Well, we've been getting snow every two to three days, and the ground
coverage is anywhere from 1 to 4 feet depending on the drifts! So, the
project has been put to bed until the spring thaw & drying out. I sure hope
the marking paint used to lay out the guy anchor points lasts through the
winter!
The good (bad?) news is this gives me plenty of time to play around on
looking at things. and I've come up with a minor question on the guying:
According to the Rohn Catalog, pg. 55G-5 (Rohn 55, 100' tower, 70 mph) the
guying is as follows:
Lower guy at 42 ft, 3/16 EHS, 400 lb pretension
Upper guy at 85 ft, 1/4 EHS, 660 lb pretension
By my figuring, the total (no wind) force on the guy anchor is as follows:
Lower guy angle = atan(42/80) = 27.7 deg
Force horizontal = 400*cos(27.7) = 354.2 lb
Force vertical = 400*sin(27.7) = 185.9 lb
Upper guy angle = atan(85/80) = 46.7 deg
Force horizontal = 660*cos(46.7) = 452.6 lb
Force vertical = 660*sin(46.7) = 480.3 lb
Total horizontal = 354.2 + 452.6 = 806.8 lb
Total vertical = 185.9 + 480.3 = 666.2 lb
Total guy anchor pull = sqrt(806.8^2 + 666.2^2) = 1046.3 lb
Angle of guy anchor force = atan(666.2/806.8) = 39.5 deg
Angle expressed as slope = 9.9 vertical / 12 horizontal
Now, here's the question: At the bottom of the drawing, anchor rod slope is
listed as 11.1 / 12
or angle of atan(11.1/12) = 42.8 deg; a significantly higher angle than I've
calculated.
Now, I realize I've neglected guy wire sag, but this would seem to lower the
angle even more from my calculation. The only thing I can figure is that
this accounts for the increased guy wire tension under wind loading. I
suppose under wind load, the tension on the upper guy increases faster than
the lower guy, causing the resulting force angle to increase. I've some
modeling that seems to indicate the tension on the guys stays pretty well
balanced, but I don't really trust the results since I'm not a mechanical or
civil engineer. Any insights?
-- Bill N7VM
Dr. William T. Ralston
14588 S Majestic Oaks Ln
Riverton, UT 84065
tel: 801-891-8358
fax: 413-254-2532
wtr@alum.mit.edu
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