From: firstname.lastname@example.org (Bruce Horn)
Date: 97-05-28 11:34:20 EDT
You recently wrote:
> A 50 MPH wind (to which the tower is rated) exerts 10 PSF wind pressure.
>A 70.7 MPH wind exerts 20 PSF wind pressure. You can see the problem
>You'll have to take the factory rating and cut it in half to get a 70 MPH
>wind load capacity.
Although you are correct in this statement that an increase from 50 mph to
70.7 mph will double the wind pressure on the antennas (pressure increases
with square of wind speed), this does not result in reducing the factory
rating by half. It reduces the overall rating by significantly more! This
is because the entire tower, not just the antennas, "feel" the additional
wind force, and it takes more of the tower's strength just to hold itself
up, let alone the antennas.
The following chart from a portion of a posting I made to TowerTalk in
August, 1996, demonstrates this effect. These are calculated values based
on the engineering calcs supplied with the 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
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
As can be seen from this chart, an increase from 50 mph to 70 mph reduces
the allowable wind load to less than 25% of the original speced load. Since
the manufacturer rates this tower (HDX-589) at 30 sq.ft at 50 mph, the
rated load at 70 mph would be slightly less than 7.5 sq.ft (allowing the
same safety margin as used when specifying the 50 mph rating).
Although the degree of reduction for a U.S. Tower tubular design may be
different than for their triangular lattice design, the idea would hold.
Unfortunately, it is not as simple as just reducing the allowable wind load
in linear proportion to the wind force.
73 de Bruce, WA7BNM (email@example.com)
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