Having just buried 6 guy anchors in concrete this past
Saturday, this issue is still accessible in my brain...
A few weeks ago, I sat down and did a quick geometric
sketch/calculation and found the angle for a 80%
spaced, 4-guy-bay tower to be somewhere around 38
degrees - guy anchor would then point to a spot on the
tower halfway between the 2nd and 3rd guys. Makes
sense, no?
Then I searched the Rohn catalog carefully and
eventually found that it stated a 1:1 angle
(45degrees) in a somewhat hidden away drawing. There
are lots of gems in the catalog if you take the time
to look through the whole thing.
So I made sure they were at or slightly less than 45
degrees (really easy to do with the cheap carpenters'
square I had with a built-in bubble level).
BTW, one of the guy rods I had was a little bent
before I started (shipping damage?). I slipped it
into a crotch of a tree and slowly straightened it
out. Looks like new. So I know they bend fairly
easily.
It's not in Rohn's interest to tell you too much about
how they do their calculations. Both for business
reasons (they charge you to run the numbers) and for
legal reasons (many people would misuse any formulae
they supplied, then sue when their tower collapsed).
73 Mike N2MG
On Thu, 06 September 2001, Stan or Patricia Griffiths wrote:
> I took another look at my Rohn Catalog and I now see where they do list the
> rod
> angle. Funny, I never noticed that before . . . Still, they don't tell you
> how
> they caluculated it . . .
>
> I learn something new every day . . . even after 48 years of ham radio.
>
> Stan
> w7ni@easystreet.com
>
> Michael Rauh wrote:
>
> > Jerry, Stan, Dave, Tom,
> >
> > I agree with Jerry and Stan - the angle is probably not critical. Rohn
> > presents the angle to 0.1 degree in their guyed tower drawings. This seems
> > impossibly precise to me - there is probably way more sag or bow in the guy
> > anchor than that.
> >
> > I also suspect that Dave is correct when he suggests taking the vector sum
> > of the loads on the guys when the tower is at its rated wind load to find
> > the guy rod angle.
> >
> > In thinking about this problem I realized I don't know how to calculate the
> > individual loads on each guy wire. I use a rigid body model for the tower
> > that will give the total load on the guys, but won't tell me how the guys
> > share the load, at least as far as I can see. To figure out the load on
> > each individual guy, I think I will need to consider how the tower deflects
> > under load.
> >
> > Consider a tower with two sets of guys and an antenna on top. I could start
> > by assuming the tower is fixed at the top and the base. I calculate the
> > wind load and how the tower will deflect under that load. Draw a curve,
> > deflection vs. load. Then assume the lower guy is attached at mid-point,
> > draw a curve of elongation vs. load. The load on the second guy is where
> > the curves meet. The reminder of the load on the tower, including the
> > antenna load, is absorbed by the top guy.
> >
> > But wait! The top guy elongates under load too, relieving the load on the
> > bottom guy. So I will have to set up a system of simultaneous equations in
> > order to get the loads on each guy. I will need equations for the
> > elongation of the top guy, the bottom guy, the deflection (bowing) and the
> > pivoting of the tower. If the tower has a pivot base that is all I need,
> > but if the base is cast in concrete I might need an equation to account for
> > the force needed to tilt the tower as the guys elongate.
> >
> > Does that sound right? Do you know of any references where this kind of
> > problem has been worked out? I know Kurt Andress has found guy loads using
> > a finite element program. I need to understand the principles involved
> > before I resort to a program!
> >
> > I think it would be very cool to know how to find the loads on the
> > individual guy wires in a tower system.
> >
> > Best Regards,
> >
> > Mike Rauh, NV7X
> >
> > Reply to mikerauh@ic.org
> >
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