replace the straight side between those two far­thest ver­tices with the newly drawn arc. Repeat this process for each ver­tex of the orig­i­nal polygon."

    HOLY CRAP — WHY???? Why would you make some­thing MORE com­plex, sim­ply because you were able to? Really, if you included that in your answer in an inter­view, I wouldn't give you the job…

  • http://sniptools.com snip­tools

    Couldn't agree more! :)

    Frankly, if I were to give an answer to this ques­tion, I would NEVER get math­e­mat­i­cal. It may be required for engi­neer­ing type inter­views, but that's not me.

  • http://sniptools.com snip­tools

    my favorite answer was from a jamaican engi­neer. after a bit of con­sid­er­a­tion he said, 'it is round because if it were a rec­tan­gle and looked up from down below, you would think you were in the grave.'

  • mas­cot­globe

    The rea­son for the cir­cu­lar con­struc­tion of man­hole cov­ers is, quite sim­ply, that cov­ers of any other shape would fall through the man­holes by virtue of their vary­ing diam­e­ters. Cir­cu­lar man­hole cov­ers do not vary in width, or in diam­e­ter, as is the case with these other shapes, thus remain­ing in place despite the street traf­fic run­ning roughshod over them.

  • Ghas­san

    Because holes are round
    you should see more holes to believe that

  • Mildew

    Not all holes are round.

  • jp

    No, but they all con­tain noth­ing but air.…

  • Wil­low

    1. The com­monly told story, which may even be true, is that man­hole cov­ers are round so some moron from the civil depart­ment of you munic­i­pal­ity won't be able to drop them acci­den­tally into the manhole.

    I explore drains. Not sew­ers, drains, which also com­monly have round man­hole cov­ers. The snide tone of the above sug­gests that you have never actu­ally had to lift a man­hole cover. They are heavy. Very heavy. The process of pulling one back over a shaft when enter­ing a drain can eas­ily result in it falling in if it is not round, which will prob­a­bly kill you and any­one else below you. I have seen non-round man­hole cov­ers fall into drains.

    Wil­low of the Syd­ney Cave Clan.

  • owen

    Another prop­erty: round is cheaper. Less mate­r­ial is required for the pipe, and there is the sim­plest align­ment exe­cise to join with the sys­tem below. It all goes together just like plumb­ing pipes. Really big, heavy, pipes. Sav­ing time in align­ing these is worth quite a bit. And yes, as men­tioned, the com­pres­sion fac­tor is bet­ter, which means less mate­r­ial again.

    I don't know I'd say a cylin­dri­cal pipe is the nat­ural shape for a human being climb­ing down a lad­der, though. Square also works well when you put the rungs down one cor­ner. The other cor­ners give good elbow, rump, & heel clearance.

    And for any­one who thinks you'd have to be an untrained twit to drop a cover down a shaft, keep in mind that the peo­ple doing it do it at all hours, in all weather, and dur­ing long over­time respond­ing to emergencies.

  • Earnon

    Round works. Anythimg else is just mas­tur­ba­tion on the customer's dime.

    It's sim­ple, ele­gant and effec­tive. Can't be dropped into the shaft. Sim­ple to fab­ri­cate. Strong. Impos­si­ble to improp­erly install.

    Jesus, this is a no-brainer.

    Only an arro­gant ego­tis­ti­cal jack­ass — more con­cerned with "mak­ing his mark" than with deliv­er­ing the best solu­tion to his cus­tomers — would choose to use a sec­ond or third rate design like a non-round man­hole cover.

  • http://sniptools.com snip­tools

    Just saw this and had to include it on this site:

    If Richard Feyn­man were asked this ques­tion:
    http://www.sellsbrothers.com/fun/msiview/#Feynman

    Hilar­i­ous! :)

  • Ran­jit Wankhede

    Well puz­zled over why its called a man­hole cover when it should be also called a femalehole

  • delta

    Why is a man­hole called a manhole?

    I know it could be called a man­hole coz the hole on a man's body emits stink­ing stuff from it & stinks to high hell (espe­cially when they've had curry, beer, kebab, FOOD in general!)

    But why is it a MANHOLE?

    answers on a postcard

  • Mark

    Well if mar­keters have their way round man­hole cov­ers won't be an issue.. here's an image of a Fol­gers cof­fee pro­mo­tion in NYC:
    http://photos1.blogger.com/blogger/353/70/1600/coffee_manhole.jpg

  • Ravichan­dran

    The rea­son could be, Sewage lines are mostly cre­ated at the cen­ter of roads. The traf­fic on the roads always press the man­holes, on a straight line par­al­lel to the traf­fic. Cir­cu­lar Cov­ers would dis­trib­ute the weight across, while square cov­ers would receive the pres­sure along the sides. Thus square cov­ers would receive more pres­sure in to the holes than the cir­cu­lar one. The prob­a­bil­ity of man­holes with square cover is more than the one with cir­cu­lar cover.

  • Ravichan­dran

    Edit:

    The prob­a­bil­ity of man­holes col­laps­ing out of traf­fic would be more with square cov­ers than with cir­cu­lar cover.