I have been trying to state same in last two posts. In no other shape the stress can be so uniformaly distributed as it can be in case of a circular disk. In case of any figure with corners, The stress will concentrate on the coners that will soon be razed to particles and there will be a defect in the main hole.

@ faseeh

will u agree with me the effect of constant radius of gyration of circular bodies ,which helps to distribute the stresses uniformly????? hope stress is different concept, usually can be observe in doom structures and manhole does not act like a doom .....

Firstly Circle have equal diameter on all side so any pressure falling on it is equally balanced by all side.
Secondly,Any object in circular shape is easy to move
Thirdly,Mainhole have been manufacture in circular shape from inception.These have been taken as best and no body tried to change.

Stress can be observed any where. The radius of gyration (r) describes the way in which the area of a cross-section is distributed around an objects centroidal axis (Centriod means mass-center). Fory symmetrical objects, like this ciruclar disc, the radius of gyration, gemetric-center & mass-center(centriod) will be the same & thus the stress will be distributed Uniformlly.

In lines bellow I will explain types of stress and then will conduct a subjective analysis of types of stresses that are found in main holes which will show how stress is the main factor in its design.

Stress:
Stress in produced by application of force. So we can safely classify stress on the basis of force causes Stress & resulting in Strain.

Tensile Stress: [Figure#1]
When a force acts on an object perpendicular to its surface & opposite in direction, the stress produced is called Tensile Stress.

Examples:
1. When children try to pull rope in opposite direction this type of stress is found in Rope.
2. When we tear paper in to two peaces we apply tensile stress.
3. When we try to expand a rubber band we are applying tensile stress.

Compressive Stress: [Figure#2]

When a force acts on an object in same direction and perpendicular to the surface, the stress produced is called Compressive Stress.

Example:

1. When we push the door the stress produced in the door is compressive stress. (Although it is not visible).
2. When we sit on our bed, the stress produced in mattress is compressive stress.

Dynamic Stress [Figure#3]

Dynamic stress is created by intermittent application of load. As the name depicts, dynamic means it varies between a minimum and a maximum value, it is often called Cyclic Stress (especially when we study “Mechanical Vibrations”, in that case.)

Examples:
1. When we press the door bell, the stress produced in the spring is dynamic stress.
2. The jumpers of Motorcycles & Cars experiences regularly Dynamic Stress.
3. Even when we use stairs, muscles in our leg experience dynamic stress when we try to step-up. (Max load when we try to step-up, when we complete the step, it is the min load of cycle).

Dead Stress: [Figure#4]
Dead stress is caused by dead-loads, the one which is still and does not move for long time.

Example:

1. To me the classic example is sagging in the power cable. Do you know why they turn into an arc? (In urdu Sag means “Latak Jana” or “Jhool A Jana”), it is because due to dead weight of cable that is starts sagging.

Shear Stress: [Figure#5]

It is a stress produced by application of force parallel to the plain. (Normal stress is applied perpendicular to the plane)

1. Stress produced by scissors blade is shear stress.
2. When we try to take butter from butter tablet the stress applied is shear stress.


Thermal Stress [Figure#6]

It is produced by the temperature difference. Thermal stress is produced in bodies that are experiences temperature gradients over their surface.

1. Glass fitted in a window and exposed to sun light.
2. Stove when fired.
3. Utensils when used in kitchens.



Which stresses are applied on Main-Holes.

Tensile Stress:
No. This type of stress certainly does not apply here.

Compressive Stress:
Yes. When a vehicle is parked over a main hole or due to red light the tyre of car rests on the main hole, the stress applied is compressive stress.

Dynamic Stress:
Yes, when a vehicle is passed over the main hole it experiences dynamic stress.

Dead Stress:
Yes, when a vehicle is parked over it. Same as in compressive stress.

Shear Stress:
No. No force is applied parallel the surface. All forces act perpendicular to the surface for main hole.

Thermal Stress:
Applicable but to negligible level.

Conclusion:
Thus by study of definations and examples above mentioned, the only reason that appears most convinceing is its unique ability to bear load (Stress). The only reason else to it that is technical is the point that it will not fall into it, while rest of the shapes can pass through it.

Thus if designed uncorrectly, it can easily be razed to particles, due to the immense load it bears on regular basis when vehicals moves, rests and bumps over it.

Your question & criticism are welcomed, but be innovative and donot repeat questions.

__________________
Thanks & Regards
Faseeh