TowerTalkians --
In followup to my post about computing guy lengths over sloping terrain,
below is an exchange with W2RU that clarifies my approach and adds important
information. Bud's question is at the bottom, my response at the top.
73, Dick WC1M
Hi Bud,
I agree that the tension must be the same in all three directions, but not
only in magnitude. It must also be the same for the horizontal and vertical
vector components of the force along each guy wire. I don't believe that,
for a given angle, simply duplicating the tension along the guy can
reproduce the relative proportions of vertical and horizontal force of a guy
that's at a different angle.
I think you can visualize this by picturing a tower with two guys at, say,
45 degrees to the tower, and a third guy at 15 degrees to the tower. That
third guy can easily be set to the same tension as the other two, but it's
going to have a much greater proportion of vertical (pull down) force than
horizontal (pull out) force. I suspect there's no value of tension that
could be used to exactly duplicated the vertical and horizontal tension
vectors of the other two guys. That proportion is a function of the angle of
the guy to the tower.
Regardless of the physics involved, one of my objectives was to follow the
Rohn-specified layout for the tower as closely as possible, on the theory
that the manufacturer knows best. Rohn specifies guy attachment heights and
the distance from the tower base for the anchors, which results in a certain
set of guy angles and horizontal/vertical vectors. I wanted to get as close
as I could to that and still allow clearance for the middle SteppIR. I had
to push the anchors out about 5% to accommodate the middle SteppIR, but
that's within the Rohn-specified tolerance for the distance. Then the task
was to reproduce the guy angles and forces over my sloping terrain.
Another way to picture it is to draw a horizontal line from the tower base
to each guy as computed for my terrain. I've done that in the attached
tower/guy diagrams. The line intersects the downhill guy well before it
reaches the ground. The line goes past the uphill guy before it intersects
the line of the guy if it were extended at the same angle into the ground.
The line hits the guy on level ground exactly at the anchor. The length of
all three lines will be 92', which is what I computed to be necessary for
middle SteppIR clearance. The horizontal line drawn from the tower base to
each guy is therefore the flat ground that's missing. The picture
essentially shows that the tower has the same geometry as it would over flat
ground.
Now, if you look closely at the guy diagrams, you'll see that what I just
said isn't true for the Northwest guy. With the extreme slope of my terrain
in the downhill direction, it's not possible to exactly reproduce the angles
for all three guy wires when attached to the same anchor position. You can
see that when the guy anchor is placed to properly reproduce the desired
angle for the top guy, the other two guys intersect the flat plane at
distances other than 92 feet. At 85 feet, the middle guy isn't too far off,
but the bottom guy is off by about 22 feet, which is a lot. When the guy
attachment points are moved to be the same as the top guy, the angles of the
other two guys must increase. If you look at the guy angles for all three
directions, you can see that the middle and bottom guys are not at the same
angle in all three directions. The middle guy is close to nominal all three
directions, but the bottom guy angles are quite different.
The correct way to deal with this is to use multiple guy anchors, either
three separate anchors or at least two: one for the top and middle guys and
another for the bottom guy. But I wasn't willing to go through the
considerable trouble and expense to do that. My feeling was that the most
important angle to get right was the top guy angle, and that if the other
two guy angles had to be different, it was at least better for the
horizontal component to be greater than spec than for the vertical component
to be greater than spec (in the horizontal direction, the other guys anchors
resist the force; in the vertical direction, the tower and base resist the
force.)
That's my story, and I'm stickin' to it!
73, Dick WC1M
> -----Original Message-----
> From: W2RU - Bud Hippisley [mailto:W2RU@frontiernet.net]
> Sent: Thursday, January 14, 2010 9:22 AM
> To: wc1m73@gmail.com
> Subject: Re: Guy article in CQ
>
> Dick --
>
> I would love to have a copy of all your drawings, calculations, and
> spreadsheets. We're in the process of locating a new house and one of
> the two finalists has small rolling hills that will necessitate guy
> wire anchors being at varying heights relative to the tower base and to
each other.
>
> You said one thing I'm not sure I understand the need for:
>
> On Jan 13, 2010, at 7:07 PM, Dick Green WC1M wrote:
>
> >
> > The idea was to ensure that the angle of the guys to the tower, when
> > placed on my hill, would be identical to the angle of the guys over flat
ground.
>
>
> Visualizing a tower with one set of guys (for simplicity), for the
> tower to be in equilibrium and vertical the horizontal tension in the
> three guys must be equal in all azimuthal directions. If one guy wire
> has a different angle relative to the tower or vertical axis, it seems
> to me it just means the tension along that guy wire must be different than
the tension in the others.
>
> However, if your objective was to ensure clearance of all guy wires by
> rotating beams, then I guess I understand why you said that.
>
> Or did I miss yet another constraint?
>
> Thanks!!!!
>
> Bud, W2RU
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