There's been recent speculation on the reflector about allowable tower wind
loads under various conditions. As an excercise in self-education, I used
the structural engineering calculations furnished by U.S. Tower to
investigate the effect of wind speed and mast height on the maximum
allowable antenna load for my crankup tower.
Before I go further, I should emphasize the following:
1) I'm not a professional mechanical engineer or a structural engineer.
2) Don't build anything based on this information. Use the data in a
relative fashion, rather than as absolute results, to get a feel for how
the maximum allowable antenna load varies. Use a professional engineer if
you really want to know the maximum allowable load for your specific
situation.
3) I didn't do anything original. I just took the rather complete tabular
data included in the engineer's 14 pages of calculations, a copy of the
wind design portions of the Uniform Building Code, and varied the mast
height and wind speed.
Engineer's Assumptions:
Design based on 1991 Uniform Building Code with no ice
70 mph basic wind speed
American Institute of Steel Construction tables
Wind exposure C (terrain which is generally open having scattered
obstructions extending 0.5 mile or more from the site
in any full quadrant, i.e. flat or gently rolling open
country and grasslands)
Antenna weighs 150 lbs
Tower:
U.S. Tower HDX-589 89-foot crankup (21-foot sections with 4-foot overlap)
Manufacturer rates tower for 30 sq.ft at 50 mph wind
My Assumptions:
2 inch O.D. mast from top of tower to height of antenna
Mast weighs 10 lbs/linear foot
Max Allowable Wind Load vs. Wind Speed: (antenna at 90 ft)
Wind Speed Max Antenna Wind Force Max Antenna Area
---------- ---------------------- ----------------
50 mph 450 lbs 44.8 sq.ft
60 354 24.5
70 209 10.6
80 48 1.9
82 0 0
Tower fails at 83 mph with no antennas
Note: Max antenna area calculated from max antenna wind force
based on UBC formula:
Area = [Max wind force] / [Cq * Ce * qsI * (2/3))
where Cq = 1
Ce = 1.57 to 1.63 depending on height
qsI = 0.00256 * [wind mph]^2
Max Allowable Wind Load vs. Antenna Height: (70 mph wind)
Mast Length Antenna Height Max Antenna Wind Force Max Antenna Area
----------- -------------- ---------------------- ----------------
1 ft 90 ft 209 lbs 10.6 sq.ft
6 95 183 9.1
11 100 160 7.9
16 105 138 6.7
1 72 and 90 120 lbs to each ant. 6.2 sq.ft/ant
Conclusions:
1) The maximum allowable antenna load drops rapidly with increasing wind
speed, particularly as the wind speed nears the failure speed of the tower.
2) While most tower manufacturers specify max antenna load a short distance
above the top of their towers (1 foot), most hams place their antennas on
tall masts mounted on the top of their towers. As can be seen from the
results in the second table, the max load significantly decreases as the
mast is lengthened.
Current engineering practice is to use the EIA standard to calculate wind
load. This standard uses different assumptions and calculations than the
UBC. However, I used the UBC method because that was the method used by the
professional engineer per California building code.
73 de Bruce, WA7BNM (bhorn@netcom.com)
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