[TowerTalk] I have a crankup tower rated at 30 sq. ft.
for a 50 mph wind. I have 20ft. of chromemolly mast with 4 ft. in the
tower. I want to derate the wind load for 70 mph. How do I do this and
what effect does the mast have on the wind loading limits of the tower? I
want to put 8 sq.ft. near the top of the mast and 8.5 sq. ft. at the top
of the tower. Can I do this, or am I dancing with disaster?
To which W6OLD wrote: Pay for a PE to work the numbers for
you, Henry. If you will be going for a building permit, you'll have to do
that anyway.
That would be the safe/correct way to go. But, to give you
a ballpark guess, I'll go though a couple equations that everyone could
apply to their particular case.
Let's start with the 50 MPH vs. 70 MPH.
New wind load = old wind load * (old wind speed)^2 / (new
wind speed)^2
= 30 * 50^2 / 70^2
= 15.3 sq.ft
By that quick calculation, your 8 + 8.5 sq.ft would be
over the rating.
I was going to go into the calculation about the mast
height, but it may be too much for most of the readers. On second thought,
it reduces pretty nicely. This applies to selfsupporting and crankup
towers, not guyed towers. I won't explain it, but will give you the
equation:
W = rated wind load
w1= wind load antenna 1
w2= wind load antenna 2
h = tower height
m = mast length (or how far above tower top that antenna 2 is mounted)
d = mast diameter, in feet (ie. 2" / 12 = 0.16666)
W*h >= w1*h + w2*(h + m) + (0.67*d*m)*(h + m/2)
So, if we were to assume that 15.3 sq.ft. is the rating at
70 MPH (remember, that is a close estimate...get a PE to verify), and a
8.5sq.ft. antenna at the top of a 70'(?) tower. How big of antenna can we
put 16 feet (20' mast, 4' in tower) above the tower on a mast?
15.3 * 70 >= 8.5 * 70 + X*(70 + 16) +
(0.67*0.1666*16)*(70 + 16/2)
1071 >= 595 + X*(86) + 208
268 >= X*86
3.1 >= X
So, as a ROUGH ESTIMATE, you could only have a 3.1 sq.ft
antenna at the top of the mast. Notice that 3.1 + 8.5 is nowhere near the
15.3 number. So, you could put up a 4 el 10M beam.
Now remember, I'm not a PE. These equations are to help
you get close to an antenna/tower configuration that might work. It is not
intended for a final check. Once you get in the ballpark of what might
work, then take all that to a PE. These equations are to save you a few
extra trips to the PE office. Choosing antennas/towers is a very iterative
process. By the very nature of the ham, we are experimenters. We always
like to try something different. If we had to check each idea with a PE,
it could become very costly.

Chad Kurszewski, WE9V
