Buoyancy Calculations

[richburton]

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.

 

[cvg]

soil depth has nothing to do with it, water level does.
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...

 

[richburton]

In other words, we can completely ignore any uplift from the underside of the slab if the bottom of the slab is at grade level? Is that true?

 

[cvg]

no, you need to determine the displaced water volume. has nothing to do with grade level. Theoretically, if the water is above the slab and there is a presumably water tight building structure attached to it, then it will become in effect, a boat. In practice, it will probably displace the building walls and damage the slab due to the upward force and the water pressure.

 

[richburton]

Hmmm, I think we are talking about general principals of buoyancy but I am not making my question clear enough.

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.

 

[cvg]

can't open your spreadsheet

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.

 

[richburton]

In lieu of the Excel spreadsheet, I have attached a jpeg illustration.

 

[richburton]

The calculations provided by the Civil Engineer indicated that the saturated soil below the slab will exert an upward force equal to the unit weight of water multiplied by the area of the concrete slab multiplied by by a soil depth BELOW the concrete slab.

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.

 

[cvg]

suggest you ask the civil engineer

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?

 

[richburton]

Somehow your rock in the swimming pool made everything "click" for me.

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!

 

[KevinEIT]

Rich, it might also help to think of it as the engineer is using the 3' depth the calculate the weight of the column of water above the pad resisting the buoyant force.