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65 gallon underglass support ?

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I've just finished gluing up my Craigslist special that failed me initially. It's an older 36x18 Aqueon/AGA framed tank with a tempered bottom glass. I removed and reseamed the bottom glass, and stripped and resealed the entire tank, meticulously. It cured for a full week dry and has been sitting with just water for several days. So far, everything looks good to go.

It will sit on a piece of 3/4" pressure-treated plywood cut and sanded to fit, and then on top of the crappy angle-iron stand that came with it. That stand was always a cause for concern so I also cut 3/4 plywood to fit snugly into the sides and back, and screwed them into place hoping to add rigidity and stability.

So here's my question for feedback: It's seems something short of miraculous that the bottom glass of one of these is only supported around the perimeter on the plastic frame, while supporting 65 gallons (542 lbs) of water. That seems nuts to put into the living room, but I don't hear of bottoms breaking so...
Not content with that, I found that 1/2" concrete backerboard placed into one of the areas under the glass, between the glass and the plywood, was about exactly the right thickness to distribute the weight better. So I cut a couple of 14"x14" pieces to nearly fill in the voids either side of the brace thereby giving support to the inner area of the tempered glass. This seems like a flash of brilliance to me but I'm easily impressed. The tank has been sitting this way filled with water for 3 days now without any leaks.

-Can anyone see any structural problem with leaving it like this?
- And should I be using support foam under the frame? The plywood/stand seem to pretty flat as checked with a 3' level.
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If the tank still has the plastic trim frame on the bottom, do not place any support between the bottom of the glass and the stand. Plastic trim framed tanks are meant to be supported on the edges or perimeter of the tank and not the glass bottom of the tank.
 

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Carpe Diem
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If you calculate the pressure per square inch, it's not that much. If your under glass support is just a bit higher then the frame, you just might have a stress point, defeating the frame's support and weight distribution - not a good thing.
You do not need a foam mat.
 

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Fully agree with the above. Adding more is a hazard rather than a benny! It sounds like you may be a bit too worried about the tank and stand. Consider how long those were the "standard" for home tanks? They are a ready-made example of the tank only needing to have the four corners evenly supported to work best. All we really need is support for the four corners and then close our eyes to not worry about how the center of the long flat bars of the old wrought iron stands bow down. All the horizontal bars need to do are to keep the four legs from moving in/out as the center of the tank simply floats much like a bridge.
Corners level? Good to go and don't risk adding a lump in the center!
 

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Discussion Starter · #5 ·
"Too worried?"
Lol, probably the basis for why I over-design things.
No apologies. 8 )

I'm trained in analyzing structures and strengths of materials. Not an expert nor professional, but enough experience to naturally look at things that way. I'm pretty sure these tanks weren't designed by an engineer.

Afa the stand, It's not compressive strength so much as torsional and lateral (racking) that worried me. I'm attaching a couple shots of how I got it, how I modified it and with the backerboard in place. I first considered different ideas for building my own, but this sits in a prominent place in a popular part of the home and I could not see myself building anything that would look better than a 6th grade 4-H project. With the stand stiffened up I'm satisfied that it's adequate.

Regarding the backerboard idea. Just to be clear, the board is exactly 7/16" thick and if you invert the tank, place a level across the frame and measure the depth to the glass surface it turns out to be exactly 7/16" inch. As well, when the empty tank is in place on these boards there is no rocking since the frame sits squarely on the stand. I'm certain there is no upward pressure from the backerboard.

Anyway, thanks for the replies. I know that's 'the way it's always been done' and all, but to my mind it just leaves very little room for error or flaw.

(In the last picture, the backerboard fits entirely within the opening of the bottom frame, even though it looks like the frame partiall sits on it.)
Rectangle Wood Gas Glass Display case
 

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While I see your point and its merits, I do see one flaw that maybe you are overlooking. The concrete backer board hopefully is as smooth as glass. If not the small imperfections, a raised grain of sand or such is going to create a pressure point. Small as it may be, it doesn't take much to shatter the glass, think of a center punch. Being also that it isn't in contact with the entire area of the bottom glass, you essentially created 2 very large low pressure points. If you look closely at the bottom frame of your tank, the entire bottom glass is only supported on the edges where it meets the vertical glass. The support crossmember under the tank is there to keep the center of the plastic frame from bowing outward, It doesn't contact the surface underneath it, and it really provides no support for the tank. You either want to support the tank around the perimeter of the plastic frame (really just the four corners will work, Or you remove the rim and fully support the bottom. All this depends on glass thickness, as the trim does, to some extent, help with the vertical glass bowing out under pressure.
And , yes, this tank design was done by engineers. The type of adhesive/sealant, thickness of glass, support structure, all of it was engineered to work properly. They tested it before it went into production to make sure it wasn't going to be a disaster waiting to happen. Don't be fooled into thinking there wasn't a lot of thought and engineering that went into making that glass box.
 

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I agree with the above. The glass tanks with plastic trim are designed specifically to be supported by the frame only. Anything else can cause pressure points that ultimately can crack the glass.

As long as the plastic edging is supported true you should have no issues. I know people have tried foam, etc. in the past and say it works but my question to them is, does it work, or have you just not had a failure yet.

Most important is keeping the plastic edges of the tank true to the stand. Also. Always level the stand, never the tank, in case you were not sure! :)

Sent from my SM-G950U1 using Tapatalk
 

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Discussion Starter · #8 ·
I just finished completely removing and reattaching the bottom glass so I understand the role the plastic cross member plays and that it does not contact nor support the glass.

Thank you for the additional thoughts, I appreciate other opinions on this but they seem to be based on an ideal situation where there's a uniform load across the glass, a tankful of water. And that's certainly not the case once you start putting stones in or loading substrate in an unbalanced way, and I've never heard of concerns for point loading (from inside) the bottom of an aquarium, and tempered glass's strength isn't one directional. In fact that would be more risky than what I'm doing since you'd then be adding force in the same direction as the existing. A bigger interest of mine is how this changes stress on the seams, and as I see it it simplifies it from a compound of shear (walls pressing out) and tension (weight pulling down, somewhat but resisted to a limited extent by the frame) to just shear. I have less confidence in the silicone than I do in tempered glass.

Believe me, I don't discount the fact that these tank bottoms aren't failing (that I'm aware of) and it's probably safe as designed and that this may be just an academic argument. I'm just acting on a hunch that I believe improves safety, and the convenience of having a couple precut scraps of backerboard lying around from the bathroom project. And yes, backerboard has a smooth and rough side, and mine are placed smooth side up.
 
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