Manhole Cover

[cindyb]

Why is a manhole cover round?

[cindyb]

but why is it round?

[andy hughes]

It's round to facilitate easy access. If you are climbing in or out of a manhole, there is less chance of you bashing yourself on a sharp edge or corner if there are none to collide with. Round manholes are smaller, so the covers are lighter and easier to lift, without edges to trap fingers under - so 'round' seems to have a number of advantages.

[Ewood27]

And a round cover can't slip down the hole.

[andjac]

and because most men aren't square

[darth vader]

The correct answer is that it cannot possibly fall down the hole, whereas a square one could

[rja211077]

I agree - the two main reasons are 1) can't fall down the hole 2) max area of hole for the amount of metal used.

[sddsddean]

The main reason (and as an ex Civil Engineer who specialised in drainage, I know) is that a round cover fits its frame any way round, thus making it miles easier to close the manhole. However, a round cover does not usually have any strenthening ribs on its underside which limits its diameter(due to inposed wheel loadings). So double triangular lids were invented. These are 2 triangular segments bolted loosely together, so that when you lift them, they should give a bit (they don't!!). They are miles harder to put back, because the frame is shaped to accept them, so they only fit one way(or 180 degrees round) and there is always a stone or something that has got in the way and you have to start again (assuming you haven't dropped it down the hole!!). They are also bl**dy heavy.

[seaoil]

As stated many manhole covers are square or made up of two triangles. As a water pollution officer I have had to lift more manholes than most unfortunately and I can state that round ones are no easier to lift or put back than any other. If you're on your own round ones are a bl**dy nightmare to put back! At least the double triangle ones mean you've only got half the weight to work with. Besides only ones that have to carry vehicular traffic need to be of the heavy cast iron variety.