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80/20 Stand - can it hold 120g with no front middle brace?

BTBarney

BTBarney

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Hello BAR,

I think the title of this post sums it up pretty well.... I'm a biologist, not an engineer, so I have no experience with this. I want to build a stand for my 120g (6' x 21" x 21" roughly...), and I would like to have no vertical brace in the front middle to disrupt access to the sump et al. I think the tank should end up weighing, what, 1500 - 2000 lbs?

Anyone with experience in 80/20 who has an idea of the weight bearing capacity of these for stand design?

Thanks!
-Bryan
 
Yes it does, but I was referring to the original post. And yes even with the calculator you do need to take into account how large you're willing to go to bypass a center brace. I looked at some 1" x 2" material (it was what I saw was available at Grainger) for a 36" long stand, and the deflection was less than 2mm which I would think would be acceptable, but who knows.
 
I looked into this when I was researching stands for the 190. I spec'd 2x4 (maybe it was 1x4) and deflection was very little as well.


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Thanks for the point to the calculator. I have a follow-up question if people are familiar with this problem:

Using 2040 (fractional, effectively a 2x4), it looks like a 72" run supporting 2500 lbs has different deflections when using either fixed two ends or supported two ends. I'm assuming I should be looking at the fixed two ends (the ends of the 72" run will be attached to something, namely the rest of the frame) and 'load evenly distributed.' With this configuration, I see a 1.73mm deflection using the 80/20 in the 'vertical' configuration and ~5.8mm deflection with it in the 'horizontal.' My question is, related to sfsuphysics comment above, how much deflection downwards is too much deflection?
 
Follow up to my follow up - have I made wrong assumptions in the calculator? all 2500 lbs will not be loaded onto this one run - the tank will weigh about that much, and be supported by all 4 peices of the top frame, not just this on 72" unsupported run.....
 
I thought fixed ends was basically having "legs" attach to the sides of the top brace (something you don't want) versus supported ends which was legs under the top brace, but I don't know what some of their terminology is.

For that configuration you definitely want vertical though, it's like stepping on a 2 by 4 in the short direction versus the long, there's a reason why they put floor joists the direction they do under your floor.

Now how much is too much... well that's the 64 dollar question, and unfortunately I couldn't give you a good answer. Deflection is all about how much of the center of the tank will be supported, but the tank does have that to take into account as well. And it boggles my mind how you can calculate a deflection without knowing the rigidity of the material you're first putting on it.
 
Hmm maybe I have that mixed up then, that fixed ends means fixed under, and supported are supported by an angle bolt attachment? Like I said, I'm not sure what exactly the definitions are.

A quick google shows an old RC thread talking about steel stands, and it seems that less than 3mm is a must, but preferably smaller is better.

Edit: ok looking at the picture I was right, although some how you get less deflection by attaching it to the ends, which I can see I guess, but I still would be more comfortable from having supported ends, maybe it takes the supported bottoms as not being fixed to the actual joist.
 
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Mark B said:
Could you not also run multiple 72" lengths under the tank instead of only supporting the tank around the perimeter?
Click to expand...
I've thought the same thing but I don't know if that actually works. I've been told it's only the perimeter that holds the weight of a glass tank and I've seen plenty of stands that are open on top to confirm that.
 
Flagg37 said:
I've thought the same thing but I don't know if that actually works. I've been told it's only the perimeter that holds the weight of a glass tank and I've seen plenty of stands that are open on top to confirm that.
Click to expand...
Are we talking about a frameless eurobraced tank?
 
In this particular case, this tank is not eurobaced. It has the typical black plastic rim you see on all cheap tanks, with cross braces every 24". Therefore the tank is basically in three 24" segments, held in place by one molded plastic rim on top. I hope that makes sense....

I would think a sheet of ply would distribute the load such that at least a portion would be held by the other three top 80/20 pieces. I could probably fix the ply to the top of the frame and screw it into the t-rail somehow (I need to look at fittings now I suppose), and I imagine that would reduce downward deflection too.

Getting to the root of the issue, is it crazy to want unfettered access to the sump area? I may be overreacting, as my current stand has a giant support in place, and the glass tank DIY sump currently installed is not going to come back out ever again (at least, not in one recoverable piece). I do have the tendency to overengineer my ideas, so many be I'm solving the wrong problem
 
Another thing to think about, is having the access on one side, can have some load distribution by having that center brace on the other side.

As far as the sump, throwing it through the sides is an option too, you don't need super thick legs for a tank. That is of course if you have a side access and it's not installed into an alcove
 
Here is a table for commercial grade woods (2x4's etc). Modulus of Elasticity for structural hardwood (2 x 4 - 4 x 4) is 1.9M psi. http://www.engineersedge.com/lumber.htm The Modulus of Elasticity for the 1020 profile of 8020 (1" x 2") is 10.2M psi.

Steven Engineering is a local distributor (S. San Francisco) for 8020 and I am sure they can help you with the math. I made my light stand out of black anodized 1020 profile and had all the pieces cut to length, so I didn't order through Grainger. Highly recommend getting everything cut to length by the factory.
 
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