[TowerTalk] Which Vertical is More Robust ?

[jimlux]

On 1/2/20 10:32 AM, Jim Thomson wrote:
> Date: Wed, 1 Jan 2020 11:13:16 -0500
> From: Gedas <w8bya at mchsi.com>
> To: TowerTalk at contesting.com
> Subject: [TowerTalk] Which Vertical is More Robust ?

> ##IF  you  know  the  yield  strength  of  the  proposed  EMT,
> plus  the  yield  strength of  the   upper  tubing, 6061-T6 is 40  ksi,
> 6063-T832 is  39  ksi.   Then  just  stuff  the  proposed  design
> into  DXEs  handy ...  yagi  mechanical  software.   It  will  spit  out
> exactly  how  far  it  will  deflect.....at  any  given  wind speed,  and
> also  what wind speed  it  will  break  at......and  also  WHERE  it  will
> break.
> .
> 
> ##  DXEs   ..yagi  mechanical  is  dirt  cheap,  easy  to use,  and  dead
> on  accurate.    Dont  guess.   It  will  also spit  out  deflection  with
> guys,  like  say  1  set  of  guys at 23 ft.   Use  the...  no-spec windload   option,
> thats  what u   get  in  a  wind  tunnel.     U  can  also  enter  the  yield  strength
> for each  individual section.


I suspect (but do not know) that DXEs program is a cantilever beam 
calculation, but might not take into account the gravity load, which 
Rick N6RK was talking about.

Once the antenna is "vertical" the loads from wind are probably going to 
dominate, as long as the deflection is small (sin(small number) is a 
small number).
However, there's a couple issues that I can think of (from practical 
experience with long floppy poles, like the spiderbeam 40 footer, and a 
bunch of 18 foot fishing poles) -

When it's being tilted up, there's more gravity load - one could use a 
calculation like that for a Yagi element or boom to determine the static 
loads and how close to failure you are.

The other one is the "weather vane" effect - It's unclear whether it 
would have a significant "pattern" effect, especially if it's anywhere 
close to 1/4 wavelength - most of the current is in the bottom, after all.

There's also a fatigue failure issue if it moves around in the wind. 
You'd want to be *way* below the yield stress to avoid this.