The water tank is the container to store water for purposes of distribution, irrigation, fire extinguish, or future use. Water can be of any shape and size as per requirement. The design of circular water tank is necessary for the economical and safe use of tanks.

The water tank can be constructed underground, on the ground, or above the ground according to its purpose. The main requirement of the water tank is, it should be watertight. Material like HDPE plastics, masonry, RCC, or steel can be used for the construction of a water tank.

Design is not a hard and fast rule, it varies greatly according to the site conditions and clients’ requirements. Therefore, beforehand the design of any engineering components one should know the basic principle and should follow the codes or standards.

Here for the design of the water tank IS code 3370 part 1 to part 4 is to be followed. It includes procedures to calculate the application of load, formulas, co-efficient factors and minimal criteria to be fulfilled.

## General Steps for the design of circular water tank

** Case 1: Tank resting on the Ground**

- Firstly, its dimension and shape need to be fixed. The size of the tank is determined according to the water demand and retention time.
- For the circular tank, the internal diameter of the tank and height of the tank is to be fixed. It can be accomplished by simple formulas of geometry. But one thing should be always taken care of that is freeboard. In every design freeboard of about 0.3-0.5m are to be considered to compensate the contingencies in design.
- The next thing is the selection of the grade of concrete, the thickness of the wall slab, and the base slab. The grade of concrete less than M20 is not preferred.
- After fixing the size of the tank, the position of the tank either to construction underground or above ground is finalized, also condition for base either rigid or flexible is also determined.
**Case 1: loads acting on the when the tank is on the ground**

- The working stress method is used here for the design of the water tank. On the basis of hoops tension area of steel required is calculated.

- The pressure diagram for the tank is in a triangular form with a base at the bottom. It means water pressure is high at the bottom and less at the top. Therefore, less amount of steel is required for the wall of the tank at the top. Again, the area of steel is calculated for the top of the wall.

After that thickness of base slab is fixed.

- Generally, the thickness of the slab is kept minimum 100mm and the minimum reinforcement provided is 0.24% for the base slab also.
- The rebars are arranged in form of mesh. Mesh of 1 layer or 2 layers is provided as required.
- For the stability check, in the case of a flexible base, check in tension is done but in the case of a rigid base check for bending moment and shear also needs to be done.

**Case 2: Tank Under the Ground**

- Firstly, its dimension and shape need to be fixed. The size of the tank is determined according to the water demand and retention time.
- For the circular tank, the internal diameter of the tank and height of the tank is to be fixed. It can be accomplished by simple formulas of geometry. But one thing should be always taken care of that is freeboard. In every design freeboard of about 0.3-0.5m are to be considered to compensate the contingencies in design.
- The next thing is the selection of the grade of concrete, the thickness of the wall slab, and base slab. The grade of concrete less than M20 is not preferred. Besides, the specific gravity of saturated soil and the angle of repose for the soil should be known.
- We should also design for worst case, since the soil is all around the water tank, the stability of the water tank depends more or less on the depth of water available inside the tank.
- As mentioned in IS codes, Permissible allowable pressure on walls of the tank is

Similarly, Max compressible stress=(P_{a}*(D/2))/A.

The obtained value is compared with the value of IS:3370 Part 2 Table-2 to check the requirement of reinforcement in the slab or not. Though the minimum percentage of reinforcement is used i.e. 0.24%

- For the thickness of wall, Hoop Stress(T)=
**γ**_{w *}H *(D/2)*K _{1}

- Where, K
_{1}is co-efficient for tension in circular ring wall, fixed base, free top and subjected to triangular load.**IS3370 Part IV clause 3.1.1, table 9**

Since the load on every point of the wall varies with the height and tension is calculated on every 1/10 portion of the height of the tank and

- Steel for bending Moment is also obtained in every 1/10
^{th }part of the height of the wall.- Again, for shear reinforcement,

K_{3} is shear in circular ring wall, fixed base, free top and subjected to triangular load and can be interpolated from table 11 of IS code 3370 part IV.

Reinforcement on the members should not be less than 0.24%. And for the detailing of the bars, IS:13920 is to be followed. Which talks about ductile detailing in Reinforced Cement Concrete.

I hope this article on “**Design of circular water tank**” remains helpful for you.

Happy Learning – Civil Concept

**Contributed by,**

**Civil Engineer – Rukshana Shrestha**

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