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" –

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:

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

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.