Both cases are fairly simple vector analyses. Deviating from the 120
degree spacing will result in increased tension on some of the guys, as
well as vertical load on the tower.. You can calculate that, and
increase the size of the guy wires and anchors, if necessary. Likewise
the distance to the guy points: the closer they are to the tower, the
greater the tension on the guys and the greater the vertical load on the
tower. If you can't do the calculations yourself, or if required by
local codes, consult a professional.
By the way, the convenient thing about the 120 degree spacing is,
assuming all the guys are anchored at the same distance from the tower
and same elevation, the worst case tension in a single guy is just the
same as the tension in one guy with the wind blowing directly from the
anchor point to the tower, i. e. , directly in line with one guy. If the
wind happens to be blowing exactly between two guys, each will see that
same tension. It's more complicated if the spacing is different.
Changing just the spacing between two guys to 130 degrees increases the
maximum guy tension by about 20 percent. (cos60/cos65)
Keep in mind that this all comes from Engineering Mechanics 101 nearly
50 years ago. We electrical types all hated that course, but it's been
one of the most useful to me.
73,
Scott K9MA
On 7/27/2021 1:00 PM, David Gilbert wrote:
The first question seem like a rather simple vector analysis task to
me. Generate a simple model for the manufacturer's recommendations
for included angles, distance from tower, and guy wire tension ... and
then calculate the lateral force on the tower due to each guy wire.
Do the same analysis for the configuration under consideration (with
the addition of translating the resulting forces to the desired
directions of 120 degrees apart) and compare the resulting lateral
forces. Decide how much deviation from the spec you're willing to
live with (5%? ... 10% ... whatever) and see if it satisfies. Keep in
mind that any change in guy wire tension to compensate for deviations
from the tower manufacturer's configuration needs to stay within the
guy wire ratings.
Seems like a better approach to me than relying on the opinion of
anyone, although my opinion says that when you do the calculation you
would find that the difference between 1 degree deviation and 2
degrees deviation is round off error.
73,
Dave AB7E
On 7/27/2021 12:47 AM, Jari Jussila wrote:
Hi ...
I'm sure these topics have been discussed earlier, but could not find
any thread ..... Sri ...
We are at our club erecting two towers. A rotating steel tower being
31 m high and a aluminium telescope mast 18+18 m = 32 meters.
A) How critical is the rule that guy wires should be "exactly" 120
degrees apart?
A professional guide says, that the anchor point can be 50 mm from
that 120 degree (119,9-120,1 degree) but some say, that a maximum of
two degrees is OK. That meaning 118-122 degree.
What's the tower community advise?
B) The steel tower height is 31 m - it's a rotating tower - but the
upper bearing is at 23 m ( leaving 8 meters for the antennas) and the
lower bearing at 11 meters.
Whats the advise of anchor point distance? Some say, that the 60 %
(distance for the tower) should be calculating from the total height
(31 x 0,6 = 18,6 meters).
Some say, it should be calculating from the upper bearing (23 m x
0,6 = 12.8 m)
Whats your opinion?
Jari, OH2BU
For OH3AC Club Station
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Scott K9MA
k9ma@sdellington.us
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