Well I personally think its something relating to the weight which is put over it.

If we step over a round manhole our weight would eventually apply equally on all sides of the manhole thus making a very strong hold. While, I think, it is not the case in the square or round cornered manholes.

dear frnds all of ur explinatins are praise worthy bt the main reason fr a circular cover of mainholes is that every where the drainage system or the passage ways fr certain purposes are made in round shapes and to cover their surfaces one have to provide such protection which should be durable and convenient,
Thats why circular shapes fit it more feasibly coz circular shapes lids or covers never fall into the circular holes because of its geomatery.

So simple and logical.

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''Always bear in mind that your own resolution to succeed is more important than any one thing''

Weight ACTS VERTICALLY DOWNWARDS.It is the force exerted on an object by the earth(the product of an object's mass and acceleration due to gravity).It always acts vertically downwards regardless of the shape of the underlying surface.

I know it sounds like fifth grade stuff but I had to put the record straight.

stresses produced by the surface load (vehicles etc) can be uniformly distributed along the circular bodies which causes the minimum strain(deformations in bodies) in overall body against the higher values of stresses..moreover, the value of radius of gyration of circular bodies causes the higher strength of circular shaped bodies.. that is why circular shaped columns are preferred in multistory buildings or even in house construction and also circular shaped manholes covers are preferred.

I think this discription will be helpfull. It is a simple case of three dimensional arch-truss. Go through this explanation.

Attached Thumbnails

 

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Faseeh

@ faseeh
Bro if it acts like an "arch" then it shud be v weak in tension and strong in compression..then how does the manhole absorb the tensile forces????

It is not actually tensile but Compressive forces that main holes suffers, another type of stresses that it may suffer is hoop stress that is specific to curved surfaces and is present is the outlined curve of the body. (Details of hoop stress is beyond the scope of this thread, any one interested may google the topic or consult Text Book of Machine Design by Khurmi & Guptta or Elements of Machine Design by Shiglay. )

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Faseeh

The logic behind this, is easy to understand if we keep ourselves in place of a sanitary worker.
1. Think, if there would be a shape of manhole, having corners then would it be an easy task to go inside it for cleaning? Circle is a shape which is uniform as it has the same distance from its centre to every point on it. So, it is easy to get inside it or to do any task.
2. Now be a civil engineer. The severiage lines are, usually, circular and manholes are cross sections on them. So, every cross section should be round even the line is not circular.

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Fixed signs.

It will be worth mentioning that a circular disc with a certain dia will have less area then a square of the sides equal to that dia. So which is easier for a sanitery worker to use???


Practically the cross section of underground lines are circular but the section that reaches ground is square or rectangular in section, with provision made for mainhole cover by placing a mainhole seat with mainhole cover. see this pic...


In my opinion it is only the unique ability of circular disc to bear extra ordinary stresses that mailhole are made in this fasion. Another example is that of Pyramids in Egypt. If you over lap the structre of a tetrahedron on a circular disc you will get the same figure that i post in my last post & since the stability of Pyramids is well known you will get the required answer for mainholes.

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Faseeh

The question's answer is simple and involves only a basic knowledge of 'Distribution of Stress (Force) among various materials' and is as follows alongwith the necessary scientific explanation at first,

Scientific Explanation:
If we apply a force or exert stress on an object then its distribution is most uniform if the object is circular in shape.

What does this mean?
It means if a car, say of 1000kg, has to pass over a manhole then the required strength of manhole will be achieved with less thinkness (and utilization of raw-material) if the every small part of manhole bears this weight equally. Since 'Circular Shape' is the one which has maximum uniform distribution of forces so we can produce the required manhole with less consumption of raw-material in a minimum cost.

Answer:
"A manhole is manufactured in a round shape because this shape offers best endurance because of even distribution of stress"

Other Illustrations:
You will definitely have seen big structures having pillars to support their roofs. Have you ever noticed that their pillars are always circular in shape? If not, then see those. May be you have a one at your own home!

From answers stated in the earlier posts, i only agree with the one that supplements the above anser is that 'Circular Shape has an added advantage of being easy in transportation' but this is not the main reason because if Square or Rectangular Shape had more proper distribution of forces then convenience of transportation would surely have been forgotten.