WHOA guys !
FULL HEIGHT fallovers are fairly common,
and probably more common than this wishful
thinking that amateur towers will fall within a
smaller (40%) circle. We know who you are
trying to fool. Don't fool yourself !
I have seen two towers fall over FULL LENGTH when the
guy wires pulled through cable clamps. Cable clamps DO
loosen with time and temperature cycling and need to be
checked and tightened annually.
K0CS lost a 100 ft tower when the ( poor quality ) main
bolt in an equilizer plate failed, dropping the tower straight
out from the base. Watch out for cheap imported hardware
in critical applications.
I lost two "self supporting" 75 year old trees, an oak and
a beech, when they uprooted during a small tornado event,
leaving a HUGE divit in the ground. They also fell "full length".
Oh yes, I did see one 80 ft self supporting Heights Aluminum
Tower fail above ground. It folded over 8 ft up, stretching out
70 ft into the street beside the owners house.
On the other hand, a PROPERLY guyed tower can survive
very high winds. NONE of my 7 tower were damaged by
the 94 tornado. In fact, the top set of (1/4 inch) guys on
one of my 130 ft towers held up two 12 inch diameter 70 ft
trees that fell against the guy. Good thing the trees were
close to the guy which prevented the trees from gaining
much momentum.
de Tom N4KG
On Thu, 22 Feb 2001 Bill Coleman <aa4lr@arrl.net> writes:
> On 2/21/01 16:09, Bob Thacker at k3gt@pgh.net wrote:
>
> >Am working with the local township on drafting new ordinances and
> would like to know if there are any concrete studies showing that a
tower
> will fall within a %age of its height. Have heard that it is either
40%
> or within the guy radius. Since we will be addressing free standing
> towers any info there would be appreciated. Also sure that plain
verticals
> will be addressed since yours truly has already requested to erect an
80m 4
> sq.
>
> Bob, I don't have an specific studies, but I've also seen the 40%
> figure
> bandied around for guyed towers. It does make sense. Even if a top
> guy
> fails, the remaining guys limit how the tower can come down. Guy
> radius
> is usually 80% of height, so 40% is certainly within 80%.
>
> For freestanding towers, while it is physically possible to have a
> base
> failure that results in a 100% fall, most failures like that are
> unlikely.
>
> Consider how trees fail. If you've ever seen downed trees after high
>
> winds or a tornado, trees rarely fail right at the ground. (Unless
> the
> soils gets so soft the roots just squish right through) Instead,
> trees
> tend to fail 10-25 feet up, with the mean being somewhere around 15
> feet.
> That's the high stress point for a freestanding structure.
>
> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
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