Reinforced concrete water tanks are vital structures used for storing water in various applications, ranging from industrial to residential purposes. The design and construction of these tanks are governed by specific standards, including IS 3370: 2009 (Parts I – IV), which provides the guidelines to ensure the durability, safety, and functionality of these tanks. The design of reinforced concrete (RCC) water tanks depends largely on their location, shape, and intended use. This article will explore the types of RCC water tanks, design requirements, permissible stresses, and reinforcement specifications.
Types of RCC Water Tanks
RCC water tanks are primarily classified based on their location and shape:
A. Based on Location
- Underground Tanks: These tanks are constructed below the ground level and are commonly used for large-scale storage or where space constraints prevent overhead installation.
- Tanks Resting on Ground: These tanks are built at ground level and are often used in industrial and agricultural settings.
- Overhead Tanks: These tanks are elevated on columns or structures and are typically used for domestic and urban water supply. Elevated tanks ensure adequate water pressure through gravitational force.
B. Based on Shape
- Rectangular Tanks: A common design for underground or ground-level tanks, providing efficient use of space and ease of construction.
- Circular Tanks: Typically used for underground tanks, they offer structural stability and uniform stress distribution.
- Spherical Tanks: This design is used for overhead tanks, providing aesthetic appeal and efficient distribution of water pressure.
- Intze Tanks: These are specialized designs with a spherical bottom and cylindrical upper portion, typically used for large-capacity water storage.
- Circular Tanks with Conical Bottom: Often used for overhead water tanks, especially when specific water storage and discharge requirements are needed.
Aesthetics and surrounding environment also influence the shape of overhead tanks, which may be designed with considerations for architectural appearance and space efficiency.
Basis of Concrete Water Tank Design
The design of an RCC water tank must ensure that the structure can resist cracking and maintain adequate strength. To achieve this, the following assumptions are made during the design process:
- The plain section before bending remains plain after bending, ensuring that the concrete behaves predictably under stress.
- Both concrete and steel are assumed to be perfectly elastic, with the modulus of elasticity of steel being used in design calculations.
- During stress calculations, tensile stresses in concrete are limited based on the values specified in relevant tables, and tensile strength in concrete is neglected during strength calculations.
These assumptions help create a design that ensures the structural integrity of the water tank, preventing leakage and failure.
Permissible Stress on Concrete
To prevent cracking and leakage, concrete used in water tanks must be carefully designed to handle stress. The permissible stresses for concrete depend on the grade of concrete used:
A. For Resistance to Cracking
- Concrete should be free of cracks to prevent water leakage, especially at the water face of the tank.
- Concrete grades M20 or higher are recommended for water tank construction.
- The wall thickness should be sufficient to keep stress levels below the permissible values specified in Table 2 for different concrete grades.
B. For Strength Calculation
- The permissible compressive stresses in concrete are specified for different grades (e.g., M25, M30, M35, etc.).
- Tables (such as Table 3 and Table 4) provide values for permissible stresses in compression, bending, and shear, which must be followed during strength calculations.
Properly designed concrete stress limits ensure that the water tank can withstand internal pressure without cracking or failing.
Permissible Stress in Steel
Steel reinforcement plays a crucial role in providing the tensile strength needed to prevent cracking in the concrete. The permissible stress in steel varies based on its position relative to the liquid:
- For steel placed near the liquid face, the permissible stress is 115 N/mm² for mild steel and 150 N/mm² for high-strength deformed bars.
- For steel placed on the face away from the liquid (for members ≥225mm thick), the permissible stress is higher: 125 N/mm² for mild steel and 190 N/mm² for high-strength deformed bars.
These stress limits prevent the excessive deformation of steel, which could lead to structural failure.
Stress Due to Temperature or Moisture Variations
Temperature fluctuations and moisture variations can introduce stresses in the concrete. However, separate calculations for temperature and moisture stresses are generally not required if the following conditions are met:
- The reinforcement is at least the minimum required, ensuring adequate strength to resist any stresses due to thermal expansion or moisture changes.
- Proper movement joints and sliding layers are incorporated to allow the structure to expand and contract without cracking.
- The tank is used for storing water at or near the surrounding temperature, and measures are taken to ensure that the concrete does not dry out during construction.
In cases where the cement content is high (between 330 to 550 kg/m³), shrinkage is minimized, reducing the potential for cracking.
Floors of Reinforced Concrete Water Tanks
The floor of an RCC water tank is subject to significant load, and proper design is essential to ensure it can support the tank’s weight without cracking.
A. Floor Resting on the Ground
- A layer of lean concrete (M15 or M20) is placed over the ground before casting the floor.
- A polyethylene sheet is placed between the lean concrete and the floor to prevent moisture loss.
- If the surrounding soil is aggressive, sulfate-resistant concrete is used.
B. Floor Resting on Support
- The floor must be designed to handle bending moments caused by dead weight and the water load.
- In multi-cell water tanks, special attention is needed to account for the interaction between individual tank sections.
- When the walls and floor are rigidly connected, the moments at the junction must be considered in the design of the floor.
Concrete Water Tank Walls
Provision of Joints
- Sliding joints can be used to allow the walls to expand or contract independently of the floor, preventing moments that could cause cracks at the base of the walls.
Pressure on RCC Tank Walls
- Gas pressure from the tank cover must be accounted for in the wall design.
- For underground tanks or tanks surrounded by earth, earth pressure must also be considered, especially when the tank is earth-embanked.
RCC Water Tank Roof
The roof of an RCC water tank must be designed to prevent cracking and ensure waterproofing. This can be achieved by:
- Ensuring movement joints in the roof correspond with those in the walls if the roof and walls are monolithic.
- Using a sliding joint between the roof and walls to allow for independent movement.
- The roof must be water-tight, especially for tanks storing water for domestic use. This can be achieved through stress limits, waterproof membranes, or sloped designs to ensure proper drainage.
Minimum Reinforcement for RCC Water Tanks
The minimum reinforcement required in RCC water tanks depends on the thickness of the structure. For walls, floors, and roofs with a thickness of 199mm or less, a minimum of 0.3% of the concrete cross-sectional area is required. For thicker sections, the reinforcement percentage reduces linearly to 0.2% for sections up to 450mm thick.
In the case of floor slabs for tanks resting on the ground, practical considerations suggest that 0.3% reinforcement is a minimum. When sections are 225mm thick or more, two layers of reinforcement should be placed, one near the top and one near the bottom, to meet the minimum reinforcement requirement.
Conclusion
The design and construction of reinforced concrete water tanks require careful consideration of structural integrity, permissible stresses, reinforcement, and material properties to ensure that the tank remains leak-proof and durable over time. By following the guidelines laid out in IS 3370: 2009 and considering the various factors affecting the tank’s design, engineers can create water storage systems that are efficient, safe, and long-lasting.