Hi Steve.
I had read Travanty's article. Travanty assumed the moment limit implied
by manufacturer's wind load restrictions, is because of a max moment
limit at the base of the tower. On page 5 he says
"Wind loading specification from the tower manufacturer. In my case, the
tower was rated for 23.3 ft2 of antenna located one foot above the topof
the tower. This is a 350-lb wind load at that point, or 19,600 ft-lb,
referred to the *base* of the tower."
and pm page 6
"The spreadsheet then returns the mast yield, total moment at the tower
*base* for the complete system and the moment of the tower without
antennas."
Travanty does no analysis of top sections. Please correct me if I'm
wrong. Like I said, I do have a set of engineering calcs on a
per-section basis for my hold HG-72HD. What surprised me is I think that
analysis is "light"..i.e. too low antenna load. (also old UBC wind analysis)
I, along with lots of you folks, know that crankup towers don't fold at
the base. There are anecdotal reports of top sections folding more than
bottom sections right? Travanty didn't have more data than I have. You
have to analyze each section separately. The manufacturers don't give us
the data to do that.
But you see my main point. A claim that anyone sleeps better than
someone else, because he ran a spreadsheet on a mast, and "that's
it"..well there's a bit of head-in-the-sand aspect to that, and doesn't
deserve a fingerpointing at Joe Ham?
Any time people load up something, you gotta understand how that load
gets transferred to the ground. And as someone pointed out, it's not
just a wind horizontal loading problem. Columnar buckling, dynamic
oscillations etc.
It would be really, really funny, if the added weight up top, adds
oscillation modes that make things worse!
If people want to berate Joe Ham (it's funny, I actually just did put in
1.9" A53 water pipe in a triex w-51, because last time I used it, I was
getting too much binding in the top with a 2" mast. Am I comfortable
with the loads it's going to see? Yes.
Now with respect to berating the use of fence post, I think that's
another thing that's wrong.
Wheatland and Allied both sell quality 1.9" O.D. Fence post with .120"
wall (down from the normal .200" wall of schedule 40 water pipe). BUT:
it's 50k psi yield (although Home Depot might not have "the good
stuff"..thinner wall and 30k psi is sometimes used too).
This SS40 and WT40 fence post, is STRONGER than schedule 40 A53 30k psi,
even though it has a thinner wall.
I've tried fence stuff for no-big-deal things, but didn't like it
because the walls were too easy to compress for the stuff I was using.
So it's fair to question local buckling issues (which is a whole nother
analysis!) with certain wall thicknesses.
So, like people say: It all depends on your situation. What we lack are
the tools for fully analyzing towers. We have a mixture of good and bad
data and models. Let's explore making that better, not making fun of
each other like this is actually fully scientific. It ain't.
(I notice there are good tower modelling programs available today. But
they're not free. Too bad. )
-kevin
AD6Z
Steve, W3AHL wrote:
> Kevin,
>
> Most of what you are asking is covered in a July/August 2001 QEX article by
> W9JCC and the accompanying spreadsheet:
>
> http://www.arrl.org/qex/1123.pdf
>
> http://www.arrl.org/qexfiles/Travanty.zip
>
>
> Steve, W3AHL
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