Worried that your stand won't hold up?

[Pharaoh]

No offense intended but there are a couple of serious errors with this thread I am sorry to say.

None taken, this is a discussion and I am not the authority on engineering. I am but a mere fish nerd.

1. What you call the "weakest link" is actually the strongest part of the stand. The vertical supports are being compressed parallel to the grain not perpendicular. The compression strength is 4800 psi not 440. The horizontal frame is being compressed perpendicular to the grain. If the weight was all on the center of the horizontal frame you would not want to exceed 440 psi. Fortunately aquariums have their weight distributed evenly across the frame or on the corners so that shouldn't be an issue. However the longer of the span you have the more weight is put on the center and the more easily it can break.

Nice catch. I believe you're the first person to see that. I will have to fix these numbers and calculate this again.

2. The green boards (or something like them) are necessary as they prevent the stand from folding. While the stand has an excessive amount of support from a force load vertically, a lateral force (pushing the stand from side to side) can cause the connectors (screws, nails, glue, etc...), if they are all coming from a direction parallel to the vertical supports, to loosen and the stand can fold over. This is because all the connecting hardware between the vertical support and the horizontal frame is in the same vector (Y axis). Add in the green boards with screws coming from the back to the front and and the connecting hardware is now in a different vector (X), 90 degrees from the other hardware and this keeps the stand from folding. The boards don't have to be there to provide any vertical support but they are a convenient way to add hardware from a different vector. There are other options to doing this besides boards as well. The most common of these being metal plates that overlap the seam between the vertical supports and horizontal frame and use 3-5 screws on each side to hold the seam together.

What boards are definitely not necessary are the purple boards facing the front and back of the stand. They are not necessary for vertical support and do not add any additional vectors of support not provided by the purple boards on the side and the green boards. Although they would make the stand look better aesthetically.

Personally I prefer dado cutting the vertical supports so they are under and around the horizontal frames. This way the screws that come in from the side provide the X axis connection and there is no need for any Y axis connection.

On this point, I will agree and disagree at the same time. Yes, the green boards will provide additional support to keep the frame from folding, but the additional purple boards will do the same. You can add additional screws between the two purple boards in order to have the screw providing the X axis connection. Either way about it, you are relying on the screws to prevent horizontal movement.

I have built countless stands without the green boards and have never had a bit of movement. Of course, the entire build depends on how you attach everything together. I do not see this being a huge issue considering the actual loading and the fact that we are talking about extreme weights that most stands will never see. If the stand is under 36" tall, I doubt you will see much movement at all, even with a 300G sitting on it.

I forgot to add that the combined compression vertical support strength of this stand is calculated by measuring the square inches from the narrowest dimension that can make a perfect square. (1.5" in the case of a 2x4). This is because the failure will happen in the vector of the narrowest dimension. A good example of this is if you take a ruler and hold it so the flat part is facing up and then bend it up and down. It bends very easily. Now hold it so the edge is facing up and try to bend it. You probably can't.
In the case of our model stand the verticals have a combined support square inches (1.5" x 1.5") x 8 = 18 square inches and can support a combined weight of 86,400 lbs or 21,600 lbs per leg. Of course this is only realistic if the stand is only about a foot or so tall.

Interesting. I will research and add this as well.

Now here is where it gets complicated. The longer the legs are the more they are going to bow from the weight applied. If the legs are really long or there is a lot of weight they can bow out in the direction facing the narrowest dimension and eventually shear. This is where the second purple leg facing the front and back of the stand becomes important. This leg, if connected to the legs on the side prevents that leg from bowing by essentially adding the strength of it's longest dimension to the narrowest dimension of the side leg. Now you essentially have a leg with a footprint of 3.5" x 3.5" which will make it considerably stronger than before with little risk of bowing from even extreme weights placed on it.

The 3.5"x3.5" would be applicable only if they were side by side making in essence, a 4x4. Please correct me if my logic is off on this one.

While I agree that a wooden frame can support MUCH more weight than most people think, it's not just the static downward force that you need to worry about.

When you think about how the stand could potentially fail, it will never be simply squashed flat, unless you run over with a bulldozer or something. It will fail by either folding over or the uprights being deformed so they no longer support the full downward force.

Watch this video.
http://www.youtube.com/watch?v=EjOvI0TOx98

OK that's a bit extreme, but the shelves were holding the static load of all those cases of vodka just fine, until someone deformed one support, and then the sideways force bought it all down like a house of cards. A relatively small earthquake can have the same effect. A few ornaments bouncing off the shelf is a nuisance, a 300 gal fish tank collapsing becomes a disaster. We recently had a mag 7.1 near hear and a lot of warehouse storage made like that video clip.
Main disaster was the big beer warehouse. :eek3:

What can you do about that? Bracing!!!!

