Water Tank - Center Column Bearing Capacity

[PEStructural]

I am designing a steel flat bottom water tank bearing on compacted fill. The geotechnical report states a 2000 psf allowable bearing pressure. To calculate the allowable bearing pressure for the center column foundation, do I need to subtract the weight of the water ( 62.4 * depth ) from the 2000 psf to get the allowable bearing capacity for the column foundation? Any help would be greatly appreciated.

 

[SlideRuleEra]

Yes, the water is applying a uniform load on the footing. This is "using up" a portion of the allowable 2000 psf that the soil can support.

 

[cbosy]

I agree although you may want to call the Geotech to see if you can increase the allowable just under the footing-2000 psf is pretty low.

 

[CarlB]

I don't agree the water is using up the allowable BC - assuming the BC is based on a shear failure mode. The water presure counterracts the shear failure - holds down the failure wedge. But if the 2000 psf is based on limiting settlement to an acceptable amount, you should consider whether the column footing may lead to unacceptable differential settlement.

 

[Focht3]

Hmmm,

The water pressure is driving the soil failure wedge - so technically it is "taking away from" the bearing capacity.

If the bearing capacity really is that low, you have very few options:[ol][li]Limit the tank height to something less than 32 feet.[/li][li]Improve the bearing capacity of the soil.[/li][li]Support the tank on driven piles or drilled piers.[/li][/ol]



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[JoeTank]

You are asking this question to the wrong group. You should refer this question to the geotechnical engineers for your project.

In the absence of any other info, the column base is sometimes designed for 1.5 times the water pressure, assuming that the part remaining for the column load is 0.5 times the water pressure.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[cap4000]

I always deduct the thickness of my concrete footings x 150#/cft from the allowable soil bearing pressure which is essentially what SlideRuleEra is saying about the uniform water pressure. I totally disagree with SteveBraune, you are in the right group that can give practical ENG-TIPS advice which you then can approach the Eng'r of Record for further discussions. Just curious how high is the tank?? Good Luck.

 

[Focht3]

Actually, the Geotech forum is more appropriate. But some of us visit this forum, too.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[connect2]

Interested to know the construction detail between the base of the column and the base of the tank and the spread footing? Does the column go thru the tank floor, ... steel tank containing water?... is this a steel column? supporting a roof structure? exposed to seasonal loads like perhaps snow? differential settlement wil be a problem depending upon the size of the footing required.
This is a structural post really once the geotechnical questions have been answered with regards to settlement, Give them your footing size based on the bearing capacity they have provided you and see what they say about settlement in the area of the central column. You may need to look at the thickness of the steel floor in this area depending upon your detailing of the column base, the tank floor and the column footing. Just how thick is the fill? The load on you column and its footing may negate the fill in settlement calcs...
Unfortunately plate comes in uniform thicknesses and adding to them creates a whole different set of challanges in structures containing liquids, so yes this is the correct post, you must limit the differential settlement between the column foundation and the tank floor to nearly zero. Give the GeoTechnique guy your design, loads and assumptions and see what they say.
Your tank can't leak thats for sure.
Add the weight of the water to your footing load.
A very difficult question with very little info to go with.

 

[Hookem]

Remember that any the center, along with any other intermediate, columns will increase the loading directly under the column footing (vs. the soil immediately around it) as that footing carries the weight of the fluid and a portion of the roof. Anytime this happens, even if the calculated load is within soil limits (both liquid and column), the soil settles. In your mind, replace all the soil with a bed (think mattress) of springs. Push down in one place more than another, you get more settlement. Trouble is, with soil, its "rebounds" only a little bit. So, do not be blinded and only look at loads, but also at settlements, and at differential settlements. Good Luck.

chesney@uei-houston.com