RCC Circular Water Tanks
Circular water tanks have the properties of the cylinder. Cylinder stress is exerted on the circular water tank. It is distributed with rotational symmetry. The stress distribution remains unchanged if the object is rotated about a fixed axis.
There are three different patterns of cylinder stress namely;
- Hoop stress or circumferential stress – it is in tangential direction
- Axial stress – parallel to the axis of the cylinder
- Radial stress – It is perpendicular to the symmetry axis but is coplanar
Some important theory for Circular Water Tanks
The base of the circular water tank has a flexible joint. The tank rests on the ground. The wall of the tank is designed for hoop tension.
Formula for Hoop tension (Ht)
The formula for calculating hoop tension is,
Ht = PD/2 ———————————— equation 1
Where,
P = water pressure on wall
D = diameter of tank in metres
The formula for pressure is given by,
P = d x h —————————————equation 2
Where,
d = density of water = 10kN/m^{3}
h = depth of water in metres
Substituting equation 2 in equation 1, we get;
Ht = d x h x D/2 ————————— equation 3
Step two:
Calculation the area of steel
Let the area of steel be denoted by Ast
Ast = (Ht)/ σst
= (d x h x D/2)/ σst
Ast = d x h x D/2 σst
Step three:
Calculating the thickness of the wall of circular tank
Ht = equivalent area of section x σct
Where,
Ht = hoop tension in kN
σct = permissible tensile stress in concrete
Equivalent area of section
= A + (m – 1) Ast
= 1000t + (m – 1)Ast
Therefore, T = [1000t + (m – 1)Ast] x σct
Important notes:
Permissible tensile stress (σct) for different grades of concrete are as follows:
Concrete
Concrete | σ ct(N/mm2) |
M15M20
M25 M30 |
1.11.2
1.3 1.5 |
In order to design a water tight tank, M20 grade concrete is generally used.
Values of σst for different grades of steel
Mild steel | Fe415 | Fe500 | |
σst (near liquid surface) | 100 N/mm2 | 170 N/mm2 | 205 N/mm2 |
σst (away from liquid surface) | 125 N/mm2 | 200 N/mm2 | 245 N/mm2 |
Minimum steel area = 0.3% of concrete area in each direction
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