BeanAnimal
Premium Member
Heck a 2x2 in each corner would easily support the vertical load... they just would not have much lateral stability 
DIY Stands Template and Calculator
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Siffy
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<a href=showthread.php?s=&postid=10436759#post10436759 target=_blank>Originally posted</a> by BeanAnimal
Heck a 2x2 in each corner would easily support the vertical load... they just would not have much lateral stability
They wouldn't have to if the back and sides were skinned with even 1/4" plywood. The only problem with 2x2s is mounting the top rails in a position that screws aren't load bearing. A couple 3/8" SS carriage bolts per corner should suffice, but that would completely change the original design.
Hmm, RocketEngineer, how do the deflection properties of a spanning member relate to thickness? IE, 2x4 vs 1x4 or 1x6 etc? I think I read a factor of 3, but I'm not certain. The goal is produce a competently stable yet lightweight stand right?
The last stand I saw a friend build was 5 layers thick when looking at it from the bottom. It was so heavy I made sure I was out of town when time came to carry it upstairs where his new 4' 120g was going.
RocketEngineer
Space is big.
If you had two boards of the same height and length but one half the thickness of the other, the beam with half the thickness would be half the strength. If you increase the height, (a 2X4 vs a 2X6) you increase the strength as a function of the height cubed (3.5^3= 42.875 vs 5.5^3=166.375). So increasing the board height is the best way to make the long spans stronger and reduce deflection.
RocketEngineer
Space is big.
Ok folks, I ran a couple numbers last night. One 2X4 on end, 24 inches long, will support 19000+ pounds before it deflects sideways enough to fail.
Oh and BUMP.
Oh and BUMP.
flyguy7150
New member
Rocket I got a few questions.... First, I will be building a stand for a 90gallon, 48"L x 18" w x 30"h. For this everything according to this design will be 2x4s. The green pieces should be one inch off the ground and one inch from the top right?? The top frame and the bottom frame, how are they attached to the purple legs?? Is everything just screwed down to the green 2x4s or really long screws from the top and bottom frame to the purple pieces or both?? Everything is bolted down with just wood screws right, no brackets?? Would adding one more leg to the back middle do any harm? What size wood screws do you use?? Sorry for all the questions .
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Siffy
New member
Yep, everything is just screwed into the green strips. 2 1/2" to 3" screws will work. They are, in a sense, brackets. They could be cut to a few inches in length and still hold things in place until you attach plywood to the whole thing. Adding another leg won't do harm, but it's not necessary at all. The only harm would be getting in the way at some point.
flyguy7150
New member
ok so no need to put like 5inch screws from the top of the top frame to the legs, thank you
RocketEngineer
Space is big.
slevesque - depends on whether you are looking at it sitting down or standing up. I don't have exact numbers but others on RC may have some suggestions. For a tank that short though, you may be able to reduce the lumber sizes a little. What size tank is it?
flyguy7150 - I think Siffy answered all you questions.
Siffy - Thanks for answering all of flyguy7150's questions.
Charlutz - The reason the screw strips are shorter is that it prevents them from being part of the load bearing structure. Because they sit above the bottom frame you are assured its the frame sitting on the floor, not the screw strips. They are below the top surface to ensure that plywood or the tank frame sits directly on the upper box, rather then on the screw strips.
checkinhawk - Thanks for the support.
flyguy7150 - I think Siffy answered all you questions.
Siffy - Thanks for answering all of flyguy7150's questions.
Charlutz - The reason the screw strips are shorter is that it prevents them from being part of the load bearing structure. Because they sit above the bottom frame you are assured its the frame sitting on the floor, not the screw strips. They are below the top surface to ensure that plywood or the tank frame sits directly on the upper box, rather then on the screw strips.
checkinhawk - Thanks for the support.
Ah. I see. So by making them shorter, you ensure the screws aren't load bearing. Otherwise, you run the risk that the actual load bearing posts are too short and aren't actually supporting the weight. I understand the principle. I found it easier to build the stands with the fastening posts the same height. I won't have the need to build another stand until probably February. I'll play with it then.
Siffy
New member
Yep, you just don't want them (the screw strips/green pieces) touching the floor when the tank weight is on them. A small piece of 3/4" plywood or 1xwhatever will shim/hold them up while screwing into them and fall off once the stand is picked up. I might suggest cutting the screw strips an inch longer if you're only able to shim them 1/2" off the floor instead of a full inch so there is plenty of material to screw the top frame into place.