What your drawing doesn't show is the bracing the stand should have to prevent it folding up. That could be diagonal braces, steel corner pieces or probably the simplest, plywood. Generally a stand is clad in something. Make that some structural plywood and you have a something that's both going handle the static weight AND any sideways stress as well.

I think this drawing does a solid job of bracing. If you build to this design, I don't think you will have much to worry about.

Ian

Responses in red.

 

[Narwhal72]

Pharoah,
When you arrange the two boards like an "L" the shortest dimension is the short leg of the L which measures 3.5". The long leg actually measures 5" but since it is a longer dimension the support will not bend in that direction when compressed. If you were to line the boards up next to each other parallel like this "ll" then you would essentially have a 4x4 although in reality it's a 3.5 x 3. The 3"x3" dimension would be the one you measure from then.

Screwing the purple boards together would bind them together and make the vertical support better but it does not address how they are connected to the horizontal frame. With all the connectors between the purple boards and the horizontal frame entering in one plane (toenailed down from above or up from below) that is the problem. You need to have them enter from two planes in order to prevent folding. Hence the reason for the green boards or something like them.

A good example is to take a marshmallow and put a toothpick through the middle of it. You can spin the marshmallow around and slide it up and down that toothpick. Now stick a second toothpick through the marshmallow on a side perpendicular from the first toothpick. You will find that you can no longer spin or slide the marshmallow on either toothpick if both toothpicks remain in the same place.

Ian, Pharoah is correct. the stand has plenty of bracing to prevent folding. That is the function of the green boards. They prevent movement laterally (as long as they are put in with more than one screw on each end).

There is no argument that the stand is way overbuilt for aquarium use. Most DIY stands generally are.
Andy

 

[Narwhal72]

Ian,
Warehouse racking collapses because it is not braced at the bottom. Most warehouses leave off the bottom shelf so they can floor stack pallets right on the ground. Because the feet of the uprights are not braced to be held equidistant in case of movement when one end of the rack is compromised it will pull the others over. If they were to put a shelf at floor level on their uprights they would eliminate the risk of that happening.

Aquarium stands are built with a horizontal frame that connects the legs together at the bottom and prevents that from happening.
Andy

 

[DrgRcr]

I'd like to see the same calculations for load capacity done for a stand built from 3/4 plywood stood on end. Figure it for 125 gallons.

 

[ianab]

"I think this drawing does a solid job of bracing. If you build to this design, I don't think you will have much to worry about."

I don't think your design will fail either, just that plywood bracing would be stronger, and if you are going to be cladding the cabinet anyway, then it serves both a structural AND a cosmetic purpose. Using the plywood would make your bracing members redundant, AND be stronger. The strength of the bracing is related to the size of the triangle it forms. The ply, that bracing is the size of the whole stand.

Ian

 

[ianab]

Ian,
Warehouse racking collapses because it is not braced at the bottom. Most warehouses leave off the bottom shelf so they can floor stack pallets right on the ground. Because the feet of the uprights are not braced to be held equidistant in case of movement when one end of the rack is compromised it will pull the others over. If they were to put a shelf at floor level on their uprights they would eliminate the risk of that happening.

Aquarium stands are built with a horizontal frame that connects the legs together at the bottom and prevents that from happening.
Andy

The ties at the bottom will protect from one mode of failure, when the legs move apart and the whole structure splays, like a weak table with too much weight on it.

But it wont help with the whole thing twisting.

Simple experiment: Get a cardboard box, and tape the bottom and lid shut, sit it on it's side and see how much weight it can hold. It will hold a surprising amount as you need so deform that bracing cardboard and tape before it can fold up.

Now open the top and bottom so you can see though the box and try again. It folds flat with almost no force as they only thing supporting it is the minimal strength of the corner joins.

So a simple 4x2 frame can hold considerable weight as long as it's directly downward. But the only resistance to sideways force is the screws / glue / nails at the corner. Start kicking the stand, those work loose, and it's going to fail eventually. The braces in the original drawing will make it much stronger as you now have a wider and overlapping joint / brace area. Think of it like taping some strips of cardboard back to the edges of our experimental box. Likewise using more advanced joints (mortice and tenon) or steel brackets will also improve things. Probably plenty strong enough to support the tank in any normal situation.

But a properly braced structure will be stronger still. If those warehouse shelves had ply sides and backs on each bay, they wouldn't have collapsed. But that costs and takes up valuable storage space, so it's never done.

A 4x2 box clad in construction ply is almost indestructible. :hitting:

There is no argument that the stand is way overbuilt for aquarium use. Most DIY stands generally are.

Over-engineered? Probably...
Going to fail? Not in my lifetime

Ian