Why are manholes made in cirlces?

Question

Why are manholes made in cirlces?

Answer 1

easier to roll them as they are quite heavy

Answer 2

Two purposes.  The first is the easiest: circles can roll.

Circles are also impossible to fit down a hole with slightly smaller diameter.  In reality, any rounded polygon with an odd number of sides would work, too.  But, rounded polygons don't roll.

Answer 3

I've seen numerous theories about this:

1. Because a circle can't fall through itself, like for example a square one could fit if turned diagonally

2. The rough cross-section of a man wearing breathing apparatus on his back , eg. an oxygen tank, is roughly circular and with no corners a circular manhole is easier to get down.

3. It's easier to cast a circle from metal than other shapes

4. You can roll a circular manhole cover, which are very heavy to lift

5. With no sharp corners workmen are less likely to injure themselves on the cover

6. They're round because they've always been round; i.e. the first one was designed round so why change it

7. manhole covers are round because a round metal plate will support weight better than a square one

8. the circular shape will use the least area and hence take the least amount of material to manufacture and so be cheaper

9. They are round because they are covers to pipes, which are round.

There seems to be no consensus on a definitive answer.  So take your pick.  Answer 1 seems to be the most poplar in quizzes and IQ type tests.  Personally though, I reckon 8 or 9 is probably the most likely.  

also some are not round.. I've seen rectangular and triangular before

Answer 4

Not all manholes are round. In fact there are more manholes that are oval or elongated rather than round. This is because manholes are intended as passage for humans. In the ships and other marine vessels most manholes are oval as it is easier for humans to go through an oval than through a round manhole. This is because humans are wider on the side but narrower across.

The ones that you see on the streets which are passages to spaces below are round because of some special consideration for the location where they are used such as:

(1). Stress considerations. Round manhole covers can take more stress and strain than oval ones.Hence they are more suitable for use in the streets where they are subjected to very rough loading.

(2). Removal and re- installation. Round manhole covers do not need to be put back exactly in the same orientation as when they were removed. This is because a circle remains oriented no matter how you rotate it.