Calculator for 2x4 strength for stands?

[Charlutz]

Hey guys -

I'm new to the forum and haven't had a reef tank in years. I joined mostly for the DIY forum because I do a lot of DIY for my cichlid tanks. Specifically, you guys have a great archive of DIY for bulkheads and plumbing.

My question is about building stands. How far of a span can you have between vertical posts on the long side of your stands? Is there a source with a definitive answer. I've searched the net, but most people guess at it.

Here's some further context. I use a simple 2x4 frame that I either leave open or for tanks out in the open will cover with plywood and make into a cabinet. My stand design involves doubling the 2x4's so that the long rails on which the tank rests are sitting on the wood. The purpose is so that no weight whatsoever is being supported by the fasteners. The screws merely hold everything in place. Here is a pic on an open stand for two 40g breeders I did. Please ignore the dust and dirty tanks. These are fry growout tanks in a storage room.



Anyway, if I had to do it again, I would have brought the short side rails so that they were resting on the vertical posts. In this design, because I put on a flat board on top of the rails, it wasn't a big deal. I point out that flaw though.

I have used this design successfully on a 7x2x2' tank. There is one center post on the front so that the 7' rails are supported in the middle, leaving open 3.5' gaps. The back has 2 posts so the gaps are even smaller. But how long can that gap be? I wouldn't do more than 48" with most tanks, but is there a way to calculate it based on tank weight? I suspect if you used 2x6's you could get even more strength, or 2x8's, etc. but have no facts on which to rely.

[MarkS]

If I remember correctly, the maximum vertical load for a single 2x2 was between 1.5 and 2 TONS. I'm not so sure about horizontal loads though. The load is, of course, greater for larger dimensions of lumber. I think that in long spans, the weight is transfered to the supports at the ends since they are load bearing. You'll probably start to see some bowing on excessively long spans (over 8' say), but that can be alleviated by doubling the 2x4s. A vertical support half way is always a safe bet, but most DIY stands are WAY over-engineered. If you look at a store bought stand, it is made up of strips of 3/4" particle board stapled to sheets of particle board. These stands are designed by engineers and will hold thousands of pounds for many years.

[RichConley]

You've got doubled up 2x4s on a 40g?


My 58g is all 1x2s, and plywood, with no doubling up, and its probably still overbuilt.

"but most DIY stands are WAY over-engineered"

That I disagree with. Being overbuilt and overengineered are two seperate things. I'd argue most DIY stands are overbuilt to compensate for being underengineered.

[jimbo78]

here is a like for DIY tank stands. just put in the size of the tank and it gives you a plan you the stand.

[RichConley]

Originally posted by jimbo78
here is a like for DIY tank stands. just put in the size of the tank and it gives you a plan you the stand.

A pretty absurd plan. Its telling me I need 4x4s for a 50g tank. Thats ludicrous.

[NO1B4ME202]

Originally posted by RichConley
A pretty absurd plan. Its telling me I need 4x4s for a 50g tank. Thats ludicrous.

LOL..

[Charlutz]

Thanks for the replies so far. Keep em coming. The concern is the bowing of the long horizontal rails under the tank. Sure, they will support the weight without breaking, but if they bow, they put a great deal of torque on the glass and that's obviously bad. I know I can build a stand for any tank, but I still haven't found anything that authoritatively states what the load limits are.

[jimbo78]

i was just passing on a like someone showed me. don't shoot the messenger.

[silverwolf72]

I think all it uses are 4x4 lumber mines 120gal same answer you got

[Charlutz]

Originally posted by RichConley
You've got doubled up 2x4s on a 40g?


My 58g is all 1x2s, and plywood, with no doubling up, and its probably still overbuilt.

"but most DIY stands are WAY over-engineered"

That I disagree with. Being overbuilt and overengineered are two seperate things. I'd argue most DIY stands are overbuilt to compensate for being underengineered.

That stand was originally a 6 footer, with the two 40g on top, and a 20g and two 10gs on the bottom. When we finished our basement, I lost tank space so I tore down the smaller tanks. I kept the two 40gs and cut the stand in half. Even still, I always use 2x4's. Not sure how much cheaper a 1x2 would be, but 2x4's are about $2.25. That stand is probably 7 or 8 2x4's, so it's cheap and I didn't have to make any notch cuts so what I spent extra in wood saved me time.

I agree about the distinction between overengineering and overbuilt.

[david86camaro]

If you can do a search, I know about 2 years ago when I was building my stand someone had a thread that had listed the shear and compressive strength of different woods.

[sjm817]

My 180G stand doesn't have a single 2x anything in it. Neither did my 90G.

