Well taken, Gerald, although I think statistically that the 4 guy wires at
90
degree intervals, if installed to exacting angular dimensions, one guy
anchor will only be taking the entire wind load theoretically at only 4
different points, less than a full degree in width. With a 3 point guying
system, spaced 120 degrees, each guy anchor will be supporting the
entire wind load for about 60 degrees. In a worst case situation, the gust
that breaks the guy will do the same thing to either system, but the 4
point guyed system stands about a 45 times less likely chance for that to
happen. The calculations for a 3 guy system vs. a 4 guy system are;
3 guys x 60 degrees (where only one is effective) = 180 degrees from
360 degrees total = 180 degrees or half the various directions that wind
can come from vs. 4 guys at 90 degrees spacing, allowing only 1 degree
where only one guy is effective x 4 guys = 4 degrees. 180/4 = 45 times
greater chance of a three wire guying system failure, all other things
being equal. You're right though, when enough wind comes from the worst
case direction, both systems will fail. My guess is that the specs. for
guying
the tilt - over tower (which I have one), were derived using some
statistical
analysis, otherwise there shouldn't be a difference in recommendations.
I'm using the 4 point system, as recommended for the entire tower,only at
the hinge, where it must be done because of the tilt - over strut, but have
extended the distance to 50 ft., where I have ground anchors with 8' ground
rods attached for increased dispersion of a potential strike of lightning.
In
order to preserve the patterns of the antenna on top, (a TH11DX), I'm using
a 3 point system of 6700# Philistran, from the top of the tower to 3
additional
ground anchors. Lots of work and extra expense, but I think well worth it.
73, -= Roger - K9RB =-
-----Original Message-----
From: Texas RF <TexasRF@aol.com>
To: wa9eka@worldnet.att.net <wa9eka@worldnet.att.net>
To: <towertalk@contesting.com>
Date: Thursday, March 12, 1998 7:49 PM
Subject: Re: [TowerTalk] GUYING
>Hi Roger, if you think through this you will see that in the worst case one
of
>the four guys is taking all the forces. When the wind is blowing directly
in
>line with one of the guy anchors, the back guy takes none and the two side
>guys are at right angles and can also take no load. This is exactly the
same
>worst case as for a three way system, one guy anchor has all the load.
>
>The foldover numbers work because the towers are short, 48, 58 or 68 feet
tall
>and the sections are designed to handle heights up to 200 feet for 25G or
300
>feet for 45G.
>
>The same numbers have to work for short regular 25G and 45G towers but for
>whatever reason Rohn has decided not to publish the data.
>
>These considerations explain how many hams get away with grossly
overloading
>(according to published data) 25G, 45G and 55G towers.
>
>Tnx/73 de Gerald, K5GW
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|