How thick should the metal be?

Rotate this [ symbol 90° clockwise . Imagine it’s a simple one person seat made from bent metal or maybe cast iron. My question is how thick should the metal be to take average human weight without sagging in the middle? There are no ridges etc to provide extra strength. Simply bent sheet or cast iron. So how thick should it be?

This all depends on the width of the span between verticals… what do you have in mind for dimensions?

you can actually buy “metal” in that shape in sizes between 20 and 14 gauge. It is used for architectural framing. Go to a building supply store (think fastenal or a contractor supply store not home depot) and ask them about steel track for wall framing.

So essentially you want to have a chair thats a perfect extrusion?

I would say you’re probably going to be in the range where it needs to be between 1/8"-1/4" as a starting point if you were using cheap steel. That would probably give you a little bit of flexiblity without collapsing. Aluminum would probably be in the same realm. but the strength depends on how it was made (a machined part will not have the same properties as a forged or extruded part).

Of course I would also say after seeing tons of metal student chair projects that finding a way to add any kind of structure to the metal, whether it be a slight crease at the edges, etc would vastly increase the strength because it would transfer the load away from the persons butt and to the uprights of the chair.

Look at how thin most car body panels are, and then think of how strong some of them are when they have strength added by bending in 2 direcitons.

Thanks for the advice. You lot are very helpful.

I think you could go with an 0.090-0.125" 6061 aluminum. If you want to be safe, 0.188" will be more than enough.

You may have to go thicker with cast iron. Not because of strength but because of the low tolerances of sand casting.

hey, here’s an answer you didn’t want to hear.

All structures deflect under load. You must define:

average human weight
allowable deflection
section thickness
material stiffness property

and enter into beam deflection equation. Vary the above inputs until you get satisfactory minimized deflection result. This is a 5 - 30 minute exercise depending on your math skills, or several minutes in FEA.

Or build, several days or weeks, and te$t.

Or, pose the question to some mechanical engineers. First, they’ll laugh at you: design it, but can’t build it! But then you can laugh at them as they argue intensely over which formula to use, conditions to apply, eventually regressing to bringing up Newton’s law…

also where are you supporting it?