Re: ATM Filling truss tubes

[Aart Olsen <aart@uiuc.edu>]

Canned foam, the common crack-filling stuff, will add some weight to the
tubes.  Also the foam won't harden properly because it has poor exposure to
air, and it may continue to flow for a time and allow the bubbles to
collapse to make a sticky drool instead of a rigid cylinder.  Without the
extra rigidity the mass will cause the tube resonance frequency to become
lower, which is undesirable, although the damping might still be better
than an empty tube.

I have used shaved-down rubber stoppers that I've rammed up or down to find
a resonance antinode and "detune" the tube.  If you do any tube stuffing
it's a good idea to put different amounts of material in each tube, and/or
to locate the material at slightly offset locations.  That way each tube
will resonate differently and the whole truss will be effectively
unsymmetrical, more lossy and quicker to damp. 

Aart Olsen           mailto:aart@uiuc.edu           217-244-4688
Library Systems Office      Univ of Illinois at Urbana-Champaign