How is the reinforcement detailing done for circular tanks?

Short Answer:

Reinforcement detailing for circular tanks is done to resist internal water pressure and maintain structural stability. The reinforcement is placed in both horizontal (circumferential) and vertical directions, along with proper cover and anchorage to prevent cracking and leakage.

Horizontal reinforcement resists hoop tension caused by water pressure, while vertical reinforcement handles temperature and shrinkage effects. The reinforcement is denser at the bottom where pressure is highest and reduces towards the top. The detailing also includes base slab reinforcement, wall-to-base junction treatment, and construction joints to ensure water-tightness and durability.

Detailed Explanation

Reinforcement Detailing for Circular Tanks

Circular tanks are widely used in civil engineering for water storage due to their efficient shape and ability to handle internal pressure uniformly. The design and detailing of reinforcement in circular tanks are critical to ensure the tank remains water-tight, crack-free, and structurally sound under hydrostatic pressure.

Reinforcement detailing involves planning and placing steel bars in specific positions and orientations to resist stresses caused by internal water pressure, shrinkage, temperature changes, and self-weight of the structure. IS 3370 and IS 456 provide guidelines for detailing such tanks.

How Reinforcement is Detailed in Circular Tanks

  1. Horizontal Reinforcement (Hoop Reinforcement):
  • This is the most important reinforcement in circular tanks.
  • Internal water pressure creates hoop tension in the tank walls, which increases with depth.
  • Horizontal bars (rings) are placed to resist this tension, especially in the inner face of the tank wall.
  • Bar spacing is kept closer at the bottom and gradually increased towards the top, as pressure decreases with height.
  1. Vertical Reinforcement:
  • Vertical bars are provided mainly for temperature and shrinkage control.
  • They also assist in resisting bending moments due to wall fixity at the base or external loads like wind or seismic effects.
  • These bars are distributed throughout the height and placed on the outer face of the wall.
  1. Base Slab Reinforcement:
  • The tank base slab is reinforced in both directions (longitudinal and transverse).
  • The slab handles vertical loads from the stored water and the self-weight of the tank.
  • Extra reinforcement is provided near the wall-to-base junction to manage stress concentration in this area.
  1. Wall-Base Junction Detailing:
  • This junction is a critical point as it experiences high stress.
  • Curtailment of vertical bars is avoided here.
  • Bent bars or cranked bars are provided to smoothly transfer forces from the wall to the base.
  1. Distribution and Cover:
  • Minimum clear cover (usually 25 to 40 mm) is maintained to protect bars from corrosion.
  • Bars are placed as per the spacing limits in IS 3370 to ensure crack width is within limits.
  • Bars must be properly lapped, anchored, and bent as per code requirements.
  1. Construction Joints and Water Stops:
  • In large tanks, construction joints are introduced at safe locations.
  • Water stops or PVC strips are installed at joints to avoid leakage through them.
  • Reinforcement continuity across joints is maintained wherever necessary.
  1. Dome or Roof Slab Reinforcement (if covered):
  • For circular tanks with domes or flat covers, the dome is reinforced radially and circumferentially.
  • Proper edge thickening and anchorage are provided at junctions with walls.
  1. Special Considerations:
  • In partially or fully underground tanks, external pressure and buoyancy must also be considered.
  • Seismic and wind loads may require additional reinforcement, especially in elevated circular tanks.
  • Quality concrete, accurate bar bending, and proper curing are essential for performance.
Conclusion

Reinforcement detailing in circular tanks involves placing horizontal and vertical bars in specific zones to resist internal pressure and other structural forces. Horizontal bars counter hoop tension, while vertical bars handle shrinkage and bending. Detailing must ensure correct bar spacing, cover, and anchorage to maintain water tightness and long-term durability of the tank.