On 05/23/2015 9:18 PM, jack wrote:
Is there a general rule(s) that describe the stiffness of a round tubing
to the stiffness of a solid rod? For instance: is 1" dia. round rod
stiffer than 1 " dia. x .125" wall tubing? How much larger dia. tubing
equals a solid rod of a certain dia.?
Is tubing inherently stiffer because it has 2 surfaces (inside and
outside) that oppose each other? Thanks.

"It all depends..." On what you really are talking about by
"stiffness" in the application. For simple deflection away from yield,
of a laterally-applied it's basically directly proportional to the area
moment of inertia; otoh, if it's used a column with axial loading that's
significant factor but the yield mechanism is different.

But, no, as a general rule a tube isn't anyways nearly as strong against
lateral loading as a solid rod of the same OD.

Ibar = pi*d^4/64 for a solid cylinder, a hollow cylinder is the same
thing excepting you must subtract out the inner area --

Icyl=pi*[do^4-di^4]/64

So, the cylinder is quite a lot less and gets that way much faster as
the wall becomes thinner.

There's truth in the weight:strength argument above and there's
complexity in a detailed answer, but the rough idea is given above.

It's the material that matters--geometry is important in comparison, but
it takes actual material to stand up.

Think about how a thin wall tube crushes whereas the solid rod bends
uniformly...

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