The document outlines the specifications and design requirements for water tanks, including types (ground, underground, elevated), materials, and permissible stress limits for concrete and reinforcement steel. It discusses the design criteria based on IS 3370, factors such as concrete imperviousness, tensile stress calculations, and minimum reinforcement percentages. Additionally, it illustrates a practical example of designing a circular water tank with specific capacity requirements, detailing each design step involved.

1. Water Tank

2. Water Tank
A water tank is used to store water to tide over
the daily requirements.
In general, there are three kinds of water tanks:
 Tanks resting on ground
 Underground Tanks
 Elevated tanks
 The tanks may have circular or rectangular
section.
 Tanks resting on ground & underground may
have flat bottom slab, while elevated ones may
have flat or conical bottom.

3. Water Tank
General Requirements (IS 3370- Part II, 1965)
1. Concrete should be impervious- Concrete should be rich in cement content i.e. water-
cement ratio should be low. The quantity of cement in mix should not be less than 3 KN/m³,
again to keep shrinkage low the quantity of cement should not exceed 5.3 KN/m³.For water
tanks concrete grade M20 or greater is used.
2. Permissible Stresses
(a) For crack resistance
(b) For strength calculation

4. Water Tank
Grade of
Concrete
Permissible Stress
in Direct Tension
(N/mm²)
Permissible Stress
in tension due to
bending
(N/mm²)
Permissible stress
in shear
(N/mm²)
M 15 1.1 1.5 1.5
M 20 1.2 1.7 1.7
M 25 1.3 1.8 1.9
M 30 1.5 2.0 2.2
M 35 1.6 2.2 2.5
Permissible Stresses in concrete for crack Resistance: IS 456-2000 does not specify the permissible
stresses in concrete for its resistance to cracking. However, IS 456-1964 included the permissible
stresses in direct tension, bending tension and shear.

5. Water Tank
Type of stress in steel
reinforcement
Permissible Stress in
(N/mm²)
Plain round mild steel
bars
High yield strength deformed
bars (HYSD)
1)Tensile stresses in the
members under direct
tension(σs)
115 150
2) Tensile stress in members
in bending(σst)
On liquid retaining face of
members
115 150
On face of away from liquid
for members
less than 225mm
115 150
On face away from liquid for
members 225mm or more in
thickness
125 190
Permissible stresses in steel for strength calculation

6. Water Tank
Circular Tank
 Circular tank with flexible joint between wall and slab
 Circular tank with rigid joint between wall and slab
Circular Tank with flexible joint between wall and floor
 When water is filled in circular tank, hydrostatic water pressure will try to increase its diameter
at any section. However, this increase in diameter all along the height of the tank will depend
upon the nature of the joint at the junction B of the wall and bottom slab. If the joint at B is
flexible (i.e. sliding joint). It will be free to move outward to a position B1. the hydrostatic
pressure at A is zero, and hence there will be no change in the diameter at A.

7. Water Tank
Circular Tank with flexible joint between wall and floor
 Let us consider the joint to be flexible so that the outward horizontal movement corresponding
to the maximum horizontal pressure is possible. Due to this, hoop tension will be induced
everywhere in the wall.
 Let D = diameter of the tank, H = height of water level in the tank
Maximum hoop tension at the bottom, per unit height of the wall = w H D/2
Taking permissible stress in steel in direct tension as σs, area of steel per meter height
at the base is given by:
 The above reinforcement is provided in the form of hoops at suitable spacing and the spacing
may be increased towards the top. The spacing should not be more than 3 times the
thickness of the wall.
2.
sh
s
wH D
A 


8. Water Tank
 Thickness of wall: Though the reinforcement has been provided to take the entire hoop
tension, the concrete has not been prevented to take part of this tension. The thickness of the
wall should, therefore, be such that the tensile stress developed in the composite section is
within safe limit. If σct is the permissible tensile stress in the equivalent concrete section, T is
the thickness of the wall, we have
( 1)
ct ct
c st st
F F
A m A A m A
  
  
 
/ 2
1000 ( 1)
ct
sh
wH D
value of T can be obtained from this equation
T m A
 
 

Thickness should also be checked by the following empirical formula:
T (mm) = 30 H +50, where H is the height of water in metres

9. Water Tank
 Minimum reinforcement: For thickness up to 100mm, minimum % of reinforcement should be
0.3%. For thickness from 100mm to 450mm, it may be reduced linearly to 0.2%
Minimum reinforcement should be ensured in both directions.
 For thickness 225mm or greater, two layers of reinforcing bars shall be placed, one near each
of the section.
 For floor slab minimum reinforcement should be 0.15% of concrete section.
 Minimum cover should be 25mm or diameter of the bar, whichever is greater.

