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Re: [TowerTalk] Aluminum mast

Subject: Re: [TowerTalk] Aluminum mast
Date: Thu, 4 Jan 2007 10:50:00 EST
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here's a link to some good information on tower stresses:

The "I" value for a circular pipe is

    I= Pi (Do^4-Di^4)/(64)

where Do=outer diameter of pipe

      Di= inner diameter of pipe

For your 2.000", 0.25" wall pipe this gives I = 0.54

The yield strength of 6061-T6 is 35,000 PSI.

The bending moment when the mast will yield is 0.54(35,000) = 19,000 PSI 

Let's say you have an antenna with a 6 sq. ft. wind area at the top of the 20 
ft. mast. The wind is gusting to 80 mph. The force per sq. foot is about 16 
lb. There are more accurate numbers then the 9 lb/sq ft I use at 60 mph. This 
is conservative and is from an old ARRL antenna handbook. For 80 mph the force 
per sq foot is (80/60)^0.5 X 9 lb. The wind force on the antenna is 96 lb. The 
bending moment at the base of the mast is (96lb)(20') = 1920 lb-ft. Muliply 
by 12 to get PSI, (1920 lb-ft)(12 in/ft) = 23,000 PSI (is this correct?). 

This mast will fail. It can stand a gust to 72 mph. The highest recorded wind 
velocity for your area can be found by a google search. This must be adjusted 
for antenna height and the terrain. The links above contain some information 
on this. Physical Design of Yagi's has all of this. I can't find my copy right 
now. Before using the formula's I have put down here one should (must) double 
check using a good written source. I could be in error. 

There is also icing to take into consideration in some areas of the world. 
Altogether this mast thing is just an engineering calculation. When performed 
with worst case numbers you can be sure that your antennas will stay in the 

     Dave WX7G;


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