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Old 08-30-2010, 12:47 PM   #1
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Tank framing in smaller tanks

Here is my 38G tank without it's bottom plastic framing.



The black strip is just black electrical tape. Now, I do have a question about this setup.
The bottom plastic framing was broken when I bought this used tank. It has this construction - showing a cross section:



That's the actual plastic frame (end view) and I drew in the aquarium glass. This framing provides a recessed bottom where only the perimeter of the bottom glass will be supported. It's plain to see that there is no bottom support to the side glass, only side support. A lot of those wrought iron tank stands that are sold have only perimeter support. That puts a lot of pressure on the bottom glass span. I would think the tank bottom would actually bow down from the weight and pressure. Perhaps that is the design of it. It confuses me.
Perhaps this info will help (or confuse), others as well.
End result is that my 38G aquarium is sitting flat on it's bottom with no recess provided by a bottom frame. It holds water just fine and the entire bottom glass is supported, not just the perimeter.
Anybody have information on just why the bottom of so many tanks have this recess area?
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Old 08-30-2010, 06:19 PM   #2
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Glass can handle a lot of pressure without deforming. On small tanks, it's more important to keep any debris from creating acute pressure points on the glass. Acute pressure points lead to cracks. That's why the glass is held up off the stand by the frame.

More importantly, the frame keeps the side glass seals from blowing out. That's the danger to your 38g. The water pressure is greatest at the bottom of the tank where the seals are. With the bottom supported, the water exerts the pressure horizontally against the sides.
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Old 08-30-2010, 08:36 PM   #3
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So, I think if I understand what you're saying is that any 'particles' trapped under the bottom glass when you fill the tank will cause stress points - is that correct?
Makes sense, because I know when a back yard pool is filled even with sand leveling the bottom, there is a lot of down pressure exposing every small little liner fold or tiny stone, that you weren't even aware of.
I don't understand why perimeter support is better than full bottom support though. (Not counting the above debris stress points.) Why would the pressure push any more horizontally be dependant upon bottom support method?
I'm missing the whole physics of this, I think.
Do you think I should drain the tank and try putting the frame back on? I'd need to bond the corners that broke. Since I've found clear tape residue near the corners, at one time the previous owner must have been trying to hold the frame together with tape. I don't think the frame broke when the tank had water, more likely it was mishandling when moving it about when it was empty, that snapped the frame at two corners.
I'm also wondering if 38 gallons of water can bow the bottom 1/4" glass. If so, I would think that would put more stress on the silicon seals than the bottom sitting flat on a piece of plywood. I'm really lost on the whole physics of this.
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Old 08-30-2010, 11:08 PM   #4
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That's correct. Anything that gets trapped under the glass will create stress points if it contacts only a small part of the glass.

Sorry if I was misleading. The water won't exert more force on the sides of the tank if the bottom is completely supported. The problem is that there's no reinforcement provided by the frame. Look at your picture and see how the frame goes up the side of the tank. That part of the frame provides a lot of lateral reinforcement to keep the tank from bowing and the silicone seams from blowing out.

To give you an idea of the power of water, the water in your tank is putting nearly 200lb of force on one of the long sides of your tank. If the tank is filled with water alone, the water is putting 300lb of force on the bottom of the tank. The pressure on the seals at the bottom of the tank is 0.7psig, which may seem insignificant, but the math doesn't lie.
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Old 08-31-2010, 09:47 AM   #5
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Structural help

In another post I stated that the plastic framing on tanks was decorative only - which caused a big stir - Ref: Will the 180 gallon tank I built leak/break once I fill it?

I should have denoted that I was refering to my broken plastic bottom frame more specifically - but I referred in a general term and that was my mistake. I never thought to mention 'top framing excluded'.
I didn't pull the information I gave out of my hat, I read it somewhere . . . here - Professional's answers on bottom glass and frame questions - Aquaria Central

The above reference is excellent reading and quite extensive. Please be sure to read post #19 on page 2.

I hope this clears things up a bit about aquarium framing
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Old 08-31-2010, 10:05 AM   #6
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Interesting. Personally, as an engineer, I'll take any extra factor of safety that frame provides. "When in doubt, build it stout." Water does funny things.
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Old 08-31-2010, 10:28 AM   #7
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I totally agree and I've felt a little leery all this week about my tank, as shown above.
When I bought the tank the bottom framing was broken at two corners and a third corner was cracked almost completely through. I tried hotmelt glue - which was a joke and didn't adhere well at all. I thought about using aquarium silicon to glue the frame back together, but then I'd need a thickness great enough to hold the pieces bonded, which I determined would just be to thick to give me a good and square tight fit around the bottom perimeter of the tank. When the frame was new and a 'one' piece extrusion, I believe it did perhaps have a structural purpose, - but broken - I see it as a very possible weakness, hence my reason to set my tank on a smooth, 'clean' flat surface. It's held water now for a week, so I think I'm home free. I have another tank - a 30 gallon that my brother had sitting dry in his basement for 35 years and it's been sitting in my living room now filled with water and happy fish for almost 3 years. I think we can put great stock in the adhesive properties of the silicon that the manufacturers glue glass aquariums together with.
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