Why are manhole covers round? This is a question that people ask fairly often, however the answers may come as a surprise. Manhole covers are almost universally round and for a variety of reasons, and some of them may surprise you.
One of the main reasons manhole covers are round is because the risers are almost universally round. These risers are pipes, large enough for a man, that connect to the sewer pipes or underground tanks. Pipes are almost universally round due to the burst and collapse properties – they are far stronger than any other shape. As a result of simplifying the fit, manhole covers are round to fit on these easily.
Another important aspect about being round is the inability for the cover to fall into the hole. The covers fit flush with the top and sit on top of a recessed flange in the frame of the cover. At no point would a round object be rotated in a manner that would allow it to pass through the hole. This is unlike a different shape, such as a square which would fall through if it were rotated slightly and dropped at an angle through the hole diagonally which is the widest part.
Circular shapes are also the strongest. However, unlike the pipes which need to prevent burst or collapse, manhole covers need to prevent bending or shear. When these covers are round, the stress is consistent when force is applied to the flange. Other shapes with corners will have stress variances due to the shape and how it’s spread on the cross sectional area. There are certain applications where other shapes are utilized, but circular manholes are typically strongest.
Lastly, round manholes are the easiest to manufacture and use. Circular manholes will fit no matter how they’re rotated and circular manholes can be forged from round bar stock. This is far less expensive than different shapes. For example, next time you’re looking at shower drains, check out the price difference between circular and square.
Needless to say, the manhole covers we’re familiar with are almost always round and for good reason. If you see one that isn’t, it was probably specifically requested as a different shape as a result of aesthetics, which with Manhole Covers Direct is possible with our custom solutions.
Manhole Covers Direct offers a wide variety of standard and custom manhole covers, replacement manhole covers, and frames. Learn more by visiting our products page.