A manhole is just what it seems, an access hole, large enough for a man to climb down to the sewer. So why aren’t they rectangular or square? Well, a round cover can’t fall through its hole. A square cover turned diagonally will slide right through its square opening.
The geometric reason is that a square cover’s diagonal distance across the length of the square is 1.414 times the length of the sides. A Reuleaux Triangle[1], which has not been used, also fits the criteria and would be just as suitable for stopping its cover from falling in.
The second reason is the ease of use. A round cover, although very heavy can at least be put on its side and simply rolled away if necessary. There is also the ease of replacing a cover. If it is square it would have to be turned to the correct positioning.
Cost also plays into the reason. There is less surface area in the same-sized round versus square opening, thus costing less to make round covers. It is also easier to dig circular holes because it holds against the compression of the earth surrounding them.
It is not tradition or any kind of rule dictating round manhole covers just that it is practical and makes sense. This is not the typical way things are done but it looks like, for once, they got something right. Manhole covers date back at least to the era of ancient Rome, which had sewer grates made from stone.
For all manholes, there is a ‘lip’ around the rim of the hole, holding up the cover, which means that the underlying hole is smaller than the cover. A round manhole cover can’t fall through its circular opening, because no matter how you position it, the cover is wider than the hole. But a square, rectangular, or oval manhole cover could fall in if it was inserted diagonally into the hole.
Larry Scheckel – in his book “Ask A Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works”
Manhole covers are often made out of cast iron, concrete, or a combination of the two. This makes them inexpensive, strong, and heavy, usually weighing more than 249 pounds. The weight helps to keep them in place when traffic passes over them and makes it difficult for unauthorized people without suitable tools to remove them.
The question of why manhole covers are typically round was made famous by Microsoft when they began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one correct answer, the problem has produced a number of alternative explanations.
Footnotes
- A Reuleaux triangle is a curved triangle with constant width, the simplest and best-known curve of constant width other than the circle. It is formed from the intersection of three circular disks, each having its center on the boundary of the other two. Constant width means that the separation of every two parallel supporting lines is the same, independent of their orientation.
Sources
Enviro Design Products
Mental Floss
Wikipedia
