"K. Smith" wrote in message
...
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. 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.

In fresh water, I guess because that seems a low number. Anyway the
force isn't "all" over the glass it's a water column 38" deep so along
the bottom of the front glass the pressure is what?? say 1.37 psi on
your figure & probably more like 1.55 psi on my figures.



It would be about 9,480 lbs of water.

The total weight of the water in the tank isn't relevant all the glass
sees is a PSI pressure starting at nothing at the surface, graduating to
say 1.55 psi along the bottom edge.

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.

How so??, did it start leaking?? what is the joint along the bottom
edge doing?? for the glass to "bow" it must be moving relative the the
tank bottom?? The bottom edge of the glass is about the only part that
sees full pressure & you should have that well tied to the bottom of the
tank (angle iron frame??? or similar) The top of the glass should have a
frame also but it's not as important as the bottom, because at the top
the glass sees very little psi of pressure.

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?

Again the total volume of water in the tank is not relevant, what does
matter is that the glass be strong enough to resist the pressure at the
deepest point, if it's OK there then it's OK at any lesser depth. At
6ft X 3ft+ it's a big sheet of heavy glass.

Most boat windows are not 6ft long in one go & if built to survey
standards are surprisingly thick, the forward facing windows in my boat
are 3/8+" thick & nothing like 6 ft in any one direction.

I'm sure you've noticed most "in survey" paying passenger carrying open
water vessels don't have large areas of unsupported glass, even
laminated etc?? There are specific rules which govern max window size vs
material, thickness & support in the USL code & it seems smaller windows
are a safer, lighter (no pun intended) & probably a cheaper choice than
trying to persist with huge picture windows that pleasure craft can get
away with.

I think 3/4" glass is just not thick enough unsupported over that
length (again I'm still wondering how it bows away from the bottom of
the tank??) & probably thicker wouldn't help much (as glass gets thicker
it doesn't get stronger in proportion, it's still the outer skin that
carries the load & it fails in tension; also it starts to have problems
supporting it's own weight), so maybe consider some better framing??

Again the pressure is mostly along the lower edge.

K

Dixon




Actually Cruise ships have large, very thick windows. Boarding a cruise
ship in Valpariso, Chile one time and they were replacing one of the side
windows that was about 6' long and was broken during a storm off the
Shetlands. The window was at least 1.5" thick. The 2 bridge windows were
not replaced at that time. I guess lack of suitable replacements. And the
glass of the aquarium will always keep bending more, until it breaks.
Glass is actually a liquid. Very high viscosity, but non the less a liquid.
Windows in 1400's buildings in Europe are actually thicker at the bottom
from the flow.