uplift pressures tank base

huangyhg

uplift pressures tank base
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
waytsh
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.
fepc,
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,
waytsh,
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.
if the hydrostatic pressre is acting upward, but is less than the tank dead weight (i.e. the tank does not float) then multiplying the hydrostatic by 1.7 may have reduced the stress in the slab since the hydrostatic is acting opposite the dead weight. thus by not multipling by 1.7 you may actually get a more conservative answer, depending on the geometry
drc1,
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).
the hydrostatic pressure acts equally in all directions. with a voided structure such as your tank which is approximately 5 ft high, the bottom of the tank would experience 312 psf (acting upward) than the top of the tank. further since the tank is 10 feet below groundwater, the bottom slab would experience 612 psf upward pressure if the interior is empty. this acts opposite the weigt of the slab and tankk walls however slab wt is only 50 psf - so hydrostatic uplift may control design of at least the lower slab.
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.