Ed Huntress wrote:

Correct me if I'm wrong, Ted (and this is stuff I'm 'way rusty about), but
the larger diameter tube made possible by lower density of aluminum will
have a higher value for the radius of gyration than will a steel tube of the
same weight, and of smaller diameter.

Correct?

Correct but misleading. Of course the radius of gyration increases with
increasing diameter but the increase in diameter is not simply "made
possible by lower density of aluminum". One could instead choose to
make the wall thinner for steel but keep the same diameter. You might
want to take a look at "Design of Weldments", Blodgett, Sec. 2.5. I'd
be surprised if you didn't have *that* book.

Ted