[TowerTalk] tower setbacks/falling trees

[Jim Thomson]

Date: Sat, 04 Feb 2012 06:53:58 -0800
From: Jim Lux <jimlux at earthlink.net>
Subject: [TowerTalk] tower setbacks/falling trees

VE7RF wrote:
The rules on setbacks  upsets me.  To have a tower  fall full length and 
still remain on  your own property is pretty tough if a ham lived on a 
typ city lot..like 50' x 120'  or  similar. Freestanding towers, like 
Trylons  are designed to break at the 40' level..and not at the 
base.They will not  fall full length.   A UST  crank up is the same 
deal, they break 3 x sections up.


--
this brings up a couple interesting issues..

You say designed to break at a particular point.  I hadn't seen that in 
any of the drawings I've seen, but then, I wouldn't think they would put 
a dashed line and arrow in a bubble saying >Break here.   But it is an 
interesting concept.

###  It's  there, plane as day, as soon as you run the freestanding trylon
through trylon's freebie software.  Start cranking up the wind speed..until
a  single section lights up red.   OR  start with too high a wind, like 140 mph,
and a big ant on top....and  several sections will light up red.  Then start reducing
the windspeed till only one section lights red.   The results are always the same.
The weak link is at the 40' level, at the junction of the 5th and 6th sections. 
OK, this is only for a trylon T-500  tower..which is 21"  wide at the top...and 
51"  wide at the base.   Other model trylons will appear to break higher still,
like on a T-400.   The point is, they will never break at the base.   We are not talking
about a guyed rohn 25/45/55  tower. 

##  on the detailed eng info on my UST HDX-689... you can clearly see from the eng
analysis, that the weak point is the 3rd section up from the bottom.  Sure, they could have
strengthened that section easily, but then the break point would then be the 2nd section from
the bottom.   They don't want them to fall full length.   Now all of this  only applies to freestanding
tower's  whose base is a lot wider than the very top.   Sure, you can free stand rohn 25/45/55/65G
per rohn specs, but those towers will break at the base...and will fall  full length.  

##  they don't advertise the weak point in any of these tapered, freestanding towers, but it's
readily apparent when you use their software to calculate wind speed survival. In the UST case,
it's clear that the 3rd section up from the bottom is far more stressed than the sections above or below it.  
City hall eng's agreed, and bought into it. 



The question would be whether it would fly in a regulatory context. It's 
well known that tall skinny freestanding things tend to break somewhere 
when falling (chimneys, pencil points), but I don't know that it's 
something that can be convincingly "designed in" in a way that will 
cover ALL circumstances.  If the tower is strong enough, it WILL fall in 
one intact piece.  (e.g. 200-300 foot trees fall over without breaking 
in the middle)

##  with tree's..the roots are always the weakest point. They uproot
in high winds.  200' tree's  are not encased in 40 yards of concrete.
Tree's  are bad news. About 11 years ago,  we had 3-4 days of heavy
rain, followed up with a huge wind storm.  Loads of tree's came down.
Tree's  in the middle of a forest have weak root systems vs
tree's  at the outer edges, and exposed to the full force of the winds.
Once developer's  remove the outer  peripheral tree's..the remaining
inner tree's are always failure prone. 

later... Jim  VE7RF