RCC WATER TANK DESIGN BASICS

RCC WATER TANK DESIGN BASICS

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, resting on the ground or underground water tanks. The tanks can be made in different shapes, but usually circular and rectangular shapes are mostly used. The tanks can be made of RCC or even steel. The overhead tanks are usually elevated from the rooftop through the column. On the other hand, the underground tanks rest on the foundation.

TYPES OF WATER TANKS

Based on the location, tanks can be classified into three categories. Those are:

o    Underground tanks (UGT)

o    Tank resting on grounds (Ground Storage Reservoir – GSR)

o    Overhead tanks (OHT)

In most cases, the underground and on-ground tanks are circular or rectangular in shape, but the shape of the overhead tanks are influenced by the aesthetic view of the surroundings as well as the design of the construction.

Steel tanks are also used, especially in railway yards. Based on the shape, the tanks can be circular, rectangular, square, polygonal, spherical and conical. A special type of tank named as Intze tank is used for storing a large amount of water for an area.

Intz Type Water Tank

The overhead tanks are supported by the column, which acts as stages. This column can be braced for increasing strength as well as to improve the aesthetic views.

BASIS OF CONCRETE WATER TANK DESIGN

One of the vital considerations for the design of tanks is that the structure has adequate resistance to cracking and has adequate strength. For achieving these, the following assumptions are made:

o    Concrete is capable of resisting limited tensile stresses. The full section of concrete, including cover and reinforcement, is taken into account in this assumption.

o    To guard against structural failure in strength calculation, the tensile strength of concrete is ignored.

o    Reduced values of permissible stresses in steel are adopted in steel are adopted in design.

CIRCULAR WATER TANK

The simplest form of the water tank is a circular tank. For the same amount of storage, the circular tank requires a lesser amount of material. Moreover, for its circular shape, it has no corners and can be made watertight easily. It is very economical for smaller storage of water up to 20000000 liters and with the diameter in the range of 5 to 8 m. The depth of the storage is between 3 to 4 m. The side walls are designed for hoop tension and bending moments.

PERMISSIBLE STRESSES IN CONCRETE

To ensure an impervious concrete mixture leaner than M 25 grade is normally not recommended to make the walls leak proof the concretes near the water face need to such that no crack occurs. To ensure this, member thicknesses are so designed that stress in the concrete is lesser than the permissible as given in table 1.

Table 1 Permissible Stresses in Concrete (For calculations relating to resistance to cracking)

Grade of Concrete	Permissible Stresses		Shear stress N/mm2
	Direct Tension N/mm2	Tension due to bending N/mm2
M25	1.3	1.8	1.9
M30	1.5	2.0	2.2
M35	1.6	2.2	2.7
M40	1.7	2.4	2.7

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.

o    When steel is placed near the face of the members in contact with liquid, 115 N/mm2 for Mild Steel Bars and 150 N/ sq mm for HYSD bars.

o    When steel is placed on the face away from the liquid for members less than 225 mm in thickness, the same as earlier.

o    When steel is placed on the face away from the liquid for members 225 mm or more in thickness: 125 N/ sq mm for M.S. bars and 190 N/sq mm for HYSD bars.

BASE FOR FLOOR SLAB

The floor slab should be strong enough to transmit the load from the liquid and the structure itself to the ground without subsidence. The floor slab is usually 150 to 200 mm thick and is reinforced with nominal reinforcement, which may be provided in the form of mesh both at the top and bottom face of the slab. Before laying the slab, the bed has to be rammed and leveled. Then, a 75 mm thick layer of lean concrete of M 100 grade should be laid and cured. This layer should be covered with tar to enable the floor slab to act independently on the bottom layer. In waterlogged soils, the bottom layer of concrete should preferably be of M 15 grade.

MINIMUM REINFORCEMENT FOR WATER TANK

The minimum reinforcement required for 199 mm thick sections is 0.3 % of the area of the concrete section which reduced linearly to 0.2 % for 450 mm thick sections. In case of floor slab for tank resting on the ground, the minimum reinforcement from practical consideration should not be less than 0.3% of the gross sectional area of the floor slab.

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.