table leg design......

cabellos

Suppose i want to make 4 table legs from aluminium square tube.......how should I go about designing the dimensions of the square tube? The table itself needs to support a load of 90kg through the four legs.

Help on this would be very much appreciated.

Cabellos

Danger

The tubing sales people, or at least the manufacturer, can tell you what sort of forces various ones can handle. Then just divide the total weight of the table by 4 to see what size you need to handle it. I'd add at least 50% to the estimated table weight as a safety factor and to compensate for any weakness introduced through your work on it, such as mounting holes. It will also have to stand up to lateral forces, particularly at the attachment points, in case the table gets pushed sideways.

FredGarvin

Metal product manufacturers are most likely not going to tell you anything other than material allowables. They won't tell you anything in regards to how much of a load something will allow. That is the designer's job to calculate.

You need to determine the load distribution and the resultant forces on each leg. You need to then calculate the stresses induced based on an initial guess of what the cross section will look like. These will be combined stresses that include the bending moments as well as compressive stresses and buckling. Compare the calculated stresses to your desired failure criteria and revise the design until the stresses are below the desired levels.

AlephZero

I would also add about 600 lb, to handle the case when a 300lb man stands on top of the table to change a light bulb or whatever (double the weight of the man to allow for the dynamic stresses when he gets on and off the table). Best to assume all that force goes down one leg, if he is standing on the corner of the table...

Finding the REAL design load cases is the first important step in any stress analysis!

Danger

Strange... I've never had any trouble with the local machine shop, where I buy raw stock, advising me as to exactly what sort of material I would need for a particular project (and it's an engineering firm, not just toolies). Maybe it's the difference of living in a small town.

I can see why you're not the one who took over my comedy badge this year. Cabellos isn't building a freeway flyover here; with a 90kg load, it's something like an entertainment unit or a cot. While I don't for a second dispute that your expertise in this matter far exceeds mine, I can also will full confidence say that on this scale 50% over-building has never failed me. Also on this scale, that amount of over-building adds very little to the cost.

AlephZero

Well, I wasn't trying to be funny (serious, but not solemn...)

Whatever it was designed for, if somebody can climb on top of it you can guarantee that somebody will climb on top of it, and will probably sue you if it breaks and damages their anatomy.

But you are quite right, the dimensions of an aluminum leg to take a 600 lb load are small (a 1/4 in diameter solid Al rod would be way over designed) so that is not a constraint on the design.

The point I was trying to make is you need to find the real constraints on the design, and "self weight" is not what matters. If it was my table (or whatever it is) I would be more concerned about its stiffness than its strength, for example - especially in the "sideways" direction.

Danger

Yeah, that last bit would be more of a concern to me as well. That's what I meant by my last statement in my first response, but I didn't express it properly. In regard to that, I would personally put more emphasis on corner bracing than actual leg material as the solution.