Design of RC tank - Uplift vs bearing pressure

[Karlos80]

Hi ,

I am designing an underground tank (7x5m by 4m deep), my question is when the tank is empty should the base slab be designed for uplift pressure (due to ground water) plus upward pressure due to self weight of walls and a roof slab and live loads? I am not sure if these two forces are additive.

I have designed all the tank to date considering the worst case load of both , but never added two together. My view was that if you add both together the tank will float. You cant have a bigger pressure than due to the total weight of the structure? That’s my understanding, I will appreciate all your advise on this.

Thank you Karol

 

[cvg]

do a google search of this forum, this has been discussed numerous times

http://www.eng-tips.com/search.cfm?...plift&t=&h=&searchSite=1&searchArea=1&d=1&s=1

 

[oldestguy]

If you put a closed bottle under water,does it matter what shape it has, orientation? Uplift is it's the weight of water displaced minus the weight of the glass. For a tank it is no different, except use concrete for glass. If the live load is always there, maybe subtract that, but the safe way is no. Pressure between bottom and soil due to operating conditions does not apply for uplift total.

 

[hokie66]

Buoyant uplift loads and gravity loads are additive, but with opposite signs. If the buoyancy magnitude is greater than the gravity load, the tank will float.

 

[dhengr]

Karlos80:
Tank DL, plus any soil overburden is a hold down force, against buoyancy. You could make the bottom slab of the tank a couple feet larger(wider) on each side to increase these hold down loads.