DIY 8x2.5x2.5 ft tank question

[Pharaoh]

Here's what runs through my mind when thinking about it the two different option.

Side panels resting on Bottom panel:

This design is based strictly on the shear strength of the silicone bond as the outward pressure of the water will want to slide the side panel off of the bottom panel.

Sides resting on the stand:

This design is based on the tensile strength of the silicone bond as the two pieces would be effectively forced apart. There would theoretically be no shear force involved as the side panel would be resting on the stand exactly as the bottom panel.

So, this comes to a debate of shear strength vs tensile strength;

I just did a quick Google Search to find this Info. As noted below, the tensile strength is much higher than the shear/tear strength. So, from a simplistic idea, it would be better to have the panes on the outside rather than sitting on top of the bottom pane. One reason that companies may do this is it makes the tank easier to assemble inside the plastic frames.

****Note - This is just a random silicone that came up. Not necessarily for aquarium use.****
http://www.dcproducts.com.au/RTV_Silicone_Solutions/Tech_Data_Sheets/RTV120S-tds.pdf

 

[MaddMaxx]

Either way, the glass/silicone/glass is a tension Do you mean friction? bond. Shear strength is going to be greater where the silicone is not stretched already by another force.

Their way, you have downward shear and outward elongation. I don't think so, take a look at my "sketch".

The better way, you have compression which keeps the silicone the most dense it can be and only outward shear on that friction/tension bond.

I'm not a physicist, so I don't know the formulas to prove my point. I do however have 35 years of keeping aquariums and have refurbished many. I can tell you that the ones that last the longest are the ones built with the glass resting on the bottom. Me neither, and I'm not trying to prove you wrong, but understand why it is...

If built as shown and supported evenly underneath, the vertical force (blue) is not applied to the silicone bond in either #1 or #2, so it can be more or less ignored. So it comes down to the shear force (red) applied perpendicular to the bond (#1) or the tension force (red) applied parallel to the bond (#2).

Yes, the vertical force (green) of the glass side helps compress the silicone bond in #1, but I would bet (relying greatly on knowledge that is too old and dusty to be trusted) that silicone performs significantly better under tension than any shear application, even if compressed.

So my guess is that the tanks like #2 you've seen fail were not uniformly supported, or the bottom pane was raised, creating a MUCH worse shear force on the silicone bond...

Sorry for the derail OP!