Why water tanks are still designed by the working stress method?
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Nowadays we use IS 3370:1965 for water tank design which permits only the working stress method.
The reason behind the design of water tanks using the working stress method only are as follows;
Working stress method is adopted as it does not account for the variation of loads and the design is mostly conservative. Water tanks are more vulnerable. As the water level in the tank is incessantly varying, there is a continuous variation in the pressure inside.
So, there is a need for high safety factors and a more traditional approach to account for this variation. Moreover, the Working stress method offers more serviceability when compared to the Limit state method which extends the lifespan of the structure. These are the reasons for adopting the working stress method.
Earlier, IS 3370:1965 code is used for the design of water tanks which allows only the working stress method. The code has been revised in 2009 which introduces the limit state method also.
Water tank still designed by working stress method because of the advantages of working stress method.
Following are the advantages of working stress method:
Limitations of working stress method:
Here, because water tanks are more vulnerable structures and plus the tank is sometime overfilled and sometime empty, the pressure inside is so varying,
It needs more safety precautions and as we all know working stress method assumes more safety factor than limit state design, which is an economical design, we prefer to use working stress method…
Water tanks are still designed using the working stress approach because of the serviceability standpoint and unpredictability of load variations.
If we look closer, we’ll find that it’s actually impractical to design water tanks (especially overhead water tanks) using the Limit state method. What limit state method does is it assumes probabilities for magnitudes of loads. These probabilities are generally normally distributed and the maximum load is considered to be even exceeded 5% of the time. That is, the z factor is 1.64 for the maximum load or there is a 5% exceedance probability of the maximum load.
In the above graph, s represents the load the structure is subjected to and R represents the structure’s response. The red area is where the problem is when the load will be exceeded by the value the structure is designed for and the response will actually be insufficient.
The structures designed by the limit state method are those where even if the maximum load is achieved, it won’t last for long since it has a low probability (ends of the normal distribution). Now, can you say that this will always happen in the case of water tanks (especially overhead ones)? No it may not. Sometimes the water tank may be full or nearly full for a week! Maybe there’s a fault in the pipelines and water withdrawal from the tank is deliberately shut to prevent flooding! Maybe there’s a fire hazard or power plant accident and continuous water has to be supplied for a day making the tank run full! Now you might argue that no problem, we always design structures with a factor of safety and it’ll take care for the time being. But, since it’s an overhead water tank, even a small failure has drastically tragic consequences. So, serviceability is extremely important. Even a little thin hairline like crack is completely unwanted.
So, we can’t rely on a factor of safety alone and that too, for how long? Hence the design of water tanks is desirably conservative and deliberately by the working stress method assuming the tank runs full all the time just to be sure that there won’t be any accident or major deficiency or the need to restrict or partially restrict the usage of the water tank till the repair is done. The repair itself is undesirable.