Why are manhole covers round?

Intro
A while ago I read an article about hiring questions . A small excerpt of this article aroused my curiosity:

I avoid asking “Microsoft” type questions. I was once interviewed for a job, and I got the litany of them from the hiring manager. For example, “Why are manhole covers round” type questions.

So WHY are they round?

Wiki to the rescue!

Manhole cover

The question of why manhole covers are typically round, at least in the U.S., was made famous by Microsoft, and Tony Martinez when they began asking it as a job-interview question.[4] Originally meant as a psychological assessment of how one approaches a question with more than one “right” answer, the problem has produced a number of find alternate explanations, from the pragmatic and satirical (“Manhole covers are round because manholes are round.”)[4] to the philosophical.

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.

Fun fact

Feminist issue with manhole name
In 1990, the city of Sacramento, California officially renamed all their manholes to “maintenance holes” out of concern for the equality of both genders [2]. The name “maintenance hole” was selected because the term shares the same initials as the word “manhole,” thereby eliminating the need to change the MH labels on the city’s utility maps.

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