Why are manhole covers round?

We pass manholes all the time. We walk over them, drive over them, and watch them explode. Have you ever wondered why the covers are round? Well, it seems Microsoft did; it was asked at job interviews in an attempt to psychologically assess how a person sees the world and approaches questions.

Just in case you ever get an interview with Microsoft, here’s the answer:

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. 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 can also be moved around by rolling, and they need not be aligned to put them back. If we look it from pure mathematical point of view, for the same surface area round will consume the least amount of metal. Hence it is cheaper to make round covers for the same amount of surface area.

Well, now you know. And much better off because of it!

via Wikipedia

Tagged: Manhole covers, Microsoft