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

[Ross Sackett <rsackett00@yahoo.com>]

--- Andy Gray <atm@misterg.fsnet.co.uk> wrote:
> 
> 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.)

My partial analysis was intented to compare single-
and multi-piece truss tubes.  Presumably length and
loading would be identical.

> 
> 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. 

Right on! (This is one of my favorite rants!)  Now
glue the top on that box.  Even with a large hole in
the face, it is still an order of magnitude stiffer
than the topless box.  But even the topless box is
stiffer than the "tube" geometry of Obsession-style
mirror boxes.  But nobody seems to listen to us...

> [The function of the connectors]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.

Is this the same as saying their function is to put
the joints into compression, like a prestressed
concrete beam?  As long as the tension on the pole
doesn't completely negate this compression and open up
the joint, then the connector shouldn't have much
impact on the stiffness of the pole in the truss. I
think your logic here is sound, but I've used
knockdown poles that were anything but stiff enough,
and the problem was clearly in the undersized joints. 

> 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.
> 

There's got to be an optimal combination of tubing
diameter and wall thickness, no?  For tubes of a given
weight, small-diameter thick walled tubes are more
likely to buckle under heavy axial loads and can store
a lot of vibrational energy, while very large
diameter/thin walled tubes are more resistant to
bending and buckling but crush more easily.  

R

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