Date: Sat, 04 Feb 2012 06:53:58 -0800
From: Jim Lux <jimlux@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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