Water-retaining structures

Tanks, reservoirs, swimming pools, sewage treatment units and basements below the water table must hold or exclude liquid as well as carry load. Strength alone is not enough: cracks that would be harmless in a building let water seep, leach lime and corrode the steel. Design of such structures in India follows IS 3370, used together with IS 456.

The governing check is usually cracking. IS 3370 (Part 2) limits the calculated surface crack width, normally to 0.2 mm, and to 0.1 mm where the exposure is aggressive or the appearance and tightness are critical. Crack width depends on the steel stress, bar diameter and spacing, cover and the strain gradient, so many small bars at close spacing are better than a few large ones. Minimum reinforcement is needed in both directions to control early-age thermal and shrinkage cracking, which in thick walls cast against a stiff base can be more severe than load cracking.

Circular tanks carry the water pressure mainly by hoop tension. For a wall free to slide at its base, the hoop tension at a depth equals the water pressure there times the radius. Real walls are usually fixed or hinged at the base, which shifts part of the load into vertical bending near the base; design tables and analysis give the split. Rectangular tanks carry the pressure by two-way plate bending and need corner steel.

Joints are planned, not accidental: construction joints with waterstops, movement joints where needed, and careful compaction and curing. A water test, filling the tank and measuring the drop in level over several days, is the final proof.