[TowerTalk] Non-Guyed Support for 80m Horizontal Loop at 35 feet

[Grant Saviers]

After my original response, I did a little research on self support 
poles.  Flagpoles have gotten a lot of engineering attention in the last 
decade as they have been pushed to astounding heights - the free 
standing record as of 6/2009 was 436 feet and flys a flag the area of 
two tennis courts!

Since the application is to support a 80m loop at 35' and not a 3 phase 
HV systemand transformer it might be worth considering some of the 
flagpole guidelines.  I purchased ANSI/NAAMM standard FP1001-07 "Guide 
Specs for design of metal flagpoles".

First to note is the wind pressure force when applied below 40' is 
modified downward a small amount towards the base, so for short masts 
using the standard wind force for the entire height is conservative.  
For 85mph that is 22psf for a flat surface (Cd=1).  So for my 4" sq 35' 
high pole that is (22psf * 35' x .3') 254 lbf over its length or a point 
load of 254lbf at 17.5' high.

A flag of 40 sq ft at 85mph wouldn't survive very long but the 
experimentally confirmed formula for flag load is

W = 0.0014(1.3*V)^2 * A^1/2 *Ch  (Ch = 1 for this)  or about 115 lbs, 
probably about the load of the wire antennas.

The base moment = 254lb * 17.5' + 115lb * 35' or 8495 ft-lbs.

The flagpole makers of stock poles up to about 60' recommend that 10% of 
the length be in the ground.  The foundation spec for a one 35' flagpole 
I found was 36" in diameter by 42" deep concrete.  So this is a long way 
from what a self support tower requires.  For ref my HDX589 foundation 
requirements are 106000 ft-lbs of base reaction force which take a 5.5' 
square by 9' deep concrete block or 272 ft^3 of concrete.  The 36" round 
x 42" deep flagpole base needs 25 ft^3 of concrete, just under 1 yard.  
Interestingly, the reaction force for my tower is 12x the flagpole and 
the concrete base is also 12x.  So a sanity test prevails on the mass of 
the base.  What about ground

Now the question is what is needed for steel poles to handle the loads 
without buckling?  This is an interesting question not addressed in any 
antenna books I have seen maybe because of the liability issues.  There 
are certainly a number of hams that don't have nature provided antenna 
supports or they are in the wrong place.  Also, a self supporting pole 
might work where aesthetics or CCR's are a concern.  I priced flagpoles 
and they are expensive, much more than $1000 for a 35' delivered on a truck.


On 2/6/2011 5:12 AM, Barry Merrill wrote:
> I recall that the Intermittent Telephone Company
> (Intermountain) pole-guy years ago told me that
> they always bury 1/7th of the above-ground pole length.
>
> So 7 feet buried for a 35 foot above ground, or total
> pole length of 42 feet; your 5 feet plus 1 per 10 of 43.5
> is a little more conservative, clearly safer.
>
> 73
>
> Barry, EI/W5GN
>
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