Code of Practice for Use of Steel in Gravity Water Tanks
1968 Edition

While IS 805 does not directly specify exact minimum thickness values in all cases, typical engineering practice guided by IS 800 and related standards suggests steel plate thicknesses generally range from 3 mm up to 6 mm depending on tank dimensions and design loads. Thickness must accommodate structural strength, welding feasibility, and corrosion protection, with corrosion allowance often adding 0.5 to 1.5 mm. Thickness calculations commonly use hydrostatic pressure, tank diameter, and allowable stresses.

IS 805 mandates adherence to IS 800-1962 and IS 816-1956 for the design and welding of joints. Welded joints must be designed considering permissible stresses and joint efficiency factors, typically around 0.85 for butt welds in tank plates. Full penetration welds are required, with careful edge preparation and inspection. Fabrication must ensure weld quality with proper reinforcement limits, and joint geometry should comply with referenced IS codes.

Mild steel conforming to IS 2062 is recommended for tank plates and structural components. Welding electrodes suitable for mild steel, such as E6013 or E7018 types, are preferred to ensure adequate weld strength and corrosion resistance. Fabrication and welding procedures should comply with IS 800 and IS 816 standards, ensuring full strength and defect-free joints, verified by water-tightness testing.

The standard requires an additional thickness of 1.5 mm for interior bracing members and plates exposed to salt or alkaline water to compensate for corrosion. Bottom plates on foundations must be painted with two coats—one applied in the shop and another in the field before installation. Seams below the tank bottom are protected by laying 25 cm wide heavily tarred paper with welded edges. Elevated tanks should be equipped with painters’ trolleys for safe and effective repainting.

Ladders must be provided internally and externally to offer safe access, positioned to avoid obstructing hatch openings and designed without outward inclination. Fixed outside ladders should be spaced at least 15 cm from the shell, mounted on brackets no more than 3 m apart, with top brackets near the tank top and bottom brackets elevated above the tank base. Ladder sides extend above the tank top and are bent down to the roof for secure fastening. Inside ladders are supported by brackets spaced similarly and extend into large riser pipes when applicable.

Water-tightness testing involves filling the tank and visually inspecting all joints for leaks, immediately lowering water levels if leaks are detected. Flat-bottom tanks undergo additional air pressure tests beneath the bottom plates using soap solutions to detect leaks. Repairs require draining or lowering water at least 60 cm below the joint. Final acceptance requires tanks to be entirely water-tight to the inspector's satisfaction before painting.

Openings must be reinforced by welding additional steel plates around their perimeters, designed to carry the full load of the removed plate area. The reinforcement ensures that the average tensile stress in the net section remains within allowable limits, preventing overstressing. Proper connection of the reinforcement to the main plate is essential for maintaining structural integrity.