On Tue, 30 Nov 2004 21:05:34 GMT, Ted Edwards
vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email

Well all I can say is that reading these posts, my radius of gyration
is becoming increasingly small, but my speed is being maintained. In
the end my brain will probably exceed Yield Stress! G

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.

I do have to say that you guys may be arguing at cross purposes. I
_thought_ that Ed was saying that tube stiffness is not the only
thing. Wall thickness is important as well, because that will prevent
buckling under certain circumstances.

I am trying to learn here, from minds that scare my dilettante little
brain! G

AFAICS the argument revolves around the fact that you can keep the
same wall thickness or even nincrease it, using aluminium, and use a
larger diameter tube to achieve the stiffness. You in fact need to do
this.

Having a smaller diameter tube would presumably make a given wall
thickness less likely to buckle. That feels right to me.

But would a steel tube of small diameter, having the _same weight_ as
a larger tube of aluminium, have as strong a _wall_ as the alum one?

If you thinned the walls of the steel pipe, in order to allow a larger
diameter for the same weight, the walls would _not_ be as strong as an
aluminium tube of the same weight as the thin-walled steel one, with
wall thickness strengthening as a cube of the thickness (?) while
weight is linear. Looking at the wall as a bar of steel, aluminium
would be stiffer than the same _weight_ bar of steel. (?????)

Hope I am saying this in a way that can be understood. trying to avoid
having to learn a whole dgree's worth of engineering terms and
conditions, by using gut feeling and laymen's terms.