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Why water tanks are still designed by the working stress method?

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Asked: September 2, 20202020-09-02T21:31:14-07:00 2020-09-02T21:31:14-07:00In: Miscellaneous
aviratdhodare
aviratdhodare

aviratdhodare

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aviratdhodare

Why water tanks are still designed by the working stress method?

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  1. Komal Bhandakkar

    Komal Bhandakkar

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    Komal Bhandakkar AUTHOR
    2020-09-03T13:15:00-07:00Added an answer on September 3, 2020 at 1:15 pm

    Dear student,

    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;

    • Variation of the load is not taken into account in working stress method.
    • Working stress method is used whenever the design is more conservative.
    • Commonly water tanks are highly vulnerable because the water level in the tank will drastically vary and there is highly continuous variation in the water pressure inside the tank.
    • Working stress method fulfils the highest requirement factor and gives one of the traditional methods to account such type of variation.
    • Serviceability of working stress method is more as compared to Limit state method and hence the lifespan of the structure will automatically extend.
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  2. aviratdhodare

    aviratdhodare

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    aviratdhodare
    2020-09-07T17:39:36-07:00Added an answer on September 7, 2020 at 5:39 pm

    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.

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  3. Komal Bhandakkar

    Komal Bhandakkar

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    Komal Bhandakkar AUTHOR
    2020-10-09T21:11:08-07:00Added an answer on October 9, 2020 at 9:11 pm
    This answer was edited.

    Water tank still designed by working stress method because of the advantages of working stress method.


    Following are the advantages of working stress method:

    • Working stress method is a very simple method.

    • Due to its simplicity, it is still used for the design of some Complex structures such as overhead water tank.

    Limitations of working stress method:

    • The assumption of linear elastic behaviour and control of stresses within specially defined permissible stresses are unrealistic to several reasons like creep, shrinkage and another long-term effect.
    • Different type of load acting simultaneously have a different degree of uncertainty
    • The actual factor of safety is not known in this method of design.

    Thank You.

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  4. nikeetasharma

    nikeetasharma

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    nikeetasharma Guru
    2020-10-15T10:00:49-07:00Added an answer on October 15, 2020 at 10:00 am

    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…

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  5. Kuldeep Singh

    Kuldeep Singh

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    Kuldeep Singh Learner
    2020-09-07T18:16:00-07:00Added an answer on September 7, 2020 at 6:16 pm

    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.

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