"dixon" wrote in message
news:Qg%qd.693019$8_6.466127@attbi_s04...



I wanted to build a 1000 plus gallon aquarium in the new house I was
having
built. I wasn't sure how thick to make the front glass. The rest of the
tank
would be fiberglass covered steel. The dimensions I arrived at were 8 ft.
across, 6 ft. front to back and 38 in. tall. This would be 1137 gallons. A
friend had a tank that was 38 in. tall but only 18 in. front to back. It
was
also 8 ft. long. (284 gallons). The glass in his tank was 3/4 in thick. My
tank would be identical except the front to back dim. would be 72 in.
At first I was concerned that there would more pressure on the front
glass.
After much research I became confident there would be no difference
regardless of front to back depth. For simplicity lets call depth the
front
to back dim. and height the top to bottom dim. I found charts that told
the
pressure at every height of water in inch increments. To figure the total
"push" on the front glass, you just need to find the pressure at the
halfway
point(19 inches in this case) and multiply by the total sq. in. of the
glass
(3,648). I believe
at 19" the pressure was about .686 or so psi. It figured out to be around
2,500 lbs. of force on the glass. It would be about 9,480 lbs of water.
As
I
was filling the tank, a straight edge laid against the front glass showed
the glass bowing outward very noticeably even at 1/4 full. I nervously
filled it full. Now it was very bowed even to the naked eye. The 2,500 lbs
was definitely there on the glass. Now, try to visualize the front glass
as
a giant rectangle piston with 2,500 lbs of force on the inside. If you put
a
4"x4" in the center of the glass that ran across the room to an opposite
wall and put a bathroom scale (quite a scale!) against the wall or better
yet, an "I" beam, the scale after filling, would read a total of 2500 lbs.
Or 25 men each pushing on the outside at 100 lbs would cancel the pressure
on the glass. Now, here's where the troubling part comes in. Lets say the
back of the tank, instead of being 72" away from the front is moved
forward
until it is just 1/8 of an inch away from the front glass. Now there is
less
than two gallons of water in the tank. I have trouble seeing the glass
(3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500 lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Dixon

You are mixing apples and oranges. The force of water at a certain depth
remains the same......Xpsi.

When you take that force and apply it over an area, the cumulative amount of
force reacts against the plane of glass or plexi...

In your thimble example....you only have 1 sq. in of force at 19" so the
force remains .686 psi.....

The plane of glass has to resist two basic forces.......shear and bending
moment.......typically the thickness required to resist the bending is
greater than that required for shear. Bending moment is a relationship of
the total force and the span of the material.