Glass thickness for plywood tank

Question

Apologies if this question is misplaced. I'm needing to know the required thickness of a glass viewing window when installed in a large plywood/steel constructed aquaculture tank (for a home / DIY aquaponics setup).

The nominal measurements for the inside of the tank are 12ft length x 8ft width x 6ft height/depth. Filling to the brim would put about 4,308 US gallons of water, but for the sake of calculations let's say the working fill is 4,200 US gallons. This means there will be about 35,050 lbs of water in the tank. This means the uniform loading at the bottom of the tank is about 2.53psi, or 365 psf.

The glass windows will be set into a framed opening that will support them all the way around and will prevent the need for the glass to flex. I've not decided how large the openings would be, it likely depends on cost of the glass.

I'm curious as to the relationship of not only the thickness of glass vs strength but also the span capabilities. E.g. I imagine if I had a 5ft x 5ft piece of glass only supported at the edges it might need to be slightly thicker than if it also had a beam running down the middle of the glass. 5ft span vs 2.5ft span.

I understand that the pressure exerted on the walls of any tank holding a liquid increases with depth so I hope I am correct in assuming that if the bottom of the viewing window did not extend all the way to the bottom of the tank I might need less strength for the window.

There is also the question of tempered vs non-tempered; I would appreciate if someone could tell me either way which one is best for this application.

Thank you!

Answer 1

It looks like the glass needs to be strong enough for 72 inches depth. But besides the pressure there is the span.

A standard aquarium glass thickness calculator only goes to a depth of 36 inches, but using the 5 foot width with a safety factor of 2, that calls for 16 mm glass, and 22 mm for a safety factor of 3.

Using a more general calculator and making all sides glass for a 256 x 366 x 183 cm aquarium using a safety factor of 3 gives 55 mm glass (which is 5.3 tonnes of glass) and an overall weight when filled of nearly 22,000 kg (24 U.S. ton).

If people will be near this structure, of course you want tempered glass.