--- "N0TT@ juno.com" <n0tt1@juno.com> wrote:
>
> Hi gents,
>
> My barbershop comments... :)
>
> I would think that if there were say, 3 guys running to the same
> anchor, that one would want to line up the anchor with
> the "middle" guy wire. If there are two guys then I
> would think the anchor would line up between the two
> guys. This is because all the guy wires should be
> tensioned the same, no one guy wire pulling more than
> the others during low wind conditions, thus averaging
> out the load.
>
Unfortunately, the calculation is not QUITE that simple, but not
terribly difficult either, and your assumption is actually pretty
close. While your assumption that the guy tensions will all be the
same is correct, the vertical and horizontal components of those
tensions will all be different since the guys are all at different
angles.
For instance, assume a tower with guys at three evenly spaced points on
the tower of 1/3, 2/3, and 3/3 of the total height. The three guys
will make angles with the ground of 22.6, 39.8, and 51.3 degrees,
respectively. Further assume that the single guy anchor for each set
of three guys is spaced eight-tenths of the height of the tower out,
and that the guys are tensioned to 300 pounds each. The vertical and
horizontal components (in pounds) of the guy tensions are:
GUY VERTICAL COMPONENT HORIZONTAL COMPONENT
------------------------------------------------------------
LOWER 115.3 276.9
MIDDLE 192.0 230.4
UPPER 234.2 187.4
Now, you can arrive at a neutral angle for the guy anchor rod two
different ways, but both give you the same answer. The quickest is to
just average the three angles above, giving 37.9 degrees. The other
method (Sanity check here. It's been a zillion years since I took
algebra and statics.) is to average the three vertical (180.6) and
horizontal components (231.6), which gives you the same 37.9 degree
angle.
But in reality, I suspect that Rohn's +/- 1 degree statement is gross
overkill, given the fact that the ultimate strength of the guy wire is
by far much, much less than the breaking strength of the guy anchor
rod. I'm really pretty surprised that they even go so far as to make
this precise +/- 1 degree statement given their necessity to cover
themselves against product liability claims.
Boy, we sure cleaned out the old cobwebs answering this one!
73,
Brad, W5CGH
Brad Bradfield, PE
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