10. Water Tank
Problem 1: Design a circular water tank with flexible base for capacity of 400000
litres. The depth of water is to be 4 m, including a free board of 200 mm. Use M20
concrete and mild steel bars.

11. Water Tank
Solution:
Step 1: Dimension of the tank
Step 2: Design of section
Step 3: Vertical Reinforcement
Step 4: Design of tank floor
Step 5: Details of reinforcement

12. Water Tank
Solution:
Step 1: Dimension of Tank
Effective depth of water 4-0.2 = 3.8 m.
If D is inside diameter of the tank
2 3
400000
3.8 (1000 1 )
4 1000
D l m
  
 400 4
11.62 11.7
3.8
D m m

   


Step 2: Design of section
Hoop Reinforcement:
Density of water w = 9800 N/m3
Max. hoop tension =
11.7
9800 3.8 217854
2 2
D
w H N per metre height at base
   
217854
1894 2 /
2. 115
sh
s
wH D
A mm m height
  


13. Water Tank
Using 20 mm dia. bars, spacing of hoops=
Provide 20 mm ϕ @ 150mm c/c
Ash (prov.) =
Thickness of the wall:
314
1000 165
1894
mm
 
2
314
1000 2093
150
mm
 
/ 2
1000 ( 1)
ct
sh
wH D
T m A

 

217854
1.2 157
1000 (13 1) 2093
T mm
T
  
 
Thickness from empirical formula T (mm) = 30 H +50
T(mm) = 30x 3.8+50 = 164 mm
Adopt T= 170 mm

14. Water Tank
Reinforcement:
 Hoop reinforcement at bottom 20 mm ϕ @ 150mm c/c
 For spacing 200mm, Area of hoop reinforcement
Height from top
 For spacing 300mm, Area of hoop reinforcement
Height from top
/ 2 115 1570
3.14
115 9800 11.7 / 2
sh
w H D
A H m

   

2
1000 314
1570
200
mm


2
1000 314
1046
300
mm


/ 2 115 1046
2.09
115 9800 11.7 / 2
sh
w H D
A H m

   


15. Water Tank
Spacing of hoops can be increased near the top. Providing a minimum reinforcement of 0.3%
at the top.
Spacing of hoops =
However, spacing of hoops should not be more than 3 times of thickness of the wall, which
gives: Spacing = 170 x 3 =510mm
Hence provide spacing 500 mm c/c at the top.
2
0.3
(1000 170) 510
100
sh
A mm
  
1000 314
600
510
mm


Since the thickness of the wall is less than 225 mm, reinforcement will be provided at the centre
of the thickness.

16. Water Tank
Step 3: Vertical Reinforcement
Distribution and temperature reinforcement is provided in the vertical direction and its area is
(by interpolation) =
170 100
0.3 0.1 0.28%
350

   
2
0.28
(1000 170) 476
100
sv
A mm
  
Using 10 mm dia. Bars, spacing =
Provide 10 mm ϕ @ 160mm c/c
78.5
1000 165
476
mm
 

17. Water Tank
Step 4: Design of tank floor
Since the tank floor is resting on the ground throughout, provide a nominal thickness of
150mm
Minimum reinforcement
Providing half the reinforcement near each face =225 mm2
2
0.3
(1000 150) 450
100
st
A mm in each direction
  
Using 8 mm dia. Bars, spacing =
Provide 8 mm ϕ @ 200mm c/c in both direction, at top and bottom of the floor slab
50
1000 220
225
mm
 

18. Water Tank
Step 4: Detailing