Code of Practice Concrete structures for the storage of liquids, Part 3: Prestressed concrete structures
1967 Edition

IS 3370 Part 3 specifies permissible stress levels based on IS 1343-1960 for both concrete and prestressing steel. Concrete tensile strengths, critical for crack prevention, are detailed in Table 1 and vary with 28-day cube strength, ranging from 16 to 22 kg/cm² for direct tension and 32 to 43 kg/cm² for bending tension. Prestressing steel stresses must not exceed 80% of the ultimate tensile strength during all construction stages and service. These constraints ensure structural durability and watertightness of prestressed concrete liquid storage tanks.

The standard incorporates guidelines from IS 1343-1960 to evaluate prestress losses arising from concrete creep, shrinkage, steel relaxation, elastic shortening, friction, and anchorage slip. All these losses must be factored in when calculating stresses during prestressing and throughout the structure’s service life. Typical loss percentages vary, with creep and friction losses potentially ranging from 5% to 15%. This comprehensive approach ensures effective prestress levels are maintained for durability and crack control.

According to IS 3370 Part 3, tanks intended for domestic water storage must have watertight roofs, achieved either by limiting tensile stresses within permissible limits or by applying waterproof membranes. Corrosion protection involves safeguarding the underside of the tank roof from condensation-induced damage, either through protective coatings or by designing it as a liquid-retaining surface with adequate concrete cover. Maintaining concrete compression above 7 kg/cm² when full and limiting tensile stress to 10 kg/cm² when empty further enhances durability and prevents leakage.

Movement joints, including sliding joints, are essential to accommodate thermal expansion, contraction, and shrinkage. Sliding joints at the wall base enable independent movement from the floor, preventing undesirable base moments. Movement joints should be spaced and detailed according to IS 3370 Part 1 Clause 8, aligning roof and wall joints in monolithic constructions to avoid cracking. A combination of contraction and expansion joints ensures structural flexibility while maintaining integrity.

Design must account for hydrostatic pressure from stored liquids, earth pressure from soil or earth covers, dead loads including self-weight and fixed covers, live loads such as maintenance and equipment, and potential upward loads from internal gas pressure. Stress limits must be observed: concrete compressive stress should not exceed one-third of the specified cube strength, shear stress should remain below 0.6 times the cube strength, and tensile stresses when empty should be limited to 10 kg/cm². These considerations prevent cracking and ensure tank integrity.