Ring Foundation Height

[mobarez]

we are designing a 600,000 bbl aboveground storage tank ring foundation with diameter 80 meters. according to settlement and also soil bearing calculations, the calculated width is 60 cm.
Also the elevation code of top of foundation is 91 meters and the outside area of tank (inside dike wall) is 90m according to contract.
How could we find the height of the foundation? There is a rule in Api 650 that seems we SHALL use at least 160 cm height(1 meter per contract and 0.6 meter per paragraph below), is such height reasonable? Is 0.6 meter according to Api compulsory? As I checked in similar storage tanks (not with such an elevation difference) usually 70 cm to 120 cm is used for the height, So is there anyway to reduce height of the foundation?
Api650- B4-2-2 says: "The depth of the wall will depend on local conditions, but the depth must be sufficient to place the bottom of the ringwall below the anticipated frost penetration and within the specified bearing strata. As a minimum, the bottom of the ringwall, if founded on soil, shall be located 0.6 m (2 ft) below the lowest adjacent finish grade"

 

[jdonville]

mobarez,

You will need to discuss with your geotechnical engineer.

Jeff

 

[Ron]

The height/depth of the foundation ringwall will be determined by the negation of lateral stress to tolerable values by the vertical load. If significant lateral load is still evident at 160cm, then the depth will need to extend a bit, preferably until lateral load equilibrium is reached.

 

[kslee1000]

Ron:

Your response puzzled me a little.

Is the lateral load equilibrium reached by sucharge and active pressure in side the ring, with the passive pressure outside? (I have difficulty to picture the passive pressure would develop under such scheme)

I also don't understand your opening statement: "The height/depth of the foundation ringwall will be determined by the negation of lateral stress to tolerable values by the vertical load." How exactly the lateral stress is negated by the vertical load?

I know, this is a small space for such questions. You may help by pointing out further study materials. Thanks in advance.

 

[BigH]

Is the 60 cm settlement of the "ring wall" or is it the settlement expected for the whole structure - steel bottomed tanks will, of course, settle more in the center than at the edges. . . . I won't go on my usual diatribe about why you want to use a ring wall . . .

 

[kslee1000]

Settlement is not given. 60 cm is the calculated ring wall width per bearing strength.

 

[BigH]

kslee1000 - you got me!! ARRGGGG

 

[kslee1000]

So that's why I get my 500 back now?

 

[mobarez]

Thanks all
According to B4-2-2 of Api 650 which I stated in the first topic, what is the meaning of “As a minimum, the bottom of the ringwall, if founded on soil, shall be located 0.6 m (2 ft) below the lowest adjacent finish grade”?

 

[kslee1000]

Not with reference to any particular code, strickly based on structural point of view:

I. Ring wall width and depth is determined by three factors:
1. Structural rigidity of tank bottom assemblage, which determines the surcharge load directly under it.
2. Bearing capacity.
3. Strength of the reinforced concrete ring to resist the applied loads, mainly ring tension, tank weight and uplift.

The process of finding solution (depth) is therefore iterative.

II. Considerations on depth of ring wall:
1. Portion above ground:
a. Mechanical/operation requirements.
b. Leakage containment requirement, incl. potential flood level.
2. Portion below ground:
Overall depth determined by "I" above less "II.1" above, but not less than local frost depth, and suggested by code (compliance issue)

Hope didn't miss anything. Check me.