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.

 

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:

• Plain section before bending remains plain after bending
• Both concrete and steel are perfectly elastic and modular ratio value has a value given in IS 456- Table 21.
• In calculation of stresses. Both for flexural and direct tension or combination thereof relating to resistance to cracking, the entire section of concrete including the cover together with reinforcement can be taken into consideration provided that tensile stress in concrete limited to values provided in Table 2.
• Neglect concrete tensile strength during strength calculation.

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.

• When steel is placed near the face of the members in contact with liquid 115 N/mm² for mild steel Bars and 150 N/mm² for high strength deformed bars.
• If steel is placed on the face away from the liquid for members 225 mm or more in thickness then permissible stress in steel shall be 125 N/ mm² for mild steel bars and 190 N/ mm² for high strength deformed bars.
• When steel is placed on face away from liquid for members less than 225 mm in thickness same as earlier.

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:

• The reinforcement provided is not less than minimum reinforcement which described in the sections below.
• IS 3370 (Part 1) recommendations regarding movement joint provisions and for a suitable sliding layer beneath the water tank are executed properly.
• The tank shall be applied for the storage of water or aqueous liquids at or near surrounding temperature
• concrete shall never dry out.
• Suitable measures are taken to prevent cracking of the concrete during the construction period and until the tank is put into use.

Nonetheless, separate computation for moisture and temperature variation shall be conducted if:

• Assumed shrinkage coefficient is .
• Permeable lining used for the water tank. In this case, possible dry out of the tank shall be taken into consideration.

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)

Floor of RCC water tank rest on the ground

• Place layer of lean concrete not less than 75 mm thick over the ground.
• Commonly, use M15 for lean concrete
• Employ M20 for lean concrete in the presence of aggressive soils or harmful water
• Consider sulfate resisting concrete if required
• Install polyethylene sheet layer between lean concrete and the floor
• Cast the floor in single layer

Floor of tanks rest on support

• It should be designed for bending moments due to dead load and water load.
• Special attention shall be practice during the design of floor of multi-cell water tank
• Lastly, when walls and floor are connected rigidly, then moment at the junction in combination with other transferred loads shall be considered in floor design.

7. Concrete Water Tank Walls

Provision of joints

sliding joints may be used if:

• It is desired to permit the walls to expand or contract separately from the floor.
• To prevent moments at the base of the wall because of fixity to the floor.

Pressure on RCC water tank wall

• gas pressure, which is developed due to the presence of fixed or floated tank cover, shall be added to the liquid pressure.
• When water tank constructed in ground or earth embanked against it, then earth pressure shall be accounted in wall design.

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.

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.