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?

 

[MiketheEngineer]

Only because the soil was better than thought or tested. Loads are loads - they got lucky!

 

[dik]

Mike... drill a hole in the base plate so that the hydrostatic pressure equalises... <G> Maybe the bolt holes provide this...

Dik

 

[MiketheEngineer]

A tank is not supposed to leak - I thought. If you are putting moisture into the soil - I believe it will probably have less capacity??

 

[Ron]

Bearing capacity usually has a FS of 3 or 4 because of the nature of geotechnical engineering. As MiketheEngineer noted...they've been lucky.

Another reason they've been lucky is that the liquid level in the tank will exert confinement for the soils around a center column, thus increasing the allowable bearing capacity. It works the same as overburden....bearing capacity increases with depth...so the liquid in the tank effectively adds "depth".