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

"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.

"P.Fritz" wrote in message
...

"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.

I don't see your meaning mixing apples and oranges. I guess I am forced to
accept the pressure at depth principle no matter how illogical it seems at
times.

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.....

No, actually with the thimble of water would be spread across the same
surface area of glass, not 1 sq. in., but 3,648. So, same "push" on the
glass with a thimble of water as with 9,480 lbs. of water?


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.

"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

"DSK" 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.
.... 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

The engineering data I read on glass was interesting. A piece of steel is
very consistant in strength from piece to piece. Not so with glass. Because
of microscopic imperfections one piece of glass might be only 10% the
strength of another. A small scratch goes a long way to weaken glass. It's
possible my aquarium is at 99% of its breaking point, and has been for
years. The bowing is not really noticeable unless sighting down the glass
from an end.
Dixon

Actually, the same holds TRUE for steel, it's just that because steel
is much more ductile than glass, the problem doesn't manifest itself as
reliably as with glass. BUT, if you take a set of, say, 3/4" dia. A325
bolts, and test them, there certainly will be fluctuations in the
tensile strength, because of those microscopic imperfections.

Is your tank absolutely level?

"Gould 0738" wrote in message
...
Is your tank absolutely level?

It was 14 yrs. ago. Now it's about 1/4 in out. It doesn't matter though
because the surface is hidden by the frame.
Dixon

dixon wrote:
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?

Here's my theory:
As you decreased the distance between front and back, and the front
galss bowed out, the volume of the tank would increase and the water
depth would lower, decreasing the water pressure on the glass.

Water depth and density determine the water pressure.

"John Wentworth" wrote in message
...
dixon wrote:
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?

Here's my theory:
As you decreased the distance between front and back, and the front
galss bowed out, the volume of the tank would increase and the water
depth would lower, decreasing the water pressure on the glass.

Water depth and density determine the water pressure.

Even with bowing you would still fill the tank to the top, it would just
take slightly more water.
Dixon