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

[atm@misterg.fsnet.co.uk (Andy Gray)]

On Tue, 19 Aug 2003 06:28:24 -0700 (PDT),  Ross Sackett
<rsackett00@yahoo.com> wrote:

>Up to loads where buckling is an issue the squashing
>or extension of truss members in tension and
>compression is controlled by the cross-sectional area
>multiplied by the modulus of elasticity.

...and length, and axial load.

(The axial load depends on truss geometry for a given
external load - tight angles bad, fat angles good.)

I seriously doubt whether the axial stiffness of the truss
members is a real concern here. The devil is in the design
of the joints to ensure the load on the truss members is
purely axial, and in making the structure at either end of
the truss stiff enough. The classic "mirror box" of an open
truss dob looks like an open topped shoe box - next time you
have your hands on one try twisting opposite sides and see
how stiff it is. Is this a good foundation for your large
diameter, tapered wall, pin jointed carbon fibre truss ?
Don't expect the truss to stiffen up the mirror box! If the
design is good, you can build it out of paper and straws and
it will be as stiff as hell (but not necessarily strong).

As to undersized connectors:

Their function should be to *clamp*, making their properties
largely irrelevant to the stiffness of the structure, since
you're reliant on friction generated by this clamping force
(or pre-load in butt[1] joints). This does require well
designed joints, though.

[1] This would not cause laughter in the UK - don't know
what US equivalent term is! 

Buckling of truss members is a real concern (more from
accidental damage / ease of handling than pure performance),
as is resonant vibration - hence large diameter, thin wall
tubes.

As to measuring the extension of a truss member:

The measurement of deflection of the whole assembly is a
reasonable target, and could be done with a weight, some
string and a dial gauge. The extension of each member can be
calculated very accurately by analysing the structure, and
plugging in relevant material properties, but if you were to
do this, you would likely find that other factors dominate.

In short, good mechanical design is always worth much more
than exotic materials, or even craftsmanship. Neither of the
latter can be exploited without the former.

I have seen some absolutely beautifully made structures that
will *never* perform well, simply because there is no
*design* behind them. As an avowed coward, I can't face
pointing this out to the proud creators! The design
principles and even the analysis of truss structures isn't
particularly difficult, but it does have to be learnt. There
are many engineering texts that cover it.

(OK, rant over...

..you can come out now...

... hellooooo.....)

Andy (actually a Mechanical Engineer!)

(Andy Gray, Dyserth, N. Wales, UK)