large acrylic tank on slightly bowed stand

[Brhino]

This is a repeat of something I mentioned in my journal but I wanted to give it its own thread for increased visibility.

I have a 300 gallon (8'x2'x30") acrylic tank that I build a stand for. Upon placing the empty tank on the stand for the first time I noted that the stand is slightly bowed such that the right and left end of the tank rests on the stand but in the middle there is a gap, approximately 1/4" wide, between tank and stand. Note that the stand is not bowing due to weight. It is solidly built but just not completely flat.

I am wondering what the ramifications of this are and how to deal with it. I am particularly hoping to hear from anyone who has first hand experience with large acrylic tanks, or who has a good understanding on the mechanical forces at play in this situation.

[mrramsey]

Brhino -

While I do not have any first hand experience with acrylic tanks, my construction / engineering background tells me that you should fully support this as much as possible. I would think at least solid contact every 12" along the seams.

It would be helpful if you link a pic of the current situation for better clarity.

[Cermet]

I had assumed that you would use the frame supports (by shimming between the vertical supports and the bottom part of the underside of the wood panel supporting the tank base) to counter that bow - that is a serious issue.

[Brhino]

yup, that would have been a good time, but unfortunately I didn't detect this until I had an actual 8 foot object on top of the stand. In hindsight I should have put the tank on for a test fit immediately after placing the top plywood on, before the glue dried.

I've talked to the same mechanical engineer that helped me design the stand and his recommendation is to place another thin piece of plywood on top of the existing top, and then shim between the two pieces so that the top one is flat and fully supported. That's the best plan I've heard so far.

[jeffs99dime]

Have you contacted the manufacturer?

[Brhino]

don't know the manufacturer, got the tank used.

[Lady Hobbs]

A new top or overlaying it with another piece of plywood would be my way of thinking, too. Sounds as the top board had a slight bow to it.

[Indian Woods Angels]

The surface of the deck that the acrylic tank sits upon must be totally flat and static.

Joists for a homes framing are non static, they are crowned with a high point at the center and flatten to level under load.

Your stand may not perform this same load flex. It must be built to resist load flex completely.

By shimming on another board it may not be possible to control this load flex.

I recommend the removal of the deck.
Planing and cross leveling the support joists to gain a level sub platform and reattaching the deck.
Be certain that no obstructions set above the deck surface and be certain to countersink any fasteners that attach the deck.
There are several products that can be used to disperse the deflection that might occur such as high density foam boards etc. A bit of research into what works best might be required.
A product to disperse load flex will only disperse very minor stresses and requires a very level deck surface.
Failure to prevent these stresses can cause catastrophic failure.

[Brhino]

interesting and a bit more extreme than most responses I've gotten so far. I'm not convinced that removing the deck and flattening underneath it (which would be difficult if not impossible due to the use of construction adhesive) would be substantially different than using furring strips to create a flat surface on top of it. If the stand is going to flex under weight it's going to flex... why does having or not having one more sheet of plywood change anything?

[Indian Woods Angels]

From the description of the problem the center is bowing down making each end high.

With 3000 pounds of force pressing the sheet of plywood down onto the sheet that is not uniform the over lay sheet would be difficult to keep from just forming to the same shape. This puts us back to square one on the problem.

If I were going to attempt to repair this without removal of the uneven deck:

I think I might try to use a floor leveling filler or something to fill in this low space on the deck. Then fit the next sheet on top of this. I would want to eliminate the deflection possibility.

In regards to plywood; Most plywood at the home store / lumber yard is a 3 or 5 layer of veneer type. The stuff is so distorted in the yard it is hard to ever get flat. There is a seven layer higher quality type of plywood for construction to finer tolerances such as cabinet making etc. I may try that as my deck material.