Reinforced Circular Water Tank | Design of RCC Structures

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;

1. Hoop stress or circumferential stress – it is in tangential direction
2. Axial stress – parallel to the axis of the cylinder
3. 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/m³

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