Steel stand tubing size?

[PotatoPig]

The entire perimeter carries the distributed load from the tank. If only the ends of the tank are supported then the front and back glass and silicone must carry and distribute that load to the ends. This puts extreme stress on the joints as well as creates significant torsional forces in the vertical panels. Think of joist blocking, without it joists under load twist.

For the tank to put loads along the long side you need strain compatibility, ie the piece you’re using to carry the load must deflect the same amount as the glass it’s carrying. This is because in order for the glass to transfer load to the frame it must be in contact with the frame. So if the frame deflects 1/16” the glass must also deflect 1/16” if it is transferring load. If it deflects less it cannot possibly transfer load, because it won’t be in contact with the frame The only way the load could be carried by the entire perimeter would be either:

1. The perimeter pieces were much more rigid than the vertical glass panel.
2. The base panel deflects independently from the vertical ones, resulting in separation of the base from the long sides.

Run the numbers for a wood frame - even if you use 2x12 pieces the vertical glass is around 10-20x more rigid than the wood that’s supposedly carrying it. By strain compatibility the wood could only be carrying 5-10% of the load.

For a steel frame:

Let’s say you want the steel beam to carry even half the load. It’s rigidity (EI) must be equal to the vertical glass panels rigidity.

Math time:

Young’s Modulus (E) for steel is 29,000ksi, for glass it’s about 10,000ksi

Say your tank is a 75 gallon. Glass is 3/8 thick and 18” deep. The I for this is about 180 in^3. For the steel to have the same EI we need it to have an I value of about 60 in^3. You’re looking at an HSS 10x2x5/16. To carry just half the load. The glass still carries half.

You want the steel to carry 90% of the load it has to be about 10x more rigid than the glass and you’re looking at something like a HSS20x4x3/8

The long sides of the tank span from end to end. This is also why the glass for a 6’ tank is thicker than in a 4’ tank the same depth, because the bending in it is greater so it needs extra thickness to reduce stresses.

If you don’t believe me check out your LFS if they have stacked tanks. The tanks will be on little angles that have no chance of being more rigid than the tank - get a piece of paper and you’ll be able to easily slip it between the angle and the tank. For bigger tanks you’ll be able to see a gap even.

/edits to correct numbers from bad phone mathing

 

[BeanAnimal]

I am on an iPad so will respond in short.

You are only accounting for rigidity in the vertical plane and that is only a piece of the picture. The math is useless if you do not apply the proper model.

You can't treat the aquarium as an ideal torsion box, it is not, even with top bracing.

Several factors from non-planar corners (even slightly) to the water pressure in the tank and seam flex and strength significantly break the ideal model you have proposed.

You can't use Young's Modulus in this context without also considering breaking point --> E x strain

You can't ignore the shear, tensile or flexural properties of the silicone used to join the panels.

The LFS example does not scale. Stress in a 20" 80 pound span != stress in a 60" 1400 pound span.

The reason for "thickness" - In an ideal model (what you proposed) 1/4" glass would work in the vertical on the longer or taller tanks. Thicker glass is used because it resists deformation against its flat plane and therefore increases overall (the desire to have a torsion box analogy) system rigidity and provides substantially more surface area for panel adhesion.

See the Red Sea failure examples - the perfectly illustrate the predicable failure mode for unsupported center spans. Seam separation in the bottom middle and 1/3 of the way up the side panels.

My wish here is not to get into a physics debate, but I strongly disagree with your understanding of the forces at play here.

 

[DDenny]

Rec

I am upgrading to a 60x24 150 gallon tank in the coming months. I have no issues building a wood stand but I'm considering having a buddy (he's a professional welder) make a steel stand for me. I would like to have a stand without a center brace. I know I can accomplish this with 2x6 top rails if using wood. What size tubing would I need in steel to accomplish this?

Thanks.

Rectangular tubing mine be a little stronger. If you buddy a welder have you asked him? Get the approximate total weight and go from there. For me I would still put the bracing but that is just me.

 

[BeanAnimal]

Rec

Rectangular tubing mine be a little stronger. If you buddy a welder have you asked him? Get the approximate total weight and go from there. For me I would still put the bracing but that is just me.

For the 60" span, corner bracing (depending on height) is far more important. 1800 pounds has a lot of potential energy that when bumped or shaken can turn into a lot of kinetic energy imparted to a very small area (the vertical welds at the corners). Even if the stand does not fail, it may be very kinetic (wiggly) with very little force.

 

[RedoubtReef]

Have you considered using extruded aluminum? Don't know if that is cheaper than what your buddy can build but might be an option if so.

 

[BeanAnimal]

Have you considered using extruded aluminum? Don't know if that is cheaper than what your buddy can build but might be an option if so.

One (or more) of the very popular aluminum profile aquarium stand builders uses Zinc plated fasteners. Anodized aluminum does resist corrosion but is not immune, especially where the hard out anodization is scratched or pitted and/or drilled/milled after anodizing.

Not a deal breaker - just something to be very aware of. I would at the very least insist on 316 stainless hardware (304 at the minimum) and treat any exposed aluminum (where anodizing is breached) to ensure a long stand life.

 

[PotatoPig]

My wish here is not to get into a physics debate, but I strongly disagree with your understanding of the forces at play here.

