[TowerTalk] tower setbacks/falling trees

[Randy]

Jim: thanks much for your input...
Things vary according to locality. I have never, ever considered 
earthquake or ice loads on my
towers...not going to happen.

If I *WAS" an engineer, nobody is gonna believe ME.

If you have a, let's say 60' foot of tower, there's a good chance that 
it *might* fall down within
its length... if you have 195' of tower, there is ZERO chance any of it 
is going to land 195' away from
the base...

73
Randy
KZ4RV



I
On 2/5/2012 9:08 AM, Jim Thomson wrote:
> 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
>
>
>
>
>
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