"dixon" wrote
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.
.... 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).

There's part of your problem. You need to size the glass for the total
force across the span of the glass, at max pressure ie along the bottom
edge.


... 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.

Yep. Just like magic! I don't see where there is any contradiction here
bewteen force on the glass and measurement of the water column.

P.Fritz wrote:
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.

Correct. The glass should be sized so that the spanwise loading results
in little or no deformation (or elongation as some term it) of the
material. The easiest way to figure it, and a way that gives a safe
result, is to figure the total force on the glass as acting on a point
at the center, and size/spec the glass to withstand this force. It might
be a little overkill though and a bit more expensive.

But it sounds to me like you've already got a piece of glass that will
withstand the force, it's just a question of how long it will continue
to do so, does it have enough safety margin, and do you mind it looking
all bulged out like that

Regards
Doug King