Why are Manholes Round?

A round shape is the only shape that can not fall through its own shape by turning it to any angle.

THIS IS THE BEST ANSWER

Raj thakral

Actually there is one other shape that would not fall through but it is not used for this. It is a Reuleaux triangle.

Very cool.

The question was about the hole, not the cover.

It is also the most economical shape to make and is stronger. Round uses less iron than an equivalent shape. Consider a round cover 30" in diameter and a rectangular cover 24" x 30".

Because a circle is the same diameter all round, even when the manhole cover is dislodged accidentally, it won't fall into the manhole.

Check this out. Imagine a 900 kG coin shaped object leaving the Earth. Operation Plumbbob

In Western Australia they are square.

Jimrat,

Really? This is in Perth. 2 drains, a square and a round.

Jon

regards.

<< ... 2 drains ... >>>

These are not manholes & their covers are not Man-hole covers but slotted welds

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to allow drain for rain water.

And on the Hard Sholder of roads farthest to the curb-side or to the Green-area between Lanes.

Haji,

I response to #7 is a picture with the comment showing 2 drains, 1 round manhole and 1 square access cover all in the same street corner in the west Australian city of Perth.

#7 said their manholes were square implying that they ALL were square.

In the US we have Round manholes, square access covers and drains too.

Jon

Round cover won't fall through a round hole.

This is a fundamental question I ask high school science students. Simple geometry. It is the only geometric surface shape that won't fall into itself. Imagine if a manhole cover was a square or rectangle. If you could pick it up (doubtful...they weigh about 250 to 300 lbs), it could be turned and fall into the hole along the longer diagonal. A falling manhole would not be good. Since a circle is equidistant from the center(unlike the square or rectangle), the diagonal (i.e. diameter) is the same length. Problem solved....eh?

Yes, Manholes are in most round shapes.The reason, they are used for sewer/storm drainage system. In case of rectangular/square shapes, there are possibilities that sludge from sewer may get stuck-up with four coners. Second reason that circular shape allows the liquid flow faster than any other shapes,(less velocity loss).However, you may observe many projects with rectangular/square shapes.

As other's have metioned, it's so they don't fall through the hole.

I was asked this question as part of my oral examination when getting my master's degree.

Langyaw,

Kabayan, you deserved to have 1 GA from me.

Regards.

Also the cover does not drop & sets itself.

I would tend to agree, but a manhole cover is not designed to contain pressure, either internal or external. Manhole covers are usually in the middle of the traffic lane where vehicle contact is minimal. Actually, there is usually a slot or opening in the top to use a specially designed tool to pick it up. I would also suspect the slot is beneficial to allow sewer gases to be vented out.

I would tend to see your point from a manufacturing view. The circular shape is king...much easier to maintain tolerances and mass produce, without trying to deal with those pesky right angles.

Thanks for your thoughts...Cheers

Regards.

You are correct to point & round Mnholes are used mostly in my country.

The point raised against it is only the problem in design of its support.

Here a reasonably strong supports are pre-fabricated & not on site built.

Your point that :

<< ... but a manhole cover is not designed to contain pressure ... >>

.. but not everywhere.

pressure no, but the things that make is good for pressure also make it good for withstanding roadway loads. rectilinear shapes have four lobes of high stress when loaded in the middle. this focuses the stress on four quadrants of the cover. round shapes however spread the forces evenly around the perimeter lowering the peak stresses. Square shapes may be a necessity for other reasons, but absent those reasons, round is always the best.

Because it's the only shape that can not fall into the hole.

Yet another good point.

My point of view is that curved or circular edge profiles are less prone to getting cut / broken. For a free entry for all shapes of persons, a circular shape is best preferred than an elliptical shape being of uniform cross section. PROTECTION FOR THE EDGES FROM CRACKING seems to a main criteria. For other square, rectangular and polygonal shapes the angular tips will be more prone to cracking. Is It?

One little reason has not been handled yet:

It is easier to get the cover away from the hole when it is round. you can simply roll it.

Another shape would need to be caried to the van.

Greetings all,

The answers received are all mostly correct as far as they go. Indeed the main reason most municipalities require round lids is because they won't fall through the opening. The fact remain however, that round lids are not the only covers used. Many vaults that contain mechanical parts functioning inside such as PRVs and pumps require rectangular lids to allow the passage of new parts for maintenance. And a rectangular lid will allow passage of a worker with tools easier than a round one. Many of these cover are hinged to prevent them from falling into the vault and damaging the components inside. As far as stresses incurred in the concrete surrounding the lids, you basically create a beam across the lid anyway so the round shape just creates another odd shape that the precaster has to reinforce. You can go to www.ejw.com and view some of the options out there. They supply us with most of our needs and are very helpful.

