Buoyancy Calculations
Buoyancy Calculations
(OP)
I am reviewing plans for a 900 sq.ft. shooting shelter to be located in the flood plain.
The Civil Engineer has provided calculations to demonstrate that the concrete pad will not float away when submerged under flood water.
Here is my question:
What is it that determines the soil depth associated with the buoyant uplift?
He has the uplift calculated as being = Unit weight of water 62.4 pcf x 3 ft x 900 sq.ft. = (-) 168,480 lbs.
The depth of buoyant uplift from the saturated soil below - appears to be equal to the depth of water above the grade line (3 ft).
Thanks. I'm always trying to learn something new.
The Civil Engineer has provided calculations to demonstrate that the concrete pad will not float away when submerged under flood water.
Here is my question:
What is it that determines the soil depth associated with the buoyant uplift?
He has the uplift calculated as being = Unit weight of water 62.4 pcf x 3 ft x 900 sq.ft. = (-) 168,480 lbs.
The depth of buoyant uplift from the saturated soil below - appears to be equal to the depth of water above the grade line (3 ft).
Thanks. I'm always trying to learn something new.





RE: Buoyancy Calculations
calculate the volume of water displaced by the structure, multiply by the unit weight of the water and that is the bouyant force. note that the concrete pad will never float away...
RE: Buoyancy Calculations
RE: Buoyancy Calculations
RE: Buoyancy Calculations
Let me refer to the excel spreadsheet (which also contains a graphic illustration) and ask if the calculations for uplift on the concrete slab are correct. There are NO building walls in this example. It is a chunk of concrete laying flat and extending six (6) inches above the ground. No footings or grade beam. No walls. We will agree that the concrete will not float away... But what is the numerical value of uplift presented by something, anything below the slab?
Thanks again.
RE: Buoyancy Calculations
if there is no building attached, still the same calculation. calculate the volume of water displaced by the slab, multiply by the unit weight of water. that is your bouyant force. it is resisted by the dead load of the slab as well as the weight of the water downward on the slab. if you really want to get tricky, you can also add some shear or cohesion resistance from the soil around the slab, but I don't recommend it.
RE: Buoyancy Calculations
RE: Buoyancy Calculations
62.4 lbs/ft^3 (unit weight of water) x (18' x 50') x 3' (soil depth?) = uplift = 168,480 lbs.
Again, I would agree that "Soil depth has nothing to do with it. Water depth does".
Alright then, why is this Civil Engineer incorporating a soil depth of 3 feet below the slab? That is what I am trying to understand.
RE: Buoyancy Calculations
soil or water below the slab do not exert a force upwards
water above the slab will exert weight on the slab
the weight of the slab itself is reduced by bouyancy
assume T = 6" thick slab
bouyancy = thickness of slab x area of slab x 62.4 pcf = 28kips upwards
dead weight of slab = thickness of slab x area of slab x 150 pcf = 67.5 kips downwards
resulting bouyant weight of the slab is 67.5 - 28 = 39.5 kips
kind of like, does a rock weigh less at the bottom of the swimming pool or just below the surface?
RE: Buoyancy Calculations
This Civil Engineer calculated the force from the saturated soil BELOW the slab as being equal to the force of the water ABOVE the slab (as if the water was not displaced by the concrete slab) as a way to negate the vertical effects of water above and below the slab.
Now it makes sense why any attention was given to a force with a depth 3 feet below the slab.
Thanks!
RE: Buoyancy Calculations