From: Ross Sackett 
To: atm{at}shore.net
Reply-To: Ross Sackett 


-+- Tom Krajci  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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