Advice? Advice on Cinder Block & I-Beam Stand

I'm going with a cinder blocks and steel I-beams (technically H-beams or wide I-beams) for my stand on an in-wall 400g. The tank is a 72x36x36 acrylic. I have 3 beams which are 72 inches long and are 6" tall x 4" wide (aka 6 x #12). The three beams will run parallel to one another supported on the ends by cinder blocks. I was originally planning on building a wooden 2x4 frame with support braces running perpendicular to the beams which would then have 2 sheets of 3/4" plywood on top of it. The tank would then sit directly on the plywood.

I'm now wondering if I actually need the 2x4 frame or if I should just lay the plywood directly on the beams. If set directly on the beams there would be 12" gaps between the beams running the width of the tank. Is that too much unsupported space? I realize the two sheets of plywood will help distribute the load but I'm still concerned.

Any thoughts from you structurally inclined people out there would be appreciated.

Thanks!
Eric

 

The beams are 6" tall and 4" wide so the space from front to back on the tank would be:
4 inch beam
12 inch gap
4 inch beam
12 inch gap
4 inch beam
Total = 36" width

I'm worried that may be too much of a gap. I don't have pics yet (I have my beams but no blocks). Tank should be delivered in a little over a month so I still have some time to get this sorted out thankfully. The 2x4 wood frame I was considering is similar to the top on your typical aquarium stand.


Obviously these aren't the right dimensions but they illustrate what I was planning.

 

Thanks. I was actually planning to use metal shims between the cinder block and H-beams.

 

Correct. I had done the deflection calcs a while back and got comfortable with these beams over that span. The cinder blocks are 8" wide so the unsupported length is only 56" with about 1250 lbs per beam. If I remember I was getting a deflection of around .006 to .009 inches.

 

Thanks all for your input. So does anyone think the 2x4 framing is necessary between the beams and plywood? I would think it would help with some of the concerns with the beams not being perfectly level under the plywood and would help minimize the unsupported gaps under the plywood. I'd love to be able to get away without this additional layer if possible but obviously I want to do it right.

 

I guess I still have one other question regarding the cinder blocks. Does anyone have any opinions on dry stacking the blocks vs dry stacking and filling the cores with concrete vs fully filled and mortared joints vs concrete/landscape adhesive? It sure seems like things would be fine dry stacked since each column of blocks will have a beam on it with a significant amount of weight on top of it.