[hyperfocal]

If I'm reading this table correctly, a stand having 4 2x4" legs can support a maximum of around 14,000 lbs (32 in^2 of loadbearing surface * ~450 psi compressive strength) -- a 1400gal tank using the 10lbs/gal rule of thumb. 1x1" lumber as legs would support a max 1800 lbs, or a 180 gal tank. Of course, I'm probably not reading it right

[RichConley]

Originally posted by hyperfocal
If I'm reading this table correctly, a stand having 4 2x4" legs can support a maximum of around 14,000 lbs (32 in^2 of loadbearing surface * ~450 psi compressive strength) -- a 1400gal tank using the 10lbs/gal rule of thumb. 1x1" lumber as legs would support a max 1800 lbs, or a 180 gal tank. Of course, I'm probably not reading it right

No, you are.

Which is why its so absurd to see a stand made of doubled up 2x4s for a 40g.

[JVJordan]

According to that table my stand for a 90 gal can hold somewhere between 47,000 and 470,000 lbs depending on the wood grain. Thats with twelve 2"x4"s as legs (6 legs, doubled up pieces). I think I over did it.

[Charlutz]

When you build a stand, is 100% of the load transferred to the legs? I'm not sure that it is, but I am not an engineer. I know that by resting the spans on the vertical posts, most of the weight is transferred there, but the span still bears weight, doesn't it? I'm not concerned about the wood collapsing or compressing. The issue is how much of a span can you have without a center brace. If that span sags a tenth of an inch, you torque the glass.

I've done some searching and reading and the calculation isn't easy. Now that I've thought about it some more, there are additional variables such as the height of the stand that don't make for an easy calculation.

[jma1978]

Building something that will simply "support the weight" is very simple. The problem comes with the fact the the tank is full of water. When you move stuff around, it starts putting force in a direction other than straight down. If you have earthquakes in your area, then an earthquake can put huge stresses on the stand. Buckling under the weight is probably never going to happen, so what you need to plan for is can the corners withstand the torque when a lateral force is applied? Kinda like when you lean back in a cheap chair and the legs break. It will withstand the weight pushing straight down; just don't change the angle of applied force... If the tank is a glass bottom with plastic trim, then you don't need to worry about the middle; just the edges. Triangle joints on the corners are one of the strongest joints and are easy to make. Then you just need some supports along the edge, and you're good to go!

But wood is cheap, so why take the chance? That's why everybody overbuilds their stands, I think.

[hyperfocal]

Building a stand out of 2x4's doesn't guaranteed lateral stability -- a poorly designed stand built out of 4x4's could be more unstable and vulnerable to shearing forces than a well-designed stand made from 1x1's.

[RocketEngineer]

Charlutz, PM me with the specks and I will be happy to run the numbers for shear failure, column buckling, and beam bending for you. I have excel calculators for these that make doing actual engineering work faster and less guessing. I agree that folks on this forum tend to overbuild DIY projects but at the same time, we can't always account for someone bumping into the tank or chasing that little fish around the tank with a net.

To those who look at those tables and think they can figure out what a 2X4 on end will take, please get a good structual engineering book and do some research. Otherwise please leave these kinds of calculations to those who do it for a living.

Hyperfocal, good point, garbage in, garbage out.

[RichConley]

Originally posted by hyperfocal
Building a stand out of 2x4's doesn't guaranteed lateral stability -- a poorly designed stand built out of 4x4's could be more unstable and vulnerable to shearing forces than a well-designed stand made from 1x1's.

Thats the whole point here. People tend to use bigger wood instead of designing the stand properly. A well built plywood stand is much stronger then the poorly built crap with 2x4s that people are pulling off of GARF, etc.

[Blackacid]

Instead of talking down our noses about the inferiority of the 2x4/4x4 designs... How about some links to superior designs? When I was searching on the subject most of, if not all, of the designs I encountered were shown using 2x4/4x4 construction. When I built my first stand... I used 2x4's. I'm sure a well built 3/4" plywood stand would a better alternative.

I never encountered plans for such a stand and my woodworking knowledge is rather limited. I'm going to guess that I'm not the only one in that boat.

[zxknight]

This is the stand I am working on right now and it’s the largest one I have ever done. The span in the front is about 44 inches now I wonder if it's too wide for the 90g.

[woz9683]

zxknight, is the tank glass or acrylic? If it's glass, the span won't be a problem. If it's acrylic you might have some issues.

[MarkS]

Originally posted by RichConley

"but most DIY stands are WAY over-engineered"

That I disagree with. Being overbuilt and overengineered are two seperate things. I'd argue most DIY stands are overbuilt to compensate for being underengineered.

I had just come home from work after more than 12 hours. What can I say?

[rsw686]

Originally posted by zxknight
This is the stand I am working on right now and it’s the largest one I have ever done. The span in the front is about 44 inches now I wonder if it's too wide for the 90g.

Its not too wide. I bult my 90 gal stand with no center supports. Heres one of the progress pics. I ended up not using the foam as it wasn't needed. My main goal was to not have the screws holding the weight but to have the weight transfered between the woods pieces. I've had the tank up and running for a week now. The stand is real sturdy and doesn't sway at all.