[TowerTalk] Aluminum tubing insert overlap...

[Jim Lux]

On 9/6/11 7:06 AM, Robert Chudek - K0RC wrote:
>
> Experimenting with this tool last night was a real eye opener... It
> reveals what you thought would be stronger element (extending the larger
> tubes) is in reality a potential failure point! Longer "whippy ends"
> help create survivability. I also learned that you model an element
> starting at the tip and working toward the boom with the larger
> sections. This mechanical modeling is separate from designing for a RF
> characteristic and resonance. The spreadsheet has a section on
> electrical characteristics although I have not looked closely at that
> section yet.
>



Yes.. that whole "make things stiff" can aggravate a stress 
concentration. When I was working with theatrical rigging (lighting and 
effects trusses) there was a whole thing about "don't stiffen the 
structure too much with tension cables, unless it's ALL really stiff)..

There's a classic homework problem with a 2" diameter manila rope and a 
1/8" diameter piano wire in parallel supporting a load that would break 
either one individually.

And, another one with a welded T joint where adding a "strengthening" 
gusset makes it weaker.