I think you may be…

Getting out of the numbers: I would hope we would agree that the only way the tank could load frame under the long side is if it is in contact? Yes? No contact and it is, I hope we agree, impossible to transfer force through an air gap. Now - if the frame is loaded it will deflect? We seem to agree on this? So - if the frame deflects the only way the tank can continue to load it is if the tank deflects along it’s span. If it doesn’t it physically cannot load the frame. It’s impossible- it won’t be in contact.

If your calculations show the frame deflecting (and I assume they do) then the tank has to deflect the same amount if it is transferring load along the span. No more. No less.
This means either the vertical panel is deflecting or the base is separating from it.

If the vertical pane isn’t deflecting vertically the same as the frame and the base isn’t separating then I hope we can agree you have a gap between the tank and the frame, and the glass is spanning from one end to the other.

 

[PotatoPig]

Post in thread 'DIY Stand - how perfectly flat must the stand be???'
https://www.reef2reef.com/threads/diy-stand-how-perfectly-flat-must-the-stand-be.861459/post-9401240
And a pic of a 150 gallon tank at my LFS. Not obvious from the photo but there’s a gap between the tank and the little angle under the entire length.

 

[X-37B]

My 120 is 2" bottom and sides
Top is 2"x3". Done this way no center brace will be needed on your 150g.
It was 1/8" tubing.

 

[BeanAnimal]

I think you may be…

I am a sucker for misunderstood physics or engineering - even if very well intentioned, especially when part of the answer is right

Getting out of the numbers: I would hope we would agree that the only way the tank could load frame under the long side is if it is in contact? Yes? No contact and it is, I hope we agree, impossible to transfer force through an air gap.

Of course.

Now - if the frame is loaded it will deflect? We seem to agree on this? So - if the frame deflects the only way the tank can continue to load it is if the tank deflects along it’s span. If it doesn’t it physically cannot load the frame. It’s impossible- it won’t be in contact.

Agreed. There are two general scenarios here.

1 - the stand bows due to indirect force from the weight of the tank and/or deterioration. Unlikely in the steel stand scenario, common with wooden stands. Again, the are numerous torsional forces at play here. In any case, the result would be leaving the tank in suspension with the bottom portion of the front and back panels under tension and the top compression. Those forces (due to the long vertical and short horizontal axis) will want to buckle the front and rear panels (torsion forces).

2 - the stand bows under the force of the sagging aquarium. The same forces as above apply (compression and tensile), but the net effect is the same. Torsional forces due to the imbalance wanting to buckle the panels.

Both instances also expose the bottom edge to significant tensile forces. However, the common failure here is usually the seam before the glass and possibly the glass following.

The very idea with a ridged perimeter support is to transfer this load to the stand and away from the panels, allowing the panels to work more in a compressive mode vertically and removing those extra forces from the seams.

 

[BeanAnimal]

Post in thread 'DIY Stand - how perfectly flat must the stand be???'
https://www.reef2reef.com/threads/diy-stand-how-perfectly-flat-must-the-stand-be.861459/post-9401240
And a pic of a 150 gallon tank at my LFS. Not obvious from the photo but there’s a gap between the tank and the little angle under the entire length.

Ohh we can find plenty of examples of tanks spanning large gaps. That is not in question, but rather the margin of safety is. We can also find plenty of examples of tanks that have failed in the same configuration.

 

[PotatoPig]

The very idea with a ridged perimeter support is to transfer this load to the stand and away from the panels, allowing the panels to work more in a compressive mode vertically and removing those extra forces from the seams.

Taking a step back - under your rigid perimeter assumption for your frame, about how much calculated deflection do you get in your frame at mid span under the tank loads?

 

[BeanAnimal]

Taking a step back - under your rigid perimeter assumption for your frame, about how much calculated deflection do you get in your frame at mid span under the tank loads?

Yard work today and posting from the ipad during iced tea breaks.

if designing this for clear span I would likely design for .05” or less deflection at midspan using a uniform load with fixed ends (assuming a gusseted design). Going to guess for a 60” span 2x2 .125“ A500 or A513 tubing would come close. For sanity, I would run the calculation with floating ends as well.

~2000 pounds design weight, 1/2 of it spread over each long member, ignoring load on short members to keep things simple and build on safety margin.

The same span with wood 2x6 (guessing) would be in the .05 to .08 range to meet l/720 200 pound live load.

 

[Amphibious Wallet]

Id pass on it myself… we’re talking over a thousand pounds sitting on it. I don’t like that thin wall material myself for anything structural but people do use it…. It reminds me of a person standing on a beer can! You can take a 200 pound person and stand on top of a beer can but the first time you move a hair or flick the can it will crumble under your foot. If it was a low boy frag tank or something sure but a large display like you’re talking is risky.

Yeah was a bit disappointed when I saw the wall thickness but my innate goblin whispered 'Yeah but free!'.
Thanks for your opinion!

 

[BeanAnimal]

It would likely hold with some gussets. The problem is welding thinwall like #16 and not creating weak spots that will tear Under stress. By the time reinforce and take the time to lay down nice well, it would be easier to start fresh with the proper steel.

I am not much of a welder and would not want to deal with structural welds in thin wall. Other people may be more comfortable…