Lecture Notes - Water Tank Design and IS Codes

Introduction

• Good evening and welcome to the lecture on water tanks.
• It’s important to revise the PDFs provided, as exam questions will be based on simple code provisions.

Key Concepts

Concrete Stress in Water Tanks

• Question: What is the permissible concrete stress in direct tension for M20 concrete?
  • Answer: 12 kg/cm² or 1.2 N/mm² as per IS 3370-1965.
  • IS Code Clause: 8.2.1 allows for direct tension in plain concrete.
  • Permissible tensile stresses:
    • M20: 1.2 N/mm² (direct) / 1.7 N/mm² (bending)
    • M25: 1.3 N/mm² (direct) / 1.8 N/mm² (bending)

Minimum Grade of Concrete for Durability

• Question: What is the minimum grade of concrete required for water tanks?

  • Answer: C, M30.

  • Importance: Water tanks are exposed to severe weather conditions, necessitating higher strength to prevent cracking.

  • IS Codes Referenced:

    • IS 456: For concrete cell.
    • IS 1343: For pre-stressed members.

  • Minimum cement content:

    • Plain Concrete: 250 kg
    • Reinforced Concrete: 320 kg
    • Pre-Stressed Concrete: 360 kg

Bending and Tension in Circular Water Tanks

• The circular ring beam in an intimate tank primarily resists hoop tension.
• Design Implications: Recognize that water pressure creates tension in circular tanks.

IS 456-2000 Compliance

• The minimum grade of concrete should not be less than M30 for structural reinforced concrete water tanks.

Allowable Stress and Design Parameters

• Tensile Stress for M25 concrete (bending): 8.5 N/m², 6.0 N/m², 2.5 N/m², 1.8 N/m² factor based on design requirements.
• For M30 concrete, the maximum allowable bending tension is assessed by understanding the permissible stress limits through IS code analysis.
• Remember: Tension and bending stresses differ, and it's essential to memorize at least a few key values from the tables.

Reinforcement Percentages

• Minimum percentage of steel reinforcement for walls in tanks: 0.24% for HYSD bars, calculated based on thickness.
• Design Calculations: For tanks thicker than 100 mm but less than 450 mm, percentages decrease linearly from 0.3% to 0.2%.

Practical Tips

• Focus on doing previous year question papers (PYQ) to familiarize with exam patterns.
• Reviewing the principles of design will enhance understanding without memorizing entire tables.

Conclusion

• The class covered significant topics concerning water tank design, reinforcing the importance of IS code compliance.
• Next class will initiate discussion on pre-stressing in water tanks.

Final Notes

• Prepare thoroughly and reach out for clarifications on challenging topics.
• Reminder: Take care during adverse weather conditions.

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• Class ends with an alert to prepare for next topic and discussion on Independence Day celebrations.
• Please utilize provided resources via the telegram group.