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Dec 12 2006
By: docking101 Treasure Hunter 8728 posts
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Cool little thing to think about

5 replies 3 views Edited Dec 12, 2006
Why are manhole covers round? I got asked this by my teacher today and I looked at wikipedia for info. Turns out there is a whole debate about this. And surprisingly, Microsoft really sparked this debate

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The question of why manhole covers are typically round, at least in the U.S., was made notorious by Microsoft, and Tony Martinez 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 "right" answer, the problem exerts a strange fascination on people, who seek to find alternate explanations, from the pragmatic and satirical to the philosophical. One practical answer being, "Manhole covers are round because manholes are round."

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.
  • Cylindrical holes are easier to dig.
  • 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 a 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

However, 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.


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Fender Bender
Registered: 09/14/2006
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Re: Cool little thing to think about

Dec 12, 2006
who really cares about the shape of manholes?
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Treasure Hunter
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Re: Cool little thing to think about

Dec 12, 2006


eman94 wrote:
who really cares about the shape of manholes?



Believe it or not that question is asked in many job interviews
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Thanks for being a part of my life PSU
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Big Daddy
Registered: 06/21/2006
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Re: Cool little thing to think about

Dec 12, 2006
Woow!!
thnx 4 theinfo




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Last Guardian
Registered: 05/09/2005
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Re: Cool little thing to think about

Dec 12, 2006
Haha that's a good question.  It is kind of easy to see the reasons, because really, the circle is perfect .

Death is not sad, only life never lived.
~Socrates
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Keyblade Wielder
Registered: 07/27/2005
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Re: Cool little thing to think about

Dec 12, 2006
Looks like im not going to work at MS
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