Mykayel
New member
This is overall a good post and good basic concept as homebuilt stands are always way overkill. And while I completely agree that loading the stand directly though the wood is better than putting the screws in shear, doing so won't be a problem at all. As there isn't a standard for a typical #8 screw, from one manufacturer, the ultimate strenght of a screw is 361 lbs in shear. If you have a 4 screw connection then that connection's good for 1444 lbs, more than you are going to put on it with any tank (of corse you'd have to check the wood for the connection strenght too but my point is screwing to the side is more than adequate).
And for those that are concerned about deflection you don't need to worry about a thing as all of the weight from your tank will be sitting at the corners. Think of it this way, in order for that top box to deflect, you would have to be putting load on it in the middle somewhere. And in order for that to happen, the glass (including the sides) from your tank would have to deflect to apply that load to the wood.
The sides of your tank are not going to deflect. Looking at the side piece of a standard 75g tank, 21"x48"x3/8 with a load of 10 lbs/ in (that would be a total load on the tank of 960 lbs which would be a conservative estimate) (and assumeing a Youngs modulus of 72GPa http://www.roymech.co.uk/Useful_Tables/Matter/Glass.html) you would get a deflection of 0.00123" barely over a thousandth of an inch, not enough to load that wooden beam.
You would have have to apply a load of 125 lbs/in (a total tank load of 12000 lbs) to get it to deflect 1/64 of an inch, and this load also corresponds to the point that that side piece would break in flexure (and the bottom would have gone way before that, but I'm trying to make a point). Even at 1/64 of an inch you would bairly begin to load that wood beam.
So, in summary, the wooden beam at the top does verturally nothing; it holds the 4 columns at the corners in place providing a little bit of stability.
And how high can you make your stand... if you are worried about tipping it over try this: It would be very difficult for a single person to push or pull 100 lbs without actually sliding on the carpet or wood or whatever first. With that assumption, use this equation as to how high the mid hight of your tank can be:
W= Weight of tank (use a conservative value of 9 lbs/ gallon which would include the weight of the tank itself and your rock and sand)
b = the width of your stand from front to back
h = hight to the middle of your tank
Equation:
h = (1/100)*W*(b/2)
So using that 75 gallon tank example:
W= 75*9 = 675
h = (1/100)*675*(18"/2)=60.75"
Moral of this story, you can build your stand just about however high you want without worries.
I don't think it's the shear of the screw that is the main problem. It's the shear strength of the wood which is low. All of the pressure of the tank will be concentrated on the screw. My concern is that it would be enough to rip the wood.
Doesn't this depend somewhat on the height of the stand? If the stand is taller, the posts will lean in under weight.
Taller stands will rock more easily due to leverage. It's the same as using a prybar. It's much easier to move a tank on a tall stand. Add in the fact that it will be top heavy, making it that much easier. Not saying the tank needs to lie on the ground, but you do need to consider height in the build.
RocketEngineer
Space is big.
Mykayel, Thanks for you comments.
But I don't agree on some points.
1) Screws in wood are likely to tear out sideways due to the shear forces on them. As the screws are in single shear, the wood is likely to fail before the screw itself. I for one would never trust a screw to take shear loading and transfer it from one piece of wood to another but this is just me.
2) You made the assumption that the tank was glass and with some simple additions such as a plywood top, this design could be used for an acrylic tank. In this case deflection IS an issue and needs to be accounted for.
3) I would like to know what that formula pertains to and what all the variables that make up that 1/100 factor come from. I have never seen it before so would like more info on it.
Thanks for the comments everyone.
But I don't agree on some points.
1) Screws in wood are likely to tear out sideways due to the shear forces on them. As the screws are in single shear, the wood is likely to fail before the screw itself. I for one would never trust a screw to take shear loading and transfer it from one piece of wood to another but this is just me.
2) You made the assumption that the tank was glass and with some simple additions such as a plywood top, this design could be used for an acrylic tank. In this case deflection IS an issue and needs to be accounted for.
3) I would like to know what that formula pertains to and what all the variables that make up that 1/100 factor come from. I have never seen it before so would like more info on it.
Thanks for the comments everyone.
Siffy
New member
It's also making the assumption the glass is tempered, isn't it?
I think the 100 comes from this statement.
I believe he's just comparing simplified moments. f1 * d1 = f2 * d2 where f1 is the weight of the tank, d1 is the width of the tank, f2 is amount of force one can push before sliding on carpet, and solve for d2 which is where that force can be applied (ie, middle of tank height). It's not exact, but gives a ballpark.
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