Analysis and Design of Steel Water Tank Using STAAD.PRO

Elevated water tanks are an essential part of water supply systems in towns, villages, industrial campuses, and residential complexes. They store treated water at a height so that it can be distributed by gravity with adequate pressure. Steel is an attractive material for such tanks because it is strong, light, quick to fabricate and erect, and can be dismantled or relocated if required. Unlike reinforced concrete tanks, steel tanks do not suffer from cracking and leakage due to shrinkage, although they require protection against corrosion. The present study deals with the analysis and design of an elevated rectangular steel water tank and its supporting staging using STAAD.Pro. A rectangular pressed steel tank of 100 cubic metre capacity, with plan dimensions of 6 m by 5 m and a water depth of 3.4 m, supported on a four-legged braced steel staging of 12 m height, is considered. The tank plates, stiffeners, bottom beams, columns, and bracing are modelled together in STAAD.Pro, with plate elements for the tank walls and floor and beam elements for the staging members. The loads considered are self-weight, hydrostatic pressure of water, wind load according to IS 875 (Part 3):2015, and seismic load according to IS 1893 (Part 1):2016 and IS 1893 (Part 2):2014 for liquid retaining tanks. The tank is analysed for both the full and empty conditions, since the full condition gives the largest seismic mass and the empty condition may govern for wind uplift and overturning. Load combinations are formed according to IS 800:2007, and the staging members are designed by the limit state method of IS 800:2007 for combined axial force and bending. The tank plates are checked for stress and deflection under hydrostatic pressure, and stiffener spacing is fixed so that the plate stresses remain within permissible limits. The analysis shows that the maximum plate stresses occur near the bottom of the side walls and at the junction of the walls and floor, where hydrostatic pressure is greatest. Seismic load in the tank full condition governs the design of the staging columns and bracing, while wind load in the tank empty condition governs the uplift check on the foundation. Hollow circular and angle sections selected for columns and bracing satisfy the strength and slenderness requirements with adequate reserve. The study demonstrates that finite element modelling in STAAD.Pro allows the tank container and supporting staging to be analysed as one structure, giving realistic force distribution and allowing the design to be checked directly against IS 800:2007. The design procedure presented can serve as a guide for students and practising engineers engaged in the design of small and medium capacity elevated steel tanks.