Uplift Pressures Tank Base
Uplift Pressures Tank Base
(OP)
I know this topic in general has been discussed quite a bit, however, I was not able to find any previous discussion on the application of forces to the base slab. I apologize if it has and I missed it.
I am reviewing some old calcs for a precast tank design to see if it is up to todays standards. The tank is 6'-10" x 15'-8" and is comprised of stackable 2'-4" high segments. In the course of my review I noticed that the base of the tank had been designed for the distributed bearing weight multiplied by the appropriate 1.4 load factor. In addition to this the hydrostatic pressure was also applied but it was not multiplied by the 1.7 load factor.
Ok, here are my questions. Is there a reason that this factor should not be applied? Do these two loading conditions need to be applied together? I would have thought that the maximum pressure that could be exerted on the base slab is the hydrostatic pressure from the ground water.
Any guidance would be much appreciated. Thanks in advance.
~waytsh
I am reviewing some old calcs for a precast tank design to see if it is up to todays standards. The tank is 6'-10" x 15'-8" and is comprised of stackable 2'-4" high segments. In the course of my review I noticed that the base of the tank had been designed for the distributed bearing weight multiplied by the appropriate 1.4 load factor. In addition to this the hydrostatic pressure was also applied but it was not multiplied by the 1.7 load factor.
Ok, here are my questions. Is there a reason that this factor should not be applied? Do these two loading conditions need to be applied together? I would have thought that the maximum pressure that could be exerted on the base slab is the hydrostatic pressure from the ground water.
Any guidance would be much appreciated. Thanks in advance.
~waytsh






RE: Uplift Pressures Tank Base
Please clarify.Is the hydrostatic pressure from water in the tank or is it from the ground under the foundation? In my opinion the load factor should be applied and there are two loading conditions. Dead Load and Dead Load plus Fluid Load.
RE: Uplift Pressures Tank Base
The hydrostatic pressure is from the surrounding ground water when the tank is empty. The distributed bearing weight pressing up on the bottom of the slab is also with the tank empty.
I guess the question could be summarized as, "Is the hydrostatic ground water pressure additive to the resisting bearing pressure or does it replace the bearing pressure?" And then of course do the full load factors apply to each?
Sorry I wasn't clear in my original question. Hopefully I did better this time.
Thanks,
RE: Uplift Pressures Tank Base
The upward soil pressure is in place prior to the hydrostatic pressure being applied. As the water table rises the pressure on the foundation slab is increased. I would use the full load factors as indicated in ACI 318-02 p.9.2.
RE: Uplift Pressures Tank Base
RE: Uplift Pressures Tank Base
Wouldn't the hydro pressure be acting in the same direction as the resisting bearing force? Or are you referring to the dead weight of the slab counteracting the hydro pressure? If this is the case then I see your point. In this situation, however, the bottom of the base slab is 10' below ground water and the slab is only 4" thick. So the hydrostatic pressure is significantly larger than the dead weight of the slab (even without the load factor).
RE: Uplift Pressures Tank Base
Any way I would multiply the net load by 1.4 or 1.7 as appropriate. Applying factors to loads acting in oppositte directions before finding the net force may lead to unintended (and incorrect) results.
Hope I have answered your question. If not let me know.