buried tank subject to hydrostatic uplif

huangyhg

buried tank subject to hydrostatic uplift
hi all, i'm a new member with a pretty basic design question.  when designing the bottom slab of a buried tank which will be subject to uplift due to high groundwater, do i add the uplift force to the soil pressure due to gravity loads or subtract it? for example, if i have a 10 ft. by 10 ft. tank that carries a total gravity load of 100,000# i will have a design soil pressure on the bottom slab of 100,000#/100sq.ft.= 1000 psf. now, if i have a groundwater table that is 10 ft. above the bottom of my tank i will have an uplift force of 10x62.4= 624psf. will my total design load for the bottom slab be 1000psf + 624psf= 1624psf or 1000psf - 624psf= 376psf???
thanks!
1-you're gonna subtract the uplift pressure from the downward pressure and apply the net on the slab and that's gonna be 624psf
2-you've got also to check uplift resistance factor of safety=downwards total force/uplift total force and it should be equals or greater than the allowable i think it's 1.50
two different cases:
1) the tank is full.  the weight of the fluid and the tank with foundation minus ten feet of water buoyancy divided by the area of the soil.
2) the tank is empty. the weight of the empty tank minus ten feet of water buoyancy must by greater than zero.  if not, the tank will pop out of the ground.  add thickness to the foundation or base and secure the tank against uplift with anchor rods to the foundation.
the bottom slab will see a service pressure of 1000psf.  of that, some will be water pressure, and the rest will be soil pressure.  if your water table is +10ft, then 624psf is water pressure, 376psf is soil pressure.  if your water table drops to +5ft, then your water pressure is 312psf, your soil pressure is 688psf.  
design the slab for factored loads minus the weight of the slab.  include whatever load factors your code calls for.  if, for instance, you're designing to aci350, you have a durability factor of 1.3, on top of a dead load factor of 1.3 and a fluid load factor of 1.7.
thanks for the replies. it appears i've been pretty conservative (ie, wrong) in my designs because i have always added the gravity and the hydrostatic loads together. in actuality, the groundwater table will fluctuate. therefore, it looks like my controlling case would be an empty tank with the maximum 10 ft. groundwater table. in this case i would have a factored design load of 1.7 x 624 plus 1.3 x 376 for a total load of 1550 psf. does everyone agree??
as jmiec has shown you, the water table level does not affect the pressure on the soil as long as the loaded tank weight is greater than the buoyant force.  and re  
hemifun-
nearly.  forget buoyancy for the moment, and just look at the net design pressure on the slab.  if your 100,000 lbs is for an empty tank, then you need to reduce your upward design pressure for the weight of the mat.
if your total gravity load of 100,000 lbs includes liquid in the tank, then you need to reduce your upward design pressure for the internal pressure on the top of the mat, in addition to the weight of the mat.
in either case, if you are designing to aci350, you need to factor your result by the durability factor of 1.3.
now, for the buoyancy issue, check out some earlier threads on this topic.  you'll read a wide variety of opinions.