The Constructor

Reinforced Concrete Water Tank Design Requirements

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Reinforced concrete water tanks are constructed for storing water. The design of reinforced concrete water tank is based on IS 3370: 2009 (Parts I – IV). The design depends on the location of tanks, i.e. overhead, on ground or underground water tanks. The tanks can be made in different shapes usually circular and rectangular shapes are mostly used. The tanks can be made of reinforced concrete or even of steel. The overhead tanks (elevated tanks) are usually elevated from the rooftop through column. In the other hand the underground tanks are rested on the foundation. In this article, Design requirements of reinforced concrete water tanks are discussed.

1.Types of RCC water tank 

Based on the water tank location and their shapes, they are classified as shown in Table 1: Table 1 types of RCC water tank based on their location and shapes
Types of water tanks
Based on water tank location Based on water tank shape
Underground tanks Rectangular tank
Tank resting on grounds Circular tank
Overhead tanks* Spherical tank
  Intze tank
  Circular tank with conical bottom
*the aesthetical view of the surroundings and the design of the construction controls shape of the overhead tanks.
 

Fig.1:above ground RCC water tank

Fig.2:under ground RCC water tank

Fig.3:overhead RCC water tank

2.Basis of concrete water tank design

RCC water tank design should be based on sufficient resistance to cracking to avoid leakage and adequate strength. For achieving these following assumptions are made:

Fig.4:Stresses in a RCC water tank

3.Permissible stress on concrete

Permissible stress for resistance of cracking

Water tank concrete shall be free of leakage. This may be achieved by selecting concrete M 20 grade and greater, and concrete near water face need to such that no crack occurs. So, to make concrete crack free at water face, water tank wall thickness shall be designed so that stress on concrete is smaller than values provided in Table 2. In members less than 225mm. thick and in contact with liquid on one side these permissible stresses in bending apply also to the face remote from the liquid. Table 2 Permissible Stresses in Concrete (For calculations relating to resistance to concrete)
Grade of Concrete Permissible Concrete Stresses
Direct Tension N/mm2 Tension due to bending N/mm2
M15 1.1 1.5
M20 1.2 1.7
M25 1.3 1.8
M30 1.5 2.0
M35 1.6 2.2
M40 1.7 2.4

Permissible stress for strength calculation

In strength calculation, permissible concrete stresses should be in accordance with values provided in Table 3 and Table 4. Table 3 permissible stresses in concrete for strength calculation
Grade of concrete Permissible stress in compression, N/mm2 Permissible stress in bond (Average) for plain bars in tension, N/mm2
Direct Bending
M25 6 8.5 0.9
M30 8 10 1
M35 9 11.5 1.1
M40 10 13 1.2
M45 11 14.5 1.3
M50 12 16 1.4
Table 4 Permissible shear stress in concrete
100*As/bd Permissible shear stress in concrete, N/mm2
M25 M30 M35 M40 and above
?0.15 0.19 0.20 0.200 0.20
0.25 0.23 0.23 0.230 0.23
0.50 0.31 0.310 0.31 0.32
0.75 0.36 0.37 0.37 0.38
1 0.40 0.41 0.42 0.42
1.25 0.44 0.45 0.45 0.46
1.50 0.46 0.48 0.49 0.49
1.75 0.49 0.50 0.52 0.52
2 0.51 0.53 0.54 0.55
2.25 0.53 0.55 0.56 0.57
2.50 0.55 0.57 0.58 0.60
2.75 0.56 0.58 0.60 0.62
3 and above 0.57 0.60 0.62 0.63
As: longitudinal tension reinforcement area

4.Permissible stress in steel

The stress in steel must not be allowed to exceed the following values under different positions to prevent cracking of concrete.

Fig.5:Reinforcement in water tanks

5. Stress due to temperature or moisture variations

It is not required to perform separate calculation for stress due to moisture and temperature variation in concrete provided that the following conditions are met: Nonetheless, separate computation for moisture and temperature variation shall be conducted if: Note: cement content with a range from 330Kg/m3 to 550Kg/m3 shall be used to reduce shrinkage to as minimum as possible.

6. Floors of Reinforced Concrete Water Tank

Movement joints

Movement joints should be provided in accordance with IS 3770 (part I)

Fig.6:Various movement joints in water tank floor

Floor of RCC water tank rest on the ground

Fig.7:Reinforced concrete water tank floor above ground

Floor of tanks rest on support

Fig.8:above head water tank floor

7. Concrete Water Tank Walls

Provision of joints

sliding joints may be used if:

Fig.9:Sliding joint in water tank

Pressure on RCC water tank wall

Fig.10:Earth fill imposed earth pressure on RCC water tank wall

8. RCC water tank roof

To avoid the possibility of sympathetic cracking it is important to ensure that movement joints in the roof correspond with those in the walls, if roof and walls are monolithic. However, provision is made by means of a sliding joint for movement between the roof and the wall, Correspondence of joints is not so important. Moreover, in case of tanks intended for the storage of water for domestic purpose, the roof must be made water-tight. This may be achieved by limiting the stresses as for the rest of the tank, or by the use of the covering of the waterproof membrane or by providing slopes to ensure adequate drainage.

Fig.11:RCC water tank roof

9. Minimum reinforcement for RCC water tank

Minimum reinforcement required for 199mm thick sections is 0.3 % of the area of concrete section which reduced linearly to 0.2% for 450 mm thick sections. Moreover, in case of floor slab for tank resting on ground the minimum reinforcement from practical consideration should not be less than 0.3% of the gross sectional area of the floor slab. Finally, if the thickness of the section (wall, floor or roof slab of the tank) works out to be 225 mm and above two layers of reinforcing steel shall be placed, one near each of the section to make up the minimum reinforcement requirements.
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