"Dave" wrote in message
news:5gtyc.307$n35.99@newsfe2-win...

"The Outcaste" wrote in message
...
mid posted

On Tue, 8 Jun 2004 16:38:11 -0400, "NetMax"
bubbled forth the following:

"The Outcaste" wrote in message
.. .
On Tue, 8 Jun 2004 10:47:31 -0400, "NetMax"
bubbled forth the following:

Personally, I think you could hold up many types of tanks by a
wood
block
in each corner. The tank's base is held by the long sides, and
the
amount of pressure needed to vertically break a pane of glass in
a
vertical position should require many more times the amount of
weight
in
the tank. The only concern is again, that the 4 corner blocks
equally
contact the glass so no twist is introduced.
While I've done this with small tanks (5 and 10 gal), I'd be a bit
leery with anything much larger.

There's one website (can't find the link now) where they recommend
that the 4 sides of the aquarium extend below the bottom panel.
This
is so you don't have to build a frame to hold the bottom panel up
off
of a solid topped stand. In this case the bottom panel is only
held by
the silicone, with not even the support of wood blocks under each
corner. However, I'm not so sure I'd want to put that much faith
in
the strength of the silicone. The bottom joint will be under more
stress this way, not only the water pressure pushing the side
panel
out, but the weight of the water and decorations pushing the
bottom
panel down.

Most DIY sites show that the side panels should be resting on the
bottom panel. All manufactured glass tanks I've seen are built
this
way. The forces on the bottom joint are mainly shear (water
pressure
pushing the side panel out), with some compression from the weight
of
the vertical panes. When you only support the 4 corners, you are
asking the silicone to hold the bottom pane up to the vertical
panes,
which places the joint in both tension and shear, and silicone is
stronger under shear than tension IIRC. While it may hold for
years,
you may be stressing the silicone to the breaking point. I have
this
mental picture of very small earthquake, or just a 2 pound rock
slipping from your hand a few inches above the substrate being the
proverbial last straw. causing the joint to fail all at once,
opening
up like a zipper, causing the bottom pane to break into many
pieces.

Plus I'm sure this would void any warranty on the tank.

I have no scientific or experiential data to say it won't work,
but
that mental picture makes me rather be safe than sorry, though as
always, ymmv

Jerry

What country are you in?

US, Portland, OR, 2878.4 miles from your store.

From Europe & Asia, I tend to see more flat
bottom tanks (glass sides sit on bottom pane as you described), but
the
bottom trim raises the bottom so it doesn't touch the stand. From
North
American manufacturers, it mostly seems to be the opposite, where
the
tank sits on the glass sides, with the bottom siliconed inside and
above
the stand. I'm not familiar with the pros & cons, but it would
surely
affect the jigging needed to build them. When I look at functional
design advantages and at manufacturing advantages, I see merit in
both
designs, at least similar enough that there isn't an obvious better
design.

Just double checked my 2 10 gal tanks. With the plastic bottom frame,
it's hard to tell, but there is enough of a gap on one to see that at
least the front pane sits on top of the bottom pane. I managed to
slide one corner off of the end table check from the bottom and the
bottom pane rests right on the plastic frame -- there is no room for
a
3rd bead of silicone, though the frame its self would add some
structural support to the bottom pane. They were manufactured by
Island Aquarium, Fontana, CA

My impression of the larger tanks I've looked at in the LFS is that
the sides sit on the bottom, but with the bottom frame it's hard to
tell, so I could well be wrong.

In case you are wondering where the edge bottom has a structural
advantage over the pane bottom, if we assume that the silcone bead
exceeds the worst case weight condtion in the edge bottom design,
then
there is a superior bond between the bottom and the side panes with
this
configuration. To explain in text is a bit labourous, but I'll do
my
best. With a pane bottom design you mentioned, there are 2 silicone
beads. Bead 1 is between the glass surfaces and bead 2 is a
chamfered
bead running inside the tank (on top). With the edge bottom design,
there are 3 silicone beads, between the glass surfaces, and a
chamfered
bead on each side of the bottom pane. An argument could be made
that the
strongest vector of concern is pushing outward on the side panes at
the
very bottom, so 3 beads are slightly stronger than 2 (even if all 3
are
in line with the side vector, while the 2 bead design has 1 bead
perpendicular to the side vector).

The 3rd bead on the bottom side could well give a structural
advantage.

If I sound like I know more than I do, then you are right ;~). I'm
not a
mechanical engineer, but it's puzzled me how 2 different designs
have
continued to co-exist. I don't think the average buyer notices one
from
the other, so it would seem to be driven by internal forces rather
than
consumers.

