Leg Strength


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So I'm planning on making a few pieces for my bedroom. I'd like them to match, but one is a stand for an aquarium, meaning it needs to support around 400-500lbs. I'm thinking of legs similar to this here for the pieces. 

 

If the legs are 3/4 walnut, would that be sufficient for supporting the weight of the aquarium? Or are there ways to get around the issue, like the actual legs hidden behind the smaller prettier ones?

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It depends how long they are.  I would think of what you show in the picture as a foot that happens to be continuous from the cabinet.  If it's all tied together well at various points it can't splay apart from the weight.  If it's just spindly legs that aren't tied together they are not going to be happy.  It's not going to hurt anything (other than your wallet) to make them out of 8/4 to be safe though.  You can always taper them down to 3/4" at the bottom to make it appear more delicate.

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Whole thing is going to be about 40" tall, about 32" wide, and 16"-18" deep

 

For tying them together I'll have tenons across the top and bottom on the front of the stand, unfortunately utility dictates that the front will be two cabinet doors with no obstructions. 

 

Front to back I'm thinking I'll do some veneered 1/2" ply in grooves cut into the legs. I can also add some cross pieces on the inside, since its going to be kinda ugly in there anyway by virtue of plumbing, equipment, and chemicals.

 

And from side to side on the back I'll do the same as the sides.

 

The top is going to be solid walnut, and underneath that I'll add several cross supports.

 

Starting with 8/4 stock and adding a taper to the thickness will work nicely I think. Does this all sound like It'll survive? Also since I'm sure it matters for the top, the bulk of the weight (water and salt), is going to be spread somewhat evenly across two tanks, one on the top and one on the bottom shelf on the inside.

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Granted this is references hickory, which is much stronger, but...

 

 

Wood is very strong in compression parallel to the grain. In fact, a hickory chair with 1.25" square legs could support a load of 32,430 lbs (I take this example from Bruce Hoadley's fine book "Understanding Wood", pg. 112). In fact, a chair with only 1/8" thick legs, if they could be supported so as not to buckle when in compression, would easily support a 250lb sitting person. Therefore, a piece of wood which has to bear loads parallel to grain either needs to be supported in such a way as to not deflect under load, or, as is more often the case, needs to be 'over-sized' (in relation to it's strength in compression parallel to grain) by some margin so as to adequately resist bending.

 

(via http://thecarpentryway.blogspot.com/2009/10/bracing-situation-ii-tension-and.html)

 

I'm sure you can find and calculate the relative strength of walnut compared to hickory, but I'd be willing to bet it's not the thickness of the legs relative to the vertical load, but the strength of the joinery so the legs don't wobble/splay.

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I'm a little concerned that the splay of the legs will create a force pushing the legs apart.  I agree that that the legs should be fine in compression, but I'd be careful to tie the legs together towards the bottom to make sure that they don't spread.

 

Would pieces going across all sides and joined with tenons be sufficient?

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I have built several aquarium stands over the years. I would go with 8/4 legs and leave them full thickness at the floor. Remember that every aquarium set up will end up leaking or experiencing spills at some time. You could line the inside of the base cabinet with shower pan liner plastic or some sort of serious waterproofing.

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