8000gal Water Tank Foundation Design

[cmhski]

OK, I know that there have been many similar threads, but I still do not think they answer my questions...

I need to design a foundation for a water tank with the following info:

Volume = 8000 gal
Height = 20 ft
Diameter = 9 ft
Flat bottomed tank w/ 6 anchors
Soil bearing capacity = 2000 psf
Design snow load = 69 psf
Live roof load = 20 psf
Basic wind speed 70 mph
Seismic zone 2A
Extreme frost penetration = 60"

Well, here are my questions:
1. Since this is an anchored tank, I would assume a ring wall type footing would be the solution?

2. AWWA D100-96 states that the minimum footing depth should equal the frost penetration level. But this gives a 6'-6" high footing??? So do I then need to go with piles instead?

3. I have used the procedures in API 650 to determine the wind and seismic overturning moments. So now I have overturning moments based on either wind or seismic forces, and gravity forces based on dead, live, and/or snow loads. But how do I size the footing based on this?

Neither AWWA D100-96, nor API 650 goes into detail when it comes to the actual footing design.

I guess I can "unfold" the circular footing and treat it as a continuous footing. But how do I distribute the longitudinal forces (tension(uplift) and compression) due to the moments along the linear footing?

4. In a previous thread, an equation (from Gaylord, et.al.) for the reinforcement required to resist the lateral earth pressure due to the surcharge was provided, so I think I have that covered. Unless there are other methodologies?

Thanks,
cmhski

 

[JStephen]

1) Normally, a 9' diameter tank wouldn't have a ringwall- it would just be cheaper to make a solid slab. Which in your case is sounding like a solid "chunk". But you COULD use a ringwall if you wanted to.
2) Use a foundation that deep, or a shallower slab and expect some movement due to freeze/thaw cycles. Check with engineers in your area. You should be able to make the slab where it moves as one piece, as opposed to breaking up under soil movement. I can't think why you'd need to make it any deeper than what other foundations or pavements in the area are.
3) Size the footing to maintain wind overturning stability and allowable bearing. Calculate moment at the base of the footing. Assume linear stress distribution, so that moment pressures become Mc/I and axial loads become P/A. Simplest way is to design so that there is no uplift on the light side, although that's not a requirement. Minimum factor of safety against overturn is usually 1.5.
4) On a tank of this size, the reinforcement based on actual loads will be minimal. Use the minimum reinforcing ratios for walls (if it's a ringwall) or slabs (if it's a slab) from ACI-318 or other sources.

Based on the info above, you might consider insulating the tank.

You might also check with the tank manufacturer to see if they do foundation design (common with larger tanks).

If this is a welded or bolted steel tank, I believe the design codes will require higher wind loading than what you indicate (typically 100mph, 18 PSF on cylinder).

 

[tincan]

We normally have the footing or footing pedestal extend 12" to 18" outside the tank for the proper anchor bolt placement. A 11' or 12' OD footing should be sufficient. Even at 6.5' depth you are only talking about 23 - 25 C.Y. of concrete.
The linear loading about the perimeter of the tank (DL LL etc )is minor and is carried/distributed by the bottom plate.
Tincan

 

[apetr26542]

One other suggesting, check your stability against wind when the tank is empty, this may be a worse case for your foundations.

 

[BigH]

cmhski - I'd say put it on a raft foundation - but, why is it anchored? You intimate that the frost level is down some 6 ft - this is rather deep but then your allowable bearing is rather low. But you could raise the footing by using insulation to minimize the depth of frost penetration. This may suggest puttin the tank on "legs" sitting on the mat foundation then the insulation could also be placed under the tank (assuming that the tank might be empty from time to time in the winter). But, too, you have insulation problems with the tank and plumbing.

 

[cmhski]

Thanks for all the recommendations/suggestions!

I guess I need to clarify a couple of issues:

1) The tank is insulated all around.
2) The manufacturer is specifying the anchorage.
3) In general, I’m only designing the footing – the tank itself is being designed by the manufacturer.

cmhski

 

[tincan]

For this size of tank, the wind loading is only
18x9x20 = 3,240# WM moment = 32,400'-# at the base of the tank = 53,784'-# at the base of the 6'-6" footing. For a 13'Diameter Footing, S = 215.7 ft^3 Fbwl = M/S = 249 psf.

For a tank weight of 9,500#, Liquid loading of 79,000#, Footing weight of 99,000# (13'OD x 6.5')

DL = 108500/132.7 = 817 psf
LiL = 79000/132.7 = 595
Misc LL 90
Total 1,502 psf

Seismic @ 438 psf
Wind Load @ 249 psf

The max loading is < 2000 psf.

The loading on the footing is uniform, reinf should be minor.

best, Tincan

 

[BFPartners]

Is the soil bearing capacity the allowable value or the value at soil shear plane development? The soil pressures developed by the tank do seem low enough when compared to the soil bearing capacity. As for foundation design, the frost penetration zone only becomes a concern when dealing with the possibilty of soil movement such as frost jacking of piles or when relying on the skin friction of the pile for resistance or if the tank is on expansive soils. Normally what would be the expected would be something like a matt slab say 1-2 ft larger (all around) than the exterior suface of the tank, top and bottom reinforcement each way, the top surface of the pad shold be sized for maintence requirements and graded to promote drainage, rubber (1-2) layers should be placed under the tank due to the top sloping surface of the slab and imperfections in the top poured surface, compacted subgrade followed by compacted gravel then the slab. Just a question: If it is a water tank what will happen to the tank when freezing temperatures come around? Is there a requirement for a double walled tank or a containment wall around the tank?