Have fun out there

Didn't any one say about the concrete pipe (used often as frame as well as used to connect, often, the manhole to the deep below storm drain) ?

As per practice and also as per the basics of design , the pipe has to be round. Now will we really want a polygonal lid (n ≠ ∞ ) over it ? I won't .

BTW: as said, most of my equipment manholes are square in shape with hinges of course, simply becuase it is just a bit easier to machine the frame, fabricate it as well as machine the cover (simply mill instead of turning on lathe)

However here we are not talking about these are we (had we been talking in general , the largest and most common manholes are always rectangular or arched, but never round - The doors and arches ) and they are stressed too, and may be quite a bit like the Pressure vessels, but then the design and stress calculations takes care of it. I have infact rarely seen machine manholes circular.

Obviously Guest is not a hobbit!

So the cover can't fall in the hole.

Yes, manhole covers/lids are round so they do not fall through the opening - ever.

However, electric system manholes are not necessarily round. When I designed manholes for primary voltage, electrical feeders we used rectangular manholes for new work. If an existing manhole needed to be rebuilt we generally used an octagonal manhole. The shape of the manhole depends on the location of conduits and the number of feeders in the manhole. It is critical that enough splicing walls exist to accommodate the number of feeders and conduit locations.

By definition a manhole contains only cables. A vault contains cables and equipment such as switches, transformers, etc.

A vault must have ground level access to replace and/or repair the equipment. Since that equipment is often bulky, large access panels are often needed. The equipment is usually square or rectangular so you do not want to try and move a square object through a round hole.

I suggest you all spend time in a manhole with 8 or 10 live 12 kV cable feeders surrounding you. It is a unique experience.

I suggest you all spend time in a manhole with 8 or 10 live 12 kV cable feeders surrounding you. It is a unique experience.

I'll take your word for it. I apparently have a knack for coming into contact with live voltage. It began when I was three and stuck a metal finger nail file in an electrical socket (my mother still has the file). Numerous shocks throughout my life sense then. Not all based on ignorance or stupidity.

There's a pentangular one nearby. The hole is covered in three isoceles triangle shaped cover pieces, two of which are identical.

Hi Raj089,

There is an opposition in Canada and mainly at my side because I am different. I like to do the opposite of others. In my profession, paint chemist and manufacturer, the "me-too" is rampant and archaic. I always have oval manholes in horizontal direction by design. This way you don't need to marque where is the right position of the cover compared to the tank. I found the oval form much better and easier to handle. Try it and let me know your opinion, Gil.

Has anyone mentioned that it's so the cover doesn't fall in the hole yet?

I see that some people are talking about different types of manhole covers. Some are talking about the ones in the middle of the street and others are talking about accesses to equipment.

I think the term more appropriate to accessing equipment or having hinges is called a hatch.

Many good points OP ask why the manhole was round many addressed just the round cover.

A manhole in many cases specifically sanitary and storm drainage is a change in direction, its easier to change direction in a round manhole structure the intersection is a degree in the circle in all cases.

Other posts touch on examples of why as well.

Manholes are not always round, but manhole frames and covers often are.

GA as nobody yet have taken spirit of the question

Haajee

Perhaps it does not qualify as a manhole if it is not round.

Jon

Langyaw,

Excellent recap.

Jon

....Second that nomination!

A round shape presents less sides to machine.

Which doesn't explain why they are cast.

That wasn't the question now was it? But I can answer your question with one word: Cheap.

Of course even if they are cast the moulds or forging tools must be machined initially. Tool and mould making can be an expensive process. So there you go.

Has anyone counted how many GA's were given out for the same answer?

My grandpa was a well digger and I can't imagine a well digger diggng anything but a round hole unless he was suicidal.

Manhole covers are round because most of the men that use them are round.

One other point so far not mentioned is that there is only one dimension to deal with.

Ogden Nash said:

"When it comes to money, my information is preciser,

I know that the round kind is nice,

But the rectangular kind is much nicer."

I hope that helps, Peter Osborne

The hole is round so the cover wont fall through AND the round hole is not easily mis-shaped by external lateral pressures from pavement migration when the cover is off.

Kuduk