Working Stress Method (WSM) is considered to be the method of earlier times. As we know, the working stress method takes the loads and the stress coming on the structure to be conservative in nature. Or in other works, it takes into account the permissible stress value.

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The WSM method works on two main aspects- stability and Serviceability. For the condition of serviceability, the section is designed such that it is not subjected cracks. In no way the cracks must affect the functionality of the structure. This design can be only attained my taking into account the permissible stresses on the structure. 

Why Working Stress Method (WSM) for Water Tanks?

Water tanks or any liquid retaining structures are vulnerable to stresses with the variation of water level in it. When compared with limit state method of design, the WSM method provide more serviceability that help to increase the durability and life of water tanks.

In simple words, WSM method designs the section to stay at the safe side, no matter how big the section be. The main disadvantage of WSM is that they derive uneconomical section. This demerit is neglected in the case of design of water tanks, bridges and chimneys.

WSM method tries to retain the stresses acting on the structure to stay within the elastic limit. Here the plastic state and limit is not considered.