Thanks Jim,
A bit of an oops in my post, I hit send rather than save as draft.
I was using buckling loosely as a term although for the really big poles
the standard does include column buckling and uses a segmented approach
for the taper for the wind load bending moments and stress.
I haven't done a decent calc for the wind load stress, I need to refresh
some very old learning or find a java app :-) .
What would be really neat is for somebody with a bit more expertise than
I have to figure out several cost optimized self supporting poles for
wire antennas. For example my steel supplier stocks 20' lengths, so
they would be good for 35' poles. One of the Metal Supermarkets or the
like has 24 footers, so they would be good for 42' or so. The question
to be answered is what is the cheapest pole design for some wind load
(e.g. 80mph) and say 200 lb of wire force at the top? Beyond 42' might
be best left to a Structural PE, soils engineer, and professional erector.
I'm partial to square tube steel since the moments are higher than pipe
and there is a variety of wall thickness available for ease of
telescoping fits. Also, the average ham could probably thru bolt the
joints rather than some structural welding.
Of course, caveat emptor, YMMV, do not rely on this post for anything,
all risks are yours, my calcs are probably incorrect, etc etc
Grant KZ1W
On 2/6/2011 9:41 AM, jimlux wrote:
>> 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.
>>
> That's because buckling loads for tapered tubes are tricky to calculate.
>
> For a straight pipe, though, it's pretty easy to determine failure load
> for a bending load. By the way, buckling refers to failure when the
> member is loaded in compression.
>
> If you know thickness of the wall and the diameter, you can figure out
> the compressive/tension load for a given load.
>
> Bending moment for failure = (yield strength)*(second moment)/(Douter/2)
>
> second momemt = pi/64 *(Douter^4 - Dinner^4)
>
> Douter = OD of tube, Dinner = ID of tube.
>
>
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