On 2/6/11 10:14 AM, Grant Saviers wrote:
> 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 :-) .
One could probably use the flagpole guidelines and add a bit for the
wires (which can have surprisingly high drag, given their small
diameter.. back when I was interested in small plane/ultralight
modeling, I was amazed to discover that a 1/4" cable had a lot more drag
than a big 1"x3" strut)
>
> 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.
It would make an excellent QST article, no? Maybe we could get some
folks on this list to collaborate on such a thing. It's a LOT of work
to generate a finished article, but if the toil is divided up among
several people, it's more tractable.
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
Actually, for a given cost (i.e. weight of steel), round tube is your
best deal. And larger diameter with thinner wall is better (that 4th
power in the second moment, and why car drive shafts are hollow sheet
metal tubes, etc.)
> and there is a variety of wall thickness available for ease of
> telescoping fits.
Yes..
Also, the average ham could probably thru bolt the
> joints rather than some structural welding.
Yes. That makes the engineering a bit more complex. I guess you could
design/calculate based on holes with a lot of clearance, etc.
While doing everything yourself is an admirable goal, sometimes it's
better to hire someone to do a specialized task. And welding fits in
that category.. A decent welding rig costs more than a drill, and you
really need some real practice and training to do it right.
The trick is in finding somewhere to get it done. If your day job is
working in a cube farm or managing the same, finding a welding
fabricator isn't a simple task. You just don't even know where to start,
because it's not like you want a whole building constructed, just an
hour's worth of welding work for $50.
So, an article about "how to find a structural welder for your antenna
project" might be useful. Around here (southern California) places that
do motorcycle frames or race cars are one place. Lots of places that do
decorative fences, but typically, those guys aren't necessarily who I'd
hire to do something structural. But it's one of those things that you
can't do by googling. It's a lot of phone calls, or going by the shop
and asking "can you do this, or do you know someone who does?" and if
you don't know where the shop is in the first place...
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