A familiar detail of the urban landscape is sewer manhole covers, almost everywhere, in all countries of the world with a round shape.

Why round – we will try to answer this question.

## Safety

Sewer manhole cover – a massive product made of cast iron. It weighs over 50 kilograms, and the weight of manholes on the roadway exceeds 78 kilograms.

It is not surprising, because the manhole cover placed on the sidewalk has to withstand a load of 12 tons, and the cover that is lying on the road – as much as 25 tons. This weight is because of the thickness of the cover, calculated based on the specified load and the strength of cast iron.

Now imagine the following situation: the manhole cover is open and lying next to the manhole, and there are people working in the manhole below. If someone accidentally moves the round cover, in the worst case, they will just close the manhole. Rectangular or square cap in this case could fall into a hole right on the heads of workers because the diagonal of a square or rectangle is always greater than any of its sides. Therefore, the round shape of the cover is the safest in this case.

## Strength

Engineers have long known that round objects have much greater durability than square ones. The sewer manhole is not only round but also slightly convex to the middle: this shape provides the greatest resistance to the mechanical load directed from above.

The internal stresses in such a manhole shape are dispersed over the metal, while in square and any other covers under load, the stress is distributed unevenly, which leads to faster wear and therefore – more frequent replacement of covers.

In addition, do not forget that cast iron – is a rather fragile material, and if you drop a rectangular lid, it may well break off a corner. To crack a round manhole cover put in a lot more effort.

Judging because almost all covers are intact for decades (and some for over a hundred years), in normal circumstances, to damage the round cover is almost impossible.

## Savings in metal

Why is the round shape more optimal in terms of metal shavings? With the same length of the perimeter of the circular and rectangular hatches, the area of the circle is always greater than the area of a rectangle or any other geometric shape.

If you translate it into economic figures, it turns out that the circle – is the most advantageous form of a manhole, because the cover for it requires much less iron (savings of up to forty percent) than a rectangular cover.

If you calculate how many sewer manholes are needed even for one small town, the scale of metal savings does not seem insignificant at all.

The optimal ratio of perimeter length to the area makes it necessary to make circular manholes not only for sewer manholes but also in much more serious constructions – for example, between the compartments of submarines or spaceships. Circular manholes are more economical and can withstand heavier loads than any other manhole shape.

## Ease of operation

A round hatch cover is much more convenient for repair or maintenance work. If the rectangular hatch should be lifted to move some distance, which will require at least two adult men, the round can easily roll over, putting it on the rib, and with this task is entirely possible to handle alone.

And in order to put the heavy lid in place, there is no need to rotate it, adjusting to the configuration of the hole. The round lid will fall into place either way, no matter which way you place it.

So, there are more than enough reasons you should make a sewer manhole cover around.