Brain Teaser
OK, here's a really easy brainteaser for all
those who have trouble with algebra.
Why are manhole covers round?
Hint: It isn't because manholes are round.
The Short Answer is . . .
Because round is the only shape that can not fall through a similarly shaped, slightly smaller hole regardless of how it is tilted. Any other shape may be tilted so that it may fall through the hole.
Our winner is . . .
Bill Blevins
Star Directory & Internet director
The Free Lance-Star
Fredericksburg, VA
But wait, there's more . . .
I received several other interesting notes, the most throrough of which came from Michelle Hoyt, an engineer at San Diego Gas and Electric.
Dick,
As I was finishing my undergraduate work at Cal Poly, the common interview question we were asked was, "Why are manhole covers round?" Applicable question to ask a future Civil Engineer, don't you think?
What I learned from the interaction of sharing answers with my peers is that the commonly accepted answer to the question is that man hole covers are round because that way, the lids won't fall into the hole, when you are returning the lid to its closed position. If you had a square man hole cover, you could easily turn the lid just a few degrees one way or the other, and you would lose your lid to the dark, black hole of darkness. I see this answer as logical. However, from an engineering standpoint, there are other answers to the same question.
The arch is a phenomenally sound structural element. It is by far stronger than anything with sharp angles, such as a square or rectangle. From a design point of view, the man hole cover and hole with an arch (or a circle), would be stronger than a rectangle. Therefore, you would have a longer design life of your man hole with less expense to pay for materials and design work. This concept of a circle being stronger than a square makes sense. The pressure distribution along the edge of the hole is much more subtle and evenly distributed. If you divide the same pressures among a rectangle, you have high stress concentrations at the corners of the manhole.
Think about famous arches - bridges, the St. Louis arch, etc. Arcs are a very fascinating concept that were discovered since the beginning of time.
Another way to respond to this question is to think about the average American who must go down into these man holes. Our society is growing larger and larger each year. Trust me, these guys don't come in a tight square shape. Most of them are quite round! Perhaps the roundness assists in letting the workers get down into the hole, and then back out. Buttering the sides of the hole aid in reducing the frictional resistance. Come on, we all know that Santa Claus gets stuck in those rectangular chimneys every year!
Cheers,
Michelle Hoyt
SDG&E - Engineer II
Civil/Structural Engineering
8316 Century Park Court, CP52G
San Diego, CA 92123
Thanks Michelle. I should point out that round manhole lids are easier to move becuase you can roll them. I suppose that's why you see so many sanitation engineers rolling manhole covers around.