At 09:42 AM 6/29/2005, jonklinkhamer@comcast.net wrote:
>Hey Ron,
>
>I'm certainly no expert but if all the cables leave the tower at the same
>height using the same bracket and have the same length and assuming the
>anchors are relatively close to 120 degrees apart with the same distance
>away from the tower - does this dictate that the angles would be very
>close to each other from vertical? Is my thinking correct or am I missing
>something.
What if you're not putting the tower up on perfectly level ground? If all
the guys are the same length, and same distance from the tower, and you're
putting up the tower on sloping ground, then the tower will be leaning at
the same angle as the slope of the ground. This might(?) be a "bad thing".
A small deflection probably isn't an issue, after all, the tower will move
in the wind some amount. But if you're looking at 10 ft out of 100, that's
about 6 degrees out of plumb. I suspect the real problem (if there is one)
might come from the fact that if you stake out your guy anchors at the same
horizontal distance (either along the ground, or true horizontal), and then
you plumb the tower, the guys will be at slightly different angles, and
will be slightly different lengths, so the forces won't be all nicely
balanced. Practically, it might make no difference.
Say you've got your guys at 45 degrees (nominal), on a 100 ft tower. The
anchors are 100 ft out. If one anchor was 10 ft higher, the guy would be
134 ft long instead of 141 ft. The angle with the tower would be 48
degrees instead of 45. Say you adjust the tensions so that the
horizontal component at the top of the tower is the same. The short guy
will have a tension that is about 95% of the other two. Or, looking at it
another way, if you tension all three guys the same, you'll be putting a
lateral load at the top of the tower.
Clearly, we're not looking at huge changes in guy tension, and hopefully,
you're not designed so close to the ragged edge of failure that a 5-10%
change is going to cause catastropic destruction.
But again, this brings up an interesting point... how much misalignment can
you tolerate from the 120 degree spacing before you have real problems. We
set up some temporary masts at field day, and one wound up with a guy
spacing of around 160 degrees, which made dialing in the tensions (on the
stretchy nylon guy ropes) to get the (fiberglass flexible) mast vertical
kind of tricky.
This kind of tolerance information would be quite useful for hams, who
rarely have the luxury of situations where you can do precise "build to
print". Obviously, the mfr isn't necessarily going to publish this sort of
"how bad can you be without catastrophe" analysis, preferring to fall back
on a nice legally defensible: If you don't do what we say, hire your own
engineer to analyze your particular case.
>Jon,KB1DC
>-------------- Original message --------------
>
> > At 09:36 PM 6/28/2005, Ron Hylton wrote:
> > >Hi Jon,
> > >
> > >When I layed out my tower, which is also on uneven terrain, I rented a
> > >transit level from the rental department of the local Home Depot. Both
> the
> > >horizontal and vertical axis of the transit are marked in degrees, so
> > >achieving the 120 degree separation on the horizontal axis was a snap. To
> > >determine the distance from the tower, to take into account the terrain,
> > >required some trigonometry, but it worked out great.
> >
> > Depending on the transit, it might have stadia marks in the telescope,
> > which, in combination with a suitable rod, will make determining distance
> > pretty easy.
> >
> >
> >
> > > I can provide you with
> > >more info about how exactly I performed the calculations, if you would
> like.
> > >I set-up an Excel spreadsheet to assist in finding the right spacing to
> > >achieve the same angle for the top guy wires with respect to the tower
> for
> > >the three anchors.
> >
> > This brings up an interesting point. I wonder how critical it is that all
> > the guys leave the tower at the same angle relative to vertical?
> >
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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