For the tank to put loads along the long side you need strain compatibility, ie the piece you’re using to carry the load must deflect the same amount as the glass it’s carrying. This is because in order for the glass to transfer load to the frame it must be in contact with the frame. So if the frame deflects 1/16” the glass must also deflect 1/16” if it is transferring load. If it deflects less it cannot possibly transfer load, because it won’t be in contact with the frame The only way the load could be carried by the entire perimeter would be either:The entire perimeter carries the distributed load from the tank. If only the ends of the tank are supported then the front and back glass and silicone must carry and distribute that load to the ends. This puts extreme stress on the joints as well as creates significant torsional forces in the vertical panels. Think of joist blocking, without it joists under load twist.
1. The perimeter pieces were much more rigid than the vertical glass panel.
2. The base panel deflects independently from the vertical ones, resulting in separation of the base from the long sides.
Run the numbers for a wood frame - even if you use 2x12 pieces the vertical glass is around 10-20x more rigid than the wood that’s supposedly carrying it. By strain compatibility the wood could only be carrying 5-10% of the load.
For a steel frame:
Let’s say you want the steel beam to carry even half the load. It’s rigidity (EI) must be equal to the vertical glass panels rigidity.
Math time:
Young’s Modulus (E) for steel is 29,000ksi, for glass it’s about 10,000ksi
Say your tank is a 75 gallon. Glass is 3/8 thick and 18” deep. The I for this is about 180 in^3. For the steel to have the same EI we need it to have an I value of about 60 in^3. You’re looking at an HSS 10x2x5/16. To carry just half the load. The glass still carries half.
You want the steel to carry 90% of the load it has to be about 10x more rigid than the glass and you’re looking at something like a HSS20x4x3/8
The long sides of the tank span from end to end. This is also why the glass for a 6’ tank is thicker than in a 4’ tank the same depth, because the bending in it is greater so it needs extra thickness to reduce stresses.
If you don’t believe me check out your LFS if they have stacked tanks. The tanks will be on little angles that have no chance of being more rigid than the tank - get a piece of paper and you’ll be able to easily slip it between the angle and the tank. For bigger tanks you’ll be able to see a gap even.
/edits to correct numbers from bad phone mathing
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