Overhead water tanks are constructed elevated, above the ground to get the desired head of water to supply at appropriate pressure. They are constructed on some supporting system called “staging”. These tanks may be designed as rectangular or circular in shape, depending upon the storage capacity requirements. For longer capacities, such as 75000 liters or above, circular tanks are designed. The supporting system for overhead water tanks may consist of various types of arrangements, some of which are explained below:
Types of Overhead Water Tanks
There are several types of overhead water tanks commonly used in both residential and commercial works.
(1) Tank Supported on Masonry Platform
In this case, the water tank is supported on a masonry platform providing a stable base for the tank. The design of the base slab for such a system is done in the same way as the tank is supported on the ground directly. Therefore, a minimum thickness of the base slab along with minimal reinforcement is provided. The platform is constructed using bricks or concrete blocks, ensuring adequate load-bearing capacity.
(2) Tank Supported on Masonry Tower
In this case, the water tank is supported by a masonry tower. The tower provides the necessary elevation for water storage, allowing gravity to facilitate water flow. The height of the tower is determined based on the required water pressure. The design of the base slab, in this case, is done by assuming the slab to be fixed along the edges and subjected to a uniformly distributed load of water all over its surface.
(3) Tank Supported on Circular Beam and Columns
In this case, the water tank is supported on a circular/ring beam which in turn is supported staging consisting of several columns. This design ensures uniform load distribution and enhances the stability of the tank. The circular shape also helps in efficient water circulation.
(4) Tank Supported on Beams and Columns
In this case, the water tank is supported by a system of beams, which may be two, four, or more in number. The beams are supported on the columns. The base slab of this tank is designed as a two-way slab with fixed edges, subjected to uniformly distributed load. This design offers flexibility in terms of tank size and shape, making it suitable for various space constraints.
(5) Rectangular Overhead Tanks
The rectangular overhead tanks are used for smaller capacities and are designed using IS 3370 (Part-4) which gives the values of the coefficients of hoop tension, moments, and shear forces for the walls of the tank for various boundary conditions. The base slab of the elevated rectangular tank is designed based on the type of supporting system. The base slab is designed for the condition when the tank is full. As the base slab is supported on a staging, it may be designed by assuming it to be fixed along the edges.
The top/roof slab may be designed as a simple slab. Due to the fixity, provided by the top and bottom slab, the walls may be assumed to be hinged at the top and fixed at the bottom for the purpose of analysis. The rectangular shape allows for efficient use of available land, making it a practical choice for densely populated regions.
Design of Circular Overhead Water Tanks
These tanks are used for larger capacities. The top/roof slab of these tanks consists of a dome. The base slab can be flat, spherical dome-shaped, or conical dome-shaped. Ring beams are provided at the junction of the tank wall and dome to take the outward thrust transferred from the dome. On the basis of the type of base slab, the following types of circular water tanks are commonly designed.
(a) Water tank with a domed top and flat bottom slab.
(b) Water tank with domed top and bottom slab.
(c) Water tank with a conical dome-shaped bottom slab (INTZE TANK).
The flat bottom slabs are generally not preferred for cylindrical tanks due to economical considerations. The thickness of this type of base slab and the area of steel reinforcement required for a flat slab is more as compared to a dome-shaped bottom slab.
The exact analysis of cylindrical overhead tanks is very lengthy, tedious, and time-consuming. All the elements or parts of such a tank are cast monolithically which results in infixity/continuity at the edges. The effect of this fixity/continuity can be incorporated into the analysis of water tanks by considering the compatibility equations of the various parts of the water tank. This analysis can be done quickly using the computer-based finite element method. The design of the overhead tank is done for membrane forces only. In the membrane analysis, various elements of the water tank are designed independently without taking into account the effect of continuity.
Dome
The roof and floor of a circular tank may be designed as a dome. A dome is a type of shell obtained by the revolution of a regular curve like a parabolic, circular, or elliptical curve about one of its axes. A dome is economical as compared to a slab as it carries the load through membrane action resulting in negligible bending moment and shear forces.
A dome may be analyzed by assuming it to consist of several horizontal rings placed one over the other with gradually reducing diameters. The equilibrium of each ring is considered independent of the rings placed above it. Any section of the ring is subjected to the following types of two forces:
1. Hoop or circumferential tension or compression.
2. Meridional thrust
The hoop tension or compression acts along the circumference of the dome and the meridional thrust acts along the meridian direction.
Construction of Overhead Water Tanks
(1) Design and Planning
Before starting the construction process, it is essential to determine the capacity, location, and required elevation of the tank. A professional civil engineer can assist in designing a tank that meets your specific requirements.
(2) Foundation Preparation
The foundation of the tank plays an important role in its stability. It should be prepared by excavating the ground and ensuring a level base. The foundation may consist of a reinforced concrete slab or a masonry platform, depending on the type of tank.
(3) Wall Construction
The walls of the tank can be built using bricks, concrete blocks, or precast panels. These materials offer durability and strength to withstand the weight of the stored water. Adequate reinforcement should be provided for additional stability.
(4) Roof and Waterproofing
The roof of the tank should be constructed using waterproof materials to prevent water leakage. A sloping roof design facilitates proper water drainage. Additionally, waterproofing measures should be undertaken to ensure a watertight structure.
FAQs
Q: Can overhead water tanks be installed in high-rise buildings?
Yes, overhead water tanks can be installed in high-rise buildings. However, additional considerations such as structural stability and water pressure management need to be taken into account.
Q: Are there any maintenance requirements for overhead water tanks?
Regular cleaning and disinfection of the tank are essential to prevent bacterial growth. It is also recommended to periodically inspect the tank for any signs of damage or leakage.
Q: What are the benefits of installing an overhead water tank?
Overhead water tanks ensure a continuous water supply, even during low water pressure or power outages. They also allow for efficient water distribution and help in managing water shortages.
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