concrete under water 6

[haldor]

I'm using concrete for a mooring for my boat, which will be moored in a salt water harbor off the Atlantic Ocean. I have approximately 700 pounds of mixed concrete in different column lengths. Does concrete lose some weight when placed under water due to bouyancy or any other issue? If so, how much will that 700 pounds weigh when submerged?

 

[cvg]

calculate the volume of water displaced times the unit weight of the water

 

[civilperson]

The 700 pounds of concrete will have about 4.2 cubic feet volume. Thus, a displacement of 301 pounds of water and a net downward weight of 399 pounds.

 

[csd72]

Isnt there pressure on it from the water above which will increase its effective weight? I imagine this would near enough negate any buoyancy as the pressure in all directions at a given depth is equal.

 

[H57]

The net downward force would be calculated as described. That is how you calculate resistance to buoyancy.

 

[concretemasonry]

It all goes back to the basics of physics and the free body diagram.

The downward pressure above is more than offset by the upward pressure below that decreases the effect of the mass.

 

[cvg]

think about when you dive in the pool and swim to the bottom. the water pressure is the highest, yet the bouyancy reduces your effective weight and it is tough to stay on the bottom.

 

[DRC1]

It will weigh about 400 lbs under water.

 

[Ussuri]

csd72, but the downward pressure on the top is less than the upward pressure on the bottom due the additional water depth. So in effect they cancel out and what is left is the (much smaller) upward component due to the difference in level between the top and the bottom ie the depth of the object. Which works out as a displaced volume when you include the plan dimensions and hence buoyant force.

 

[csd72]

I see what you mean, but I am surprised that it makes that much difference.

 

[cvg]

http://en.wikipedia.org/wiki/Bouyancy

http://en.wikipedia.org/wiki/Fluid_statics

another good example which illustrates the affect of bouyancy is an aircraft carrier. It is constructed almost entirely of steel. It can carry a fleet of 85 airplanes and three thousand men. It weighs literally thousands of tons, yet it floats due to the bouyant force of the water which is displaced by the hull. In fact, the displacement is actually specified at full load for a Nimitz Class carrier at about 97,000 tons.

http://www.navy.mil/navydata/fact_display.asp?cid=4200&tid=200&ct=4

 

[Lion06]

WOW- I am surprised there is such confusion on this issue. In my fluid mechanics classes I learned that very simply the buoyant force is the unit weight of the displaced fluid times the volume of that fluid that was displaced (as civilperson correctly states).
I didn't run the numbers, but civilperson's analysis looks correct.

 

[concretemasonry]

StructuralEIT - I agree and I learned (in engineering mechanics) the same thing over 40 years ago before I got my EIT.

Both civilperson and DRCI had good numbers, depending on what you assume for the unit weight of the concrete (that is another thread). The other answers were all good, but maybe they did not post numbers because of liability and other professional reasons. Since I have retired, I am now considered an amateur that cannot calculate, but I still have my honorary "Doctors" degree from somewhere in China.