I did finally track down the web site that mentions the edge bottom
method, it's http://www.garf.org/tank/buildtank.asp
Here is their only reason for this type of construction:
"It is important that the weight of the aquarium is supported by the
front, back and sides. There should never be any support under the
bottom glass as this could fracture it."

They don't go into much detail on applying the silicone, basically
apply to edges and stick together. If it leaks, then run a bead along
the inside, nothing about adding a bead on the bottom of the bottom
pane

Conversely, after checking several other DIY sites, they all say the
front, back and sides must sit on the bottom pane, but never say
*Why*
this is so.

I did some digging on the GE Silicone web site and found the
following
info for their Construction SCS1201 silicone I found under
construction/glazing/aquarium


http://www.gesilicones.com/silicones...zing/aquarium/

It shows the tensile strength is 470 psi, and the peel strength is 40
lb/in. From what I can find on how peel strength is determined, this
is what I have been calling shear stress, or the downward force on
the
bottom plate of the edge bottom style tank.

Now keep in mind that my back ground is electronic engineering, not
mechanical, so I may be way off on these numbers. The peel strength
of
40 lb/in means 40 lb force per inch of bond line at 180 ° angle. I
don't know if this requires the bond line to be 1 in width for each
in
of length (ie, 1 sq in) but the tests are done with a 1 sq in patch.
I
doubt if the strength is linear with the width of the bond, but if we
assume it is, a 1/4" bond would give 10 lb/in, so:

Take a 48"x18" piece of 1/4" glass. It's perimeter is 132 inches. At
10lbs/in, it can support 1320 lbs. The depth of water to equal 1320
pounds (158 gal) would be 42 inches. With a 1/4" base I wouldn't go
more than 12"-14" deep, so there appears to be a safety factor of at
least 3 (42/14). And this doesn't include the additional strength of
the 2nd and 3rd bead. As far as the outward force, 1/4*48 gives 12
sq
in of bond*470 psi= 5640 pounds -- that would take 676 gallons with
all the weight acting solely on the one edge. So looks like the peel
strength is the limiting factor.

So if my assumptions and calculations are correct, while the bottom
plate construction may be stronger, in practical application the
silicone is strong enough that it doesn't make any difference which
way you assemble it.

Comments and corrections of my assumptions are welcome.

Jerry


Nice calculations.... I believe the prime bonding strength to be the
edge to
edge
contact area, with a very thin layer of silicone sandwiched by the
glass.
This
area takes the bulk sheer. I am led to believe the chamfered beads
placed
inside/
outside supply only water proofing where the prime contact bond is
perforated.

The two construction types each have their own
advantages/disadvantages.
The
bottom supported type has to be supported equally otherwise undue
stresses
are
applied. The force of the water at the bottom of the tank pushes
outwards,
therefore
the stress here is outwards, walls pushed off base = peel/sheer.

The side wall supported tank is different as this can be supported
anywhere
so long as the supports are flat. The disadvantage here is when an edge
is
chipped/
micro fractured and is only supported by the corners. Think the
stiletto
effect of women's
shoes. The tensile strength of glass along its edge is stronger. The
SAME
force of the
water at the bottom of the tank pushes downwards, therefore
the stress here is downwards, base pushed off/down walls = peel/sheer.

The two stress vectors are of equal magnitude, just looking at it from
different angles. IMHO the
best design is just up to personal preference.

Dave.

Jerry, yes, good stuff on the calculations. Unfortunately, my background
is also electrical engineering and not mechanical. While all my tanks at
home have a raised bottom, I'm going to look more closely at the newer
ones at work. You are right about the plastic trim interfering with
being able to see their construction. I could be wrong about the
European vs North American trend as they could have started changing over
and I haven't been watching for it. It's when I carry tanks around the
fish room that I notice the gap, and I *think* the manufacturers I carry
are mostly raised bottom, but I'm going to start paying more attention to
it.

Dave, on your comments about which beads have the most structural
function, this has been my understanding as well. The inside bead is a
secondary element for the structure's integrity, proving an additional
layer of safety for waterproofing and helping prevent the main seal from
damage due to the occupants. I'd also read that the physical limit of
silicone occurs at around 48" of depth, so tanks are usually not
constructed taller than 36" to maintain a safety margin (but I think I
read it on a web site, so who know?). I suppose keeping a stainless
steel frame around the base or the top would address this. Before my
120g split open, I was thinking of putting a pipe clamp on the end about
halfway up (I could see a lot of bubbles in the silicone). Of course,
the silicone gave out before I added the pipe clamp *argh*.
--
www.NetMax.tk