...and what happens to those numbers when the wind starts blowing at 50 mph?
Brad Bradfield wrote:
> --- "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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