Last reviewed 16 Sept 2026 · 6 min read
Classification
| Basis | Types |
|---|---|
| Position | Resting on ground, underground, elevated (on staging) |
| Shape | Circular, rectangular, spherical, Intze, conical |
| Base joint (ground tanks) | Flexible (sliding) base — wall free to move; rigid (fixed) base — wall monolithic with floor |
Design philosophy — watertightness
A liquid-retaining structure must be strong and also impervious. Cracks let water seep, corrode the steel and cause leakage and efflorescence. So design emphasises:
- limiting crack width (or avoiding cracking on the liquid face),
- dense, low-permeability concrete (low w/c ratio, good compaction and curing),
- adequate minimum reinforcement to distribute shrinkage and temperature cracks,
- proper joints and water bars.
- Design by the limit state method with checks at the limit state of serviceability for cracking; the calculated surface crack width is generally limited to 0.2 mm, and to 0.1 mm where appearance or aggressive exposure demands it.
- Minimum grade of concrete for reinforced liquid-retaining structures: M30.
- Minimum reinforcement in each of two directions: 0.24% of the cross-sectional area for HYSD bars and 0.35% for mild steel bars (sections up to about 200 mm thick; different rules for thicker sections with surface zones).
- Working stress design (with reduced steel stresses) remains permitted as an alternative.
Uncracked-section approach (textbook method)
Many textbooks and exam problems use the older approach (based on IS 3370:1965) in which the section is designed so that concrete tensile stress on the liquid face does not exceed a permissible value, computed on the uncracked transformed section:
| Grade | Permissible concrete tension: direct (N/mm²) | In bending (N/mm²) |
|---|---|---|
| M15 | 1.1 | 1.5 |
| M20 | 1.2 | 1.7 |
| M25 | 1.3 | 1.8 |
| M30 | 1.5 | 2.0 |
Direct tension: . Steel stresses in this approach were kept low (typically about 115 N/mm² for mild steel and 150 N/mm² for HYSD bars) to limit crack widths.
Circular tanks resting on ground
Flexible (free) base
The wall is free to slide and rotate at the base, so it acts purely as a series of rings in hoop tension with no vertical bending. At depth below the water surface, for internal diameter and unit weight :
Hoop steel , distributed in rings (often on both faces). Hoop tension varies linearly, so steel can be reduced towards the top.
A common empirical minimum wall thickness: mm ( in m), not less than about 150 mm.
Fixed (rigid) base
The base restrains the wall, so near the bottom the wall bends vertically as a cantilever and hoop tension reduces. Hoop tension peaks somewhere above the base and vertical moments cause tension on the liquid face near the base. Coefficients for hoop tension and moment are tabulated (e.g. IS 3370 Part 4 tables) in terms of .