Reduction of bearing capacity due to hydrostatic load?

[arbitrader]

I am seeking peer opinion/discussion as to a statement that is presented in a book entitled "Aboveground Storage Tanks" by Philip E. Myers, in regard to calculating design loads for storage tank columns that are within the tank. The column baseplate would be subject to hydrostatic load.

The text states on page 153:
"If hydrostatic load caused by the liquid level is deducted from the allowable soil bearing capacity, then this approach often leads to large foundations or bearing plates because the net soil bearing capacity is much reduced. There are many installations where no deduction has been used that are adequately supported Although some form of multiplier may be used to reduce the soil bearing capacity due to the presence of the hydrostatic load, a reduction is most often not used in practice, and the results have been acceptable."

Can anyone justify this statement from a technical perspective? If it can be technically justified, wouldn't you leave yourself open to violating code since hydrostatic loads would likely be included almost any governing design code?

I also posted this in the geotechical/foundations forum for their take on this.

 

[Duwe6]

They are WAAAAy overthinking the problem. Storage tanks sit on either;
a] compacted soil and a sand pad
b] soil with a bearing capacity established by a geotechnical engineer, and a sand pad
c] concrete or asphalt base
d] unknown soil and maybe some sand

For a, b, and c the column bears on its baseplate and spreads the load, then baseplate-to-floor [more spreading] then into a bearing pad with 5,000 10K psf (++) capacity. 5k - 10k psf = 35 - 70 psi. Assuming a tall, 40-ft high tank full of water, the hydrostatic load is 17.3 psig. Lots of bearing capacity left over for the column baseplate load.

 

[JStephen]

I'm not aware that the issue is addressed one way or the other in the different tank standards.

One thing to note is that if the bearing capacity of a base plate is limited by shear in the soil, then adding a uniform surcharge over the entire tank bottom will not affect the shear capacity of that plate.

 

[IFRs]

API 650 will address this soon, if the agenda item gets to publication.