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UNDERGROUND RECTANGULAR/SQUARE TANKS
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SAMYNATHAN
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PostPosted: Fri Aug 01, 2014 11:21 am    Post subject: Under ground rectangular water tank Reply with quote Reply with quote

Sir,

      The book mentined  Design of Foundations and Detailing by PWD is out
of print.It may be substituted  by any other book related to the subject. It is
good to refer easily  available resources.
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es_jayakumar
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PostPosted: Sat Aug 02, 2014 2:29 pm    Post subject: Reply with quote

As the water tank wall cannot move in the direction of the earth pressure unlike a retaining wall, the EP acting on the wall will be EP at rest and not active EP, as we normally assume. How to work out the same for submerged clayee soil ?

E S Jayakumar
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ibu_4_u
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PostPosted: Thu Jul 26, 2018 8:05 am    Post subject: RCC Rectangular Tank with size 14m length and 3.5m height Reply with quote

Dear All,

I am Designing the RCC wall with 3 edges fixed and Top free condition.
the size of wall is 14m length and 3.5m height,

As per 3370 Part 4, the coefficient are given for boundary condition with b/a ratio maximum upto 3, whereas in my designing the b/a ratio is coming upto = 14/3.5 = 4

is there any other references where i can get coefficient for b/a ratios more than 3.
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es_jayakumar
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PostPosted: Thu Jul 26, 2018 4:59 pm    Post subject: Reply with quote

When the b/a ratio increases, the cantilever action in vertical direction becomes prominent for the wall. Neither Reynolds Hand Book, nor Theory of Plates by Timoshenko has given the coefficients for bending moments for b/a ratio more than 2. IS 3370 Part-IV covers this only up to a b/a ratio of 3. The ratio in your case being 4, you can assume even a pure cantilever action in the vertical direction , though a bit conservative. Thus, the vertical moment at the base for hydrostatic loading will be wh3/6 or 0.167wh3 (w = 10kN/m3 for water and h = height of the wall). I carried out plate analysis in STAAD and found the fitting expression for the same as 0.143wh3. (the coefficient 0.143 is computed from the vertical moment value at base of 61.5 kNm, obtained from the plate analysis carried out for a wall of size 14.00m x 3.50m with the two vertical and bottom edges fixed & top free support conditions, subjected to hydrostatic load)

E S Jayakumar
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mainak.mallik
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PostPosted: Thu Jul 26, 2018 6:15 pm    Post subject: Re: RCC Rectangular Tank with size 14m length and 3.5m height Reply with quote

ibu_4_u wrote:
Dear All,

I am Designing the RCC wall with 3 edges fixed and Top free condition.
the size of wall is 14m length and 3.5m height,

As per 3370 Part 4, the coefficient are given for boundary condition with b/a ratio maximum upto 3, whereas in my designing the b/a ratio is coming upto = 14/3.5 = 4

is there any other references where i can get coefficient for b/a ratios more than 3.
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ibu_4_u
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PostPosted: Fri Jul 27, 2018 8:32 am    Post subject: Reply with quote

Dear es_jayakumar Sir,


Presently as per my query i got the coefficient of Ratio of b/a upto 4, from Reynolds handbook 11th edition table 2.78 & 2.79, the ratio is coming 0.152wh3 for vertical Moment with base fixed and top free.

Sir actually i am design CCT for an Sewage treatment plant, i had made a complete staad model Base raft with plate element and applying sub grade modulus Considering SBC 150kn/m2 and 40 mm settlement.

From Staad results 14m length and 3.5m height wall, Results are 83.26kNm (Factored) where as, from manual as per reynolds handbook its coming around 102.85kNm (factored), the variation in moments are there.

I created another model with removing base raft, the Vertical moments are 98kNm, near by with manual calculation results.

sir i guess the values of moment are getting changed due to raft created at bottom with applied subgrade modulus, is this method of designing with raft is proper or raft should be seperatly designed,

if we want to model as a whole with raft which support condition to be applied.

The staad model with raft and without raft is attached. kindly give your suggestions.



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es_jayakumar
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PostPosted: Fri Jul 27, 2018 3:13 pm    Post subject: Reply with quote

You may adopt the highest of the moments obtained by the different sensible methods, to be on safer side.

E S Jayakumar
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ibu_4_u
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PostPosted: Sat Jul 28, 2018 5:08 am    Post subject: Reply with quote

Dear es_jayakumar Sir,


AT present, i have considered the higher value which is coming from Reynolds Handbook Coefficient.


Still will be eager to know if any mistake is there in applying support condition or subgrade modulus, as per the previously uploaded Staad model.




Thank you sir.
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es_jayakumar
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PostPosted: Sun Jul 29, 2018 12:57 pm    Post subject: Reply with quote

I don't find any conceptual mistake in your STAAD model with the support modeled as Plate mat. If the accurate value of Sub-grade modulus cannot be assessed, use a range of values of the same in different trials, and find the maximum moments.

E S Jayakumar
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