Interview question: Why are manhole covers always round?

ballz2wallz · 07-25-2007, 09:56 AM

So in interviews, particularly in the business world like consulting or venture capital, the interviewers often ask questions that are strange, ones that you've never thought about, and ones that are really off-the-wall. They do this with the intent of seeing how quickly you think, and also how out-of-the-box you can think. One example is:

Why are manhole covers always round?

So I was curious to know what everyone else thought? My answer is 'so they couldn't fall down the holes', but I'm curious to get some other answers.

Linzyhop · 07-25-2007, 10:00 AM

so it's easier to put back. It's really damned heavy, so if it's perfectly round, then the cover can fall back into place anyway. if it was square you'd have to make sure the corners go over the corners.

WickedLou9 · 07-25-2007, 10:14 AM

I thought it was an established fact that the are round so that they can't fall into the holes...?

ballz2wallz · 07-25-2007, 10:29 AM

Originally Posted by Linzyhop

so it's easier to put back. It's really damned heavy, so if it's perfectly round, then the cover can fall back into place anyway. if it was square you'd have to make sure the corners go over the corners.

That was also one that I thought of...

ballz2wallz · 07-25-2007, 10:29 AM

Originally Posted by WickedLou9

I thought it was an established fact that the are round so that they can't fall into the holes...?

I don't think so.

Donkey® · 07-25-2007, 10:57 AM

They are not all round.

Roonie · 07-25-2007, 11:01 AM

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 aperture.

Round covers are much easier to manufacture.

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.

Round castings are much easier to machine using horizontal boring mills.

A round manhole cover can be more easily moved by being rolled.

If an automobile rolls over a dislodged manhole of another shape, the sharp corners could puncture the automobile's tire, whereas a circle doesn't have corners.

Some answers to the question. I was asked this exact question when I interviewed for Microsoft. I answered with the first explanantion.

ballz2wallz · 07-25-2007, 11:03 AM

Interesting. Are they really easier to manufacture?

ballz2wallz · 07-25-2007, 11:04 AM

Originally Posted by Donkey®

They are not all round.

Are you sure?

WickedLou9 · 07-25-2007, 11:12 AM

Originally Posted by ballz2wallz

Are you sure?

The only square covers I have seen are for storm drains.

Ardentfrost · 07-25-2007, 02:10 PM

Manhole cover - Wikipedia, the free encyclopedia

Reasons for the shape include:

* 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 aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
* 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.
* 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 horizontal boring mills.
* Circular covers do not need to be rotated to align them when covering a circular manhole.
* Human beings have a roughly circular cross-section.
* A round manhole cover can be more easily moved by being rolled.
* If an automobile rolls over a dislodged manhole of another shape, the sharp corners could puncture the automobile's tire, whereas a circle doesn't have corners.
* Tradition
* Aesthetics

Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards, and larger triangles cannot be found.

Stylerod · 07-25-2007, 02:16 PM

I remember learning that in 2nd grade.

Roonie · 07-25-2007, 03:16 PM

Originally Posted by ballz2wallz

Interesting. Are they really easier to manufacture?

Don't know. The answers I posted above were the results of research done on the question, not necessarily my answer.

I will look around as I wrote down a lot of the strange questions I was asked during a Microsoft interview. I will see if I can locate them.

Publius · 07-25-2007, 03:26 PM

I thought it was common knowledge that it was so that the manhole cover could not fall through the hole?

DosEquis · 07-25-2007, 04:12 PM

manhole covers? thats what i call my underwear.

ballz2wallz · 07-25-2007, 04:14 PM

Originally Posted by DosEquis

manhole covers? thats what i call my underwear.

Publius · 07-25-2007, 04:20 PM

Originally Posted by DosEquis

manhole covers? thats what i call my underwear.

Joe_Cool · 07-29-2007, 11:19 AM

I've never been down a manhole, but I'm guessing it's probably the same shape as the tube it's covering.

Also, round gives you the largest opening for a given amount of material, so it's the most efficient shape.

ballz2wallz · 07-29-2007, 04:53 PM

Originally Posted by Joe_Cool

Also, round gives you the largest opening for a given amount of material, so it's the most efficient shape.

Wow, I didn't know that...my favorite answer

RMNIXON · 07-29-2007, 05:24 PM

Originally Posted by Ardentfrost

Manhole cover - Wikipedia, the free encyclopedia

Nerdy, Nerdy, Nerdy!

Oh shit!

I forgot you are as tall as me and might be young enough and in good enough shape to kick my ass!