Water tank, ground water tank, overhead tanks, design example of ground water tank & water tank design excel sheet

Tank

Water tanks are crucial infrastructure for storing and supplying water in residential, commercial, and industrial settings.

Their structural design and material integrity ensure safe and reliable water storage.

Types of Water Tanks

Water tanks are primarily categorized based on their placement and function:

1. Ground Water Tanks

• Description: Installed at ground level, these tanks store water for immediate use or as a buffer before transferring to overhead tanks.
• Applications: Rainwater harvesting, fire-fighting reserves, irrigation systems, and general water storage.
• Advantages: Easier to construct and maintain compared to elevated tanks, with relatively lower costs.

2. Overhead Water Tanks

• Description: Positioned above ground, typically on rooftops or elevated platforms, to utilize gravity for water distribution.
• Applications: Residential and commercial buildings for potable water supply.
• Advantages: Ensures consistent water pressure without additional pumping systems.

Design Example: Ground Rectangular Water Tank

Design a rectangular ground water tank with a storage capacity of 150,000 liters. Assume the following:

• Water depth = 3 meters
• Freeboard = 0.3 meters
• Length-to-width ratio = 2:1
• Concrete mix = M30
• Steel grade = Fe500

Step 1: Determine Dimensions

1. Total Volume of Tank
Volume of tank = Storage capacity + Freeboard allowance
Volume of tank = 150,000 liters × 0.001 + (10% of storage capacity)
Volume of tank = 150 + 15 = 165 cubic meters

2. Area of Tank Base
Volume of tank = Area × Water depth
Area = Volume of tank / Water depth
Area = 165 / 3
Area = 55 square meters

3. Dimensions of Tank Base
Assuming the length-to-width ratio is 2:1, let the width = W and the length = 2W.
Area = Length × Width = 2W × W
55 = 2W^2
W^2 = 27.5
W = sqrt(27.5)
W ≈ 5.24 meters
Length = 2 × W = 2 × 5.24 = 10.48 meters

Tank Dimensions:

• Length = 10.48 meters
• Width = 5.24 meters
• Water Depth = 3 meters
• Freeboard = 0.3 meters

Step 2: Structural Design

1. Wall Thickness
Assume a wall thickness of 200 mm (0.2 meters).

2. Hydrostatic Pressure on Walls
Hydrostatic pressure at the base of the tank wall = w × H
Where:

• w = Unit weight of water = 9.81 kN/m^3
• H = Water depth = 3 meters

Hydrostatic pressure = 9.81 × 3
Hydrostatic pressure = 29.43 kN/m^2

3. Bending Moment on Longer Wall
Maximum bending moment on the longer wall = (w × H^2 × Width) / 6
Substitute values:
Bending moment = (9.81 × 3^2 × 5.24) / 6
Bending moment = (9.81 × 9 × 5.24) / 6
Bending moment = 77.2 kN·m

4. Steel Reinforcement for Walls
Using the formula for reinforcement:
A_s = M / (0.87 × f_y × d)
Where:

• M = Bending moment (in N·mm) = 77.2 × 10^6 N·mm
• f_y = 500 MPa = 500 N/mm^2
• d = Effective depth = Thickness - Clear cover = 200 - 25 = 175 mm

Substitute values:
A_s = (77.2 × 10^6) / (0.87 × 500 × 175)
A_s = 1012 mm^2/m

Choose 12 mm diameter bars @ 150 mm spacing.

5. Base Slab Design
Base slab thickness is assumed as 300 mm to resist uplift and bearing pressures.

Summary of Dimensions

• Tank Length: 10.48 meters
• Tank Width: 5.24 meters
• Wall Thickness: 200 mm
• Water Depth: 3.0 meters
• Freeboard: 0.3 meters
• Base Slab Thickness: 300 mm

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