Re: ATM 2 Piece Poles - and how trusses really work

[Ross Sackett <rsackett00@yahoo.com>]

--- Tom Krajci <tkrajci@san.osd.mil> wrote:
>  The
> 'bending test' is not appropriate [for testing
multi-part truss members].  A
> tension/compression test is
> far better...in my humble opinion.

Tom,

Caveat emptor: this is just off the top of my head and
I may be far wrong about all this.

I think your analysis is correct, but a flexure test
provides a good rough-and-ready indicator whether a
built-up tube will be stiff enough.  Break-down poles
have a funny property we don't encounter in monolithic
ones--they can be stiffer in compression than in
tension.  Imagine two tubes connected by a bungee
cord.  If kept in alignment, it will be just as stiff
in compression as a continuous one-piece tube, but in 
tension it is much more elastic.  

The connector assembly is the problem, of course.  You
are right that undersized bolts and assorted hardware
can be at fault here.  I think our goal ought to be to
make the connector assembly stiffer in tension than
the poles themselves; in this way the built-up truss
pole should have the same stiffness in both tension
and compression as a single-piece pole.

While testing the pole in compression would be ideal,
I doubt this could be done in a practical way with the
kind of weight/measurement devices available to most
of us.  But by flexing the pole, you put the connector
under great tension.  If it is more flexible than the
pole material, then their will be an inflection in the
curve of the pole at the point of the connector; if it
is as stiff or stiffer than the pole material the
curve of the pole ought to be continuous.

Ross



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