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[valleveti_sumanth]

Dear people,
i have a doubt regarding design of underground tanks.While designing a  circular tank for water pressure we take coefficients for hoop tension and design it. Can we apply same method for the external soil pressure acting on the walls in case of underground circular tanks. Can we consider same co-efficient and permissible tensile stress to calculate the steel required. As the pressure acting from outside acts as hoop compression what is the exact procedure to calculate the area of steel required.

[es_jayakumar]

Dear Er.Sumanth,
I hope, you can make use of the same coefficients for hoop compression also. While designing UG tanks for hoop tension (case of water pressure from inside), we consider the contribution of steel only, towards the strength. But when it comes to hoop compression (case of pressure of submerged soil from outside-being extreme), both the compressive strength of concrete and steel will be effective. Thus, in this case, the compressive stress will be  safe as well, even though the pressure in this case may be around 50% more than the former case. The compressive strength of the wall can be calculated similar to that of an axially loaded RCC column. If required, you can design the section for hoop tension and check it for hoop compression. But this check is not very essential.

Regards,
E S Jayakumar

[es_jayakumar]

Please browse the link given below and see my postings, which are in similar connection :
http://www.sefindia.org/forum/viewtopic.php?t=16484

E S Jayakumar

[valleveti_sumanth]

Dear sir,
Thanks a lot for your reply. I went through the link and it was very helpful. I also saw that staad file you attached. I designed the water tank manually(6.1m dia by 3.5m height). the total capacity requirement of the tank is 1 lack liters. how will i know what dimensions will give me the optimum economical design? i never modeled a tank in staad sir. Is there any link for gudelines or Staad notes for modeling a tank. would be very helpfull for me.

with regards,
v.sumanth

[es_jayakumar]

Dear Sumanth,
You can find out the optimum (economical) dimensions of the circular tank by differentiation, as illustrated below:
Consider a cylindrical tank without top slab and of uniform thickness for wall and base slab. Let r, h, A & V be its radius, height, surface area and volume respectively.
A = 2πrh+πr²   ----------------(a)
V=πr²h,    h=V/πr²
Substituting in (a), A=2πrV/(πr²)+πr²= 2V/r+πr²
For finding minimum A for a fixed V, set dA/dr = 0
dA/dr = -2v/r²+2πr= 0.  r=(V/π)^1/3 = (πr²h/π)^1/3
This, upon simplification, gives r=h. This illustrates that for economy, choose diameter of the tank twice the height of it and accordingly work out the dimensions. In your  case, h=3.17m and D=6.34m for maximum economy.
For small to medium sized tanks, you can adopt IS code coefficients for working out the forces /moments with reasonable degree of accuracy and no need to go for software analysis.
If you intend to useSTAAD, the structure can be modeled by Structure wizard -cylindrical surface -platemodels method, with hydrostatic load applied on the walls.


Regards
E S Jayakumar

[valleveti_sumanth]

dear sir,
Here is what i did,

> i took the coeffecients for the hoop tesion and moments and calculated the maximum values for water pressure from inside.
> calculated the steel required and provided in the outer side of the section(tension side)
> same way for maximum hoop compression i calculated steel by dividingthe hoop compression value by permissible COMPRESSION stress in steel(140).
> which side should i place this steel now?? is in of the outer side of the tank or the inner side(wer tension steel is already present). or should i provide the maximum of tension or compression steel.
> i checked the tank for base shear, floating, and crack with. is anything else required??
I'm attaching the photos with this post, please refer them if needed.

[es_jayakumar]

Dear Sumanth,
I advise you to strengthen your theoretical background. You mention that you have provided the hoop tension steel outside, which is the tension side. How did you arrive at the conclusion that tank wall outside is tension side ? The hoop tension is the tensile force that the entire cross section is subjected to (horizontal for a water tank wall). That means the full section is under tension only. Don't get confused with the behaviour of bending moment, which produces tension and compression in the same cross section. For hoop forces, you can provide steel any where in the wall cross section. If your tank wall thickness is 200mm or more, you can provide the hoop steel in two layers on both the faces of the wall (half of the total steel on either face). Hoop tension steel will take care of hoop compression also, as concrete will also be effective in resisting the same. In the vertical direction, there will be bending moments for the wall (both negative and positive). In that case, you have to discern the position of placing of tension reinforcement.
You can thoroughly study the RCC design books by H.J.Shah (CPH) and M.L.Gambhir (PHI) for getting your basics right.

I would like to advise the youngsters that they shall master the fundamentals of structural engineering by going through standard books, before referring their doubts to this forum.

E S Jayakumar

[valleveti_sumanth]

thank you sir, will go through the books