Why are manhole covers round?

On February 8, 2011, in Curiosity, Interview, Raj Murthy, Why is that so?, by Raj Murthy

A seemingly simple question. Right? This question gained a little attention back when Microsoft apparently used it as an interview question to judge the candidate’s ability to answer questions with more than one correct answer. Here are several takes on this question, please note that there are possibly many other correct answers as well.

1) Manhole covers are round because manholes are generally round – pragmatic, and simple.
2) A round cover would never fall inside the hole whereas a square or rectangular cover could potentially fall down the hole – also a practical and clean approach to this question.
3) Circular covers do not need to be aligned when covering a circular manhole.
4) It is easier to move a round cover by rolling it around on its side.

Lets geek it up a little

5) It is easy to drill round holes (drills are round) therefore it makes sense to cover round holes with a round cover.
6) Round or cylindrical surfaces 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 would assume a similar circular shape.

The Engineering answer

7) The purpose of the cover is to keep things above the cover from falling inside. A round manhole cover has two advantages. It can never fall inside the hole, potentially injuring someone or making it difficult to recover. A similar argument could be potentially made for an equilateral triangle shaped cover however, a circular or round surface is better at dissipating the stress move evenly at all points than any other shape. So, manhole covers are round because it is the most efficient and effective shape.

Some other interesting answers

8 ) It is more aesthetically pleasing
9 ) It breaks up the straight lines of the road
10)It is cheaper to make this way

Answers that ensure you will never be called again

11) It looks prettier that way
12) Anything that beings with ….”I think it really depends…….existentially speaking”

2 Responses to Why are manhole covers round?

  1. Alex says:

    Great post! It gets me thinking… it the round shape is so much more efficient (and it is), why are cookies round? It doesn’t make much sense… given the wasted material and the difficulty in storing and packaging…

  2. Raj Murthy says:

    Allright the next post will discuss and tackle that issue.



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