Why Are Round Manhole Covers Round?
About 25 years ago the question “Why are manhole covers round?” appeared in the book “When Do Fish Sleep?” by David Feldman. Since then, this question has gained some notoriety, even warranting a series of answers on the Wikipedia page regarding manholes. Microsoft and Google candidates have reportedly been asked this question in interviews–presumably to demonstrate a thought process. Or to watch how a candidate does with hemming and hawing.
Unfortunately, the question itself is flawed because it presupposes that manhole covers are inherently round. As we shall see, manhole covers can be nearly any shape with round being the most common followed by rectangular shapes including squares. Factoring out this discrepancy the question becomes, “Why are round manhole covers round?”
The common wisdom answers given to this tautological question by most people are effectively wrong in a way which illustrates how we handle brain-teasers and puzzlers. So, let’s take a look at the list of reasons shown on the Wikipedia entry.
Why Aren’t They Square? Problems with the most common answers.
- A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
While it is true that a round manhole cover cannot fall into a smaller sized manhole and heavy steel plates dropping into a hole poses a safety hazard, there are plenty of other ways to get around this problem. The first is that a square cover would not drop into the hole if it were wider than the hole’s longest span. For instance, a square cover that is 45″ wide could not fall into a square hole <32″ wide although the 6″ lip on either side would constitute a waste of material. On the other hand, a 45″ square cover could not fall in a <45″ diameter round hole.
This is looking like a reasonable answer until you realize there are plenty of square manhole covers. Most electrical boxes and other forms of underground cabling have square covers. Although we think of the traditional round cover for sewage and storm drains, there are many that are square.
Therefore the falling-in factor is an advantage of a round manhole cover, but it is not the inherent reason for their roundness. The question asks why they are round and does not seek additional benefits or advantages. Thus, because there are other ways to prevent the cover from falling in, and the frequent presence of non-round covers, this is not a valid answer to the question.
- The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
True, a lathe does circles terrificly well. However, there are two flaws with this answer. The first is that the machining process uses special equipment that can handle a square, round, or any shape cover. You can see that at 4:15 in this clip from How It’s Made. In fact, using a lathe to produce manhole covers would be extremely inefficient.
The second flaw is that round covers pre-date mills and lathes, so the causality we seek just isn’t in this answer.
- Circular covers do not need to be rotated to align them when covering a circular manhole.
True but so what? The inconvenience of aligning a non-round cover with the hole’s supporting edges is minimal.
- Human beings have a roughly circular cross-section.
Again, so what?. If our roundness were a factor, we might consider living in round houses. We don’t, of course, because a square gives the most available space in the same width. This goes as well for a manhole. If the holes were square we would have even more room to move around and bring equipment with us.
- A round manhole cover can be more easily moved by being rolled.
This is not only true, it is a liability. Utility personnel are warned not to roll manhole covers because they can roll away.
A similar point not mentioned is that a round cover can be dragged/wiggled more easily. However, that probably didn’t influence the initial design. “I want somethin’ I can wiggle! I want to wiggle somethin’ in that manhole!”
- It’s easier to dig a circular hole.
It’s easier than what? A star-shaped hole? Yes. But unless you have an auger, squarish holes are easier to dig with shovels or backhoes.
An important point is that manholes are not simply dug. They access something such as a sewage system that requires significantly more excavation than the circular hole the manhole is in. In other words, the excavation for the plumbing to which the manhole accessway connects has already been completed. They don’t just dig holes in the ground and put in a manhole.
- Tradition
The Romans gave us square manhole covers, and the British gave us roughly trianglular covers. Yet, the supposed tradition of a round manhole exists all over the world.
Although this is a silly answer, there is history behind the shape of these covers. But the covers are not round simply because they was round in granpappy’s day, and by dingy, they’s gonna stay that way.
A variant on this answer is slightly more valid: Interchangeability. If all the manhole covers on one system are the same shape then they can be swapped around as needed. Again, this does not provide a resolution for the cover’s roundness.
- Supply. Most manhole covers are made by a few large companies. A different shape would have to be custom made.
- Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
At the bottom of the list is the closest to a correct answer for this question.
The actual reason why manhole covers are round is because they cover round holes.
Sewage systems usually comprise large round concrete pipes that fit together. The T-junctions used to create the manhole accessways are the same T-junctions used to join other non-manhole pipes together. The simplest way to cover a round pipe is with a round cover.
For many people this pragmatic answer is unsatisfactory. They want a more clever reason for the inherent roundness they perceive. I believe this stems from looking at the cover as an item unto itself and not looking at the cover’s part in a much larger system. Unless you’ve climbed into a manhole, been on a construction site, or have somehow otherwise seen the entire system, your perception is limited to the only object we see: The manhole cover.
This perceptual bias has caused enough people to attempt rationalization of the supposed purpose behind round manhole covers. They found advantages (and some disadvantages) of round covers, but never really answered the question. In fact, they have become so biased that they discount other non-round covers.
What’s even more interesting is that these incorrect answers have been codified. They’re posted on Wikipedia, and they are considered “good answers” by Microsoft and Google. The pragmatic answer is considered uncreative, yet, it’s the hardest answer to ascertain! I’ve recently discovered that Richard Feynman has also answered this question in the same way as above and has also rejected the common answers. I wonder if he would have been rejected from Google or Microsoft in the interview process?