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


-+- Andy Gray  wrote:
>
> On Tue, 19 Aug 2003 06:28:24 -0700 (PDT),  Ross
> Sackett
>  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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