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Table legs - Strength

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Forum topic by Stephen_Adelaide posted 476 days ago 978 views 0 times favorited 6 replies Add to Favorites Watch
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Stephen_Adelaide

4 posts in 478 days


476 days ago

Hi,

I’m building a coffee table. It is a laminated slab (very strong – think workbench lamination) and I’m going to attach some legs on an angle (eg. 85 degrees). I’m wondering how strong the attachment to the apron needs to be. I tried thinking about the leg as a simple lever but I’m not sure that it is as basic as that. Eg. A leg set at 45 degrees is weaker than a leg (in my mind) than one set at 85 degrees. This suggests to me that this isn’t just an issue of leverage with the joint unaffected by anything other than simple leverage.

So, what is going on and how are the forces interacting? Anybody? Or is it a simple lever?

My thoughts at the moment are that it is obviously a lever but leg is some how anchoring to the floor more solidly with a 85 degree leg than a 45 degree leg through friction. But, I really don’t know.

Thanks,
Stephen


6 replies so far

View pjones46's profile

pjones46

208 posts in 1248 days


#1 posted 476 days ago

The force on the leg is actually the resulting force of one directly perpendicular to the attachment point on the table toward the floor and one at right angle to the perpendicular force. In other words the resulting force on the leg is the hypotenuse of a right triangle of these two forces. They call this if my memory is correct, vector analysis of forces but over and above that I think you need a mechanical or architectural engineer to give you some guidance. However, the 85 degree leg from the floor will be able to support greater weight.

-- God is great, the Beer is good and people are Crazy. www.pauljoneswoodworks.com

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Stephen_Adelaide

4 posts in 478 days


#2 posted 476 days ago

Thanks. That is the type of answer that I was looking for.

View crank49's profile

crank49

3343 posts in 1576 days


#3 posted 476 days ago

From a mechanical point of view pjones is exactly correct.
From a woodworking point of view, it sounds like you are attaching, or are planning to attach, the legs to the aprons for support. I would not expect that to be sufficient for a heavy slab top..
The leg should be attached to the slab with a mortise and tennon joint. Then the aprons just serve to stiffen the connection and ties the legs together. But the primary load is carried by the shoulders around the tennons.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Lee Barker's profile

Lee Barker

2163 posts in 1455 days


#4 posted 475 days ago

I’m loving this.

As I am attempting to visualize this I am thinking that the mortise being at right angles to the surface of the top is stronger than one that would be at the 5o angle. Theoretically. Am I right, or is that irrelevant.

I hope I’m not hijacking what is a noble and erudite thread.

Kindly,

Lee

-- "...in his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

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pjones46

208 posts in 1248 days


#5 posted 475 days ago

Correct, Lee. Maximum reduction of shear forces on the tennon as long as the leg is also at right angle to the top.

pj

-- God is great, the Beer is good and people are Crazy. www.pauljoneswoodworks.com

View bondogaposis's profile

bondogaposis

2448 posts in 956 days


#6 posted 475 days ago

I think that obviously a splayed leg is going to have both tension and compression forces working on it where as a right angle leg is going to be a pure compression force. The splayed leg configuration is going to be more stable due to it’s wider foot print. The question for joinery is, how strong is strong enough? Think of the use, a coffee table; unless you plan on dancing on it, doesn’t need to support much beyond itself. Therefore ordinary mortise and tenon joints for an apron to legs is going to be more than adequate. Simple buttons to attach the top to the apron will be sufficient there as well. The size of the timbers used to make the legs and apron are going to weigh in too, but if they are proportional to the top they are going to be plenty strong enough.

-- Bondo Gaposis

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