Design of Water Tank Using Limit State Method

Abstract

IS 3370 (Part-II):2009 provides choice to structural engineers for design of liquid retaining structures using either working stress method (WSM) or limit state method (LSM). Earlier IS 3370 (Part-II):1965 allows design of liquid retaining structures by only working stress method. While adopting limit state method of design for liquid retaining structure, IS 3370 (Part-II):2009 restricts the crack width of 0.20 mm in limit state of serviceability. Design for limit state of collapse is similar to IS 456. Criteria for minimum reinforcement in wall, slab and floor are modified in IS 3370 (Part-II):2009. Further permissible stresses in reinforcing steel used for working stress method of design are modified. So, it becomes necessary to evaluate structural design of liquid retaining structures using WSM and LSM from economy and safety point of view.

Earthquake is major cause of damage of liquid retaining structures. When liquid storage tank is subjected to earthquake forces, liquid is accelerated and due to this acceleration storage tank wall is subjected to hydrodynamic pressure in addition to hydro static pressure on the wall. Hence, it need to consider the additional hydrodynamic pressure in the design of the water retaining structures. IS 1893 (Part 2):2014 gives the criteria for calculating the seismic forces in the water storage tank.

This major project includes the design of ground supported tank and elevated water tank using working stress method and limit state method. Seismic analysis of the ground supported water tank with circular as well as rectangular shape with different capacity is carried out. Results of seismic analysis of ground supported tank in terms of base shear, moment at wall base, overturning moment at bottom of the base slab and hydrodynamic pressure on the tank wall due to earthquake is presented graphically. Analysis of the ground supported water tank is carried out using finite element software SAP2000. Comparison of the design is presented in terms of material quantities derived from the working stress method and limit state method for the ground supported tank.

Design of elevated tank (Intze shape) is carried out by varying height of the lower supply level for different capacities. Wind load analysis as per IS 875 (part-3):2015 and seismic analysis as per IS 1893 (part-2):2014 is carried out. Maximum forces out of both (wind and seismic) analysis is taken into account for the design. As a result variation of the base shear and base moment due to wind and earthquake is presented graphically. Comparison of the structural design for the 5 lac, 7 lac and 10 lac litre capacity elevated tanks are presented in the tabular form which includes section size of the elevated tank, reinforcement schedule and material quantities i.e. steel and concrete. General drawings of elevated water tank with reinforcement detailing are presented in this report.