Sniptools

“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.

This post is tagged Musings