[TowerTalk] de-rating tower wind loading limits

[Dick Flanagan]

At 7:34 PM -0800 on 1/15/98, you wrote:

>I have a crank-up tower rated at 30 sq. ft. for a 50 mph wind. I have 20
>ft. of chrome-molly mast with 4 ft. in the tower. I want to de-rate 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?

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.

I have a US Tower with similar specs to yours:  30 sq ft at 50 mph.  I live
in a standard "low wind" area of the country, so the only thing the county
building folks wanted was engineering proof it would survive the lowest
specs:  80 mph winds with 1/2-inch of radial ice!  (The radial ice thing is
called "Exposure C" which is the minimum they usually spec to.)

Well, my engineer worked the numbers and told me my tower, without any
antennas on it at all, wouldn't meet that requirement!  At 70 mph it would
sustain something less than 6 sq ft.  I got my building permit with the
proviso I placard the tower that it had to be retracted when the winds
exceeded 70 mph.

Your mast will handle the wind load fine, but be careful your rotor, rotor
shelf and tower top can hand the side pressure placed on it by that 16-foot
lever arm with the 8.5 sq ft at the far end of it.

Again, I suggest you pay a PE and be prepared for him to tell you some
depressing facts, but rather he tell you than experience show you.

Best of luck and please let us know how things turn out.

73, Dick

--
Dick Flanagan W6OLD CFII Minden, Nevada DM09db (South of Reno)
Visit http://www.qsl.net/w7di/



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