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The theory is.........

Posted By: Martin Thoene <martin.thoene@lakenheath.af.mil>
Date: Monday, 30 August 1999, at 6:10 p.m.

In Response To: Re: Anyone ever had a tank that cracked? (Scarlett)

......that old-fashioned metal framed tanks relied on their strength from the actual frame itself. All glass tanks only have glass for strength, and are inherantly at risk from uneven stresses being imposed upon them. This can take the form of being stood on an un-level surface, or an uneven one. Any small object (grit, or whatever) trapped under the tank can cause a pressure point that can lead to a fracture. By placing a thin sheet of styrofoam under the tank, any uneveness can be alleviated, and any small particles trapped will deform the styrofoam, and essentially be absorbed by it (unless they are too big) relieving contact pressure concentrations. Obviously one should ensure as far as possible that nothing gets trapped under the tank.

That's the theory. In practice, the tank I had in my bedroom stood on 4mm of styrofoam on a large, antique chest of drawers. A seriously hefty piece of furniture, which never the less, bowed slightly when the tank was filled. There was a discernable slight gap in the middle of the tank, so there's no way that the styrofoam was actually supporting the centre of the tank.

Now the tank resides on the bottom of the purpose made steel aquarium stand, with my River-tank above it. Both tanks stand on 4mm of styrofoam on a half-inch thick wooden shelf. I added the shelf as otherwise the only tank support is peripheral on the tank by the stand frame. Again, there's a slight bowing of the stand and shelf, so there's not much support in the middle!

In each scenario the actual stands are perfectly levelled before the tank is placed on them. I think that tanks are stronger than their manufacturers maybe believe they are. Probably, they say that styrofoam is necessary as extra insurance for their product, and as a liability thing too. If you don't use it, and crack the tank...tough!

Both these tanks are 15" deep, and 18.5" wide, so there weight is supported over a wide area with full contact assumed, but I know that it isn't full contact of course! At this depth the water pressure loading is much smaller than in my other tank which is 26" deep. At this depth I would consider that styrofoam is essential as extra insurance because of the huge increase in stresses imposed on the tank by the water column depth.

Martin.


 

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