RCC WATER TANK DESIGN BASIS



Reinforced Concrete Water tank design 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 RCC or even of steel. The overhead tanks are usually elevated from the roof top through column. In the other hand the underground tanks are rested on the foundation.

TYPES OF WATER TANK

Basing on the location of the tank in a building s tanks can be classified into three categories. Those are:

  • Underground tanks
  • Tank resting on grounds
  • Overhead tanks

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

concrete-water-tank

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

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

BASIS OF CONCRETE WATER TANK DESIGN

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

  • Concrete is capable of resisting limited tensile stresses the full section of concrete including cover and reinforcement is taken into account in this assumption.
  • To guard against structural failure in strength calculation the tensile strength of concrete is ignored.
  • Reduced values of permissible stresses in steel are adopted in steel are adopted in design.

CIRCULAR WATER TANK

The simplest form of water tank is circular tank. For the same amount of storage the circular tank requires lesser amount of material. More over for its circular shape it has no corner and can be made water tight easily. It is very economical for smaller storage of water up to 20000000 liters and with 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 impervious concrete mixture linear than M 20 grade is not normally 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 then the permissible as given in table 1.

Table 1 Permissible Stresses In Concrete (For calculations relating to resistance to concrete)

Grade of Concrete

Permissible Stresses

Shear stress N/sq.mm

Direct Tension N/sq.mm

Tension due to bending N/sq.mm

M15

1.1

1.5

1.5

M20

1.2

1.7

1.7

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

 

THE 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/ sq mm for ms Bars and 150 N/ sq mm for HYSD bars.
  • When steel is placed on face away from liquid for members less than 225 mm in thickness same as earlier.
  • 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 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 act independently on the bottom layer. In water logged soils the bottom layer of concrete should preferably be of M 15 grade.

MINIMUM REINFORCEMENT FOR 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. 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.

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.