>From Tom, N4KG:
<FB Kurt !
< You really get into the fine points of mechanical design.
We all worry the most about the things we rmost readily recognize as
problematic!
< I would like to encourage you to copy your last message
< to the TowerTalk reflector, to share that insightful information.
Copy to follow.
< Would you mind if I sent copies to a few friends as well?
Please do so, I'd really love to have input from others on their
analyses.
< You have "heightened" my awareness ! (pun intended).
We all know, intuitively, that the tall stuff falls harder tham the
short stuff!
Per Tom's request, Following copy of previous private mail:
Hi Tom,
> Thanks for sharing the interesting guyed tower analysis Kurt.
My pleasure. I was sure it would help clear up the misunderstanding.
> How long do you think it would have taken to compute the
> solution using only paper and a calculator?
I'm sure that's why nobody has tried it! I need to get really bored and
spend a bunch of time modeling the Rohn EIA configurations to see what
the safety margins are. I expect to find that the EIA specs have allowed
for the guy stretch and resulting tower moments.Those fellows shouldn't
have missed it!
> The bending > moments obviously make things much more complex!
It gets downright scary sometimes!
FYI, the solid steel rod diameter I'm using in the models to get
equivalent wire rope stretch are:
1/4" 1x7 EHS = .1018" Dia
3/16" 1x7 EHS = .0764 Dia
These are from the MacWhyte Co. formulas.
It kind of makes sense from a sniff check. First, the cross sectional
area of the actual material is less than the nominal wire size, due to
the fact that it is comprised of several smaller dia wires. Reduction
caused by the gaps in the bundle and gaps between the imaginary nominal
size circle.
Then we add the cable twist.
> Would you conclude that the best base configuration is
> a base plate mounted on a flat concrete pad with a single
> pier pin? This is my guess after hearing about my friend's
> 200 ft Rohn 45 tower sheering off at the base when a tornado
> passed nearby. He had 4 Cushcraft 4L20's side mounted on
> the tower. The tower remained standing, but rotated slightly
> from where the legs had been implanted in concrete.
Absolutely! The recent discussion about pier pin vs fixed bases was what
started winding me up about the whole tower subject. Then everybody said
that guys don't stretch, and I lost it! Just had to open my big mouth!
I had not heard of this failure, heard of them twisting out from
underneath themselves.
I'm not the least bit surprised.
Not knowing more, I would guess that your friend was unfortunate enough
to get caught with his side mount swingarms broadside to the wind
direction, generating max torque on the tower. The consolation is, I'd
guess it is impossible to keep them pointed in the right direction as a
tornado passes by. To much rapid wind direction change in a short time,
when one needs to be in the basement.
In my opinion, when it howls, all sidemount owners need to put the
swingarm in line with the wind (same for TIC owners), and have boom/mast
attachment at the physical center of the boom. There are a few other
caveats, but this should be the least torque condition.
For torque, the pier pin footing is by far the best.
The one that has me worried right now is the stretching guys and tower
base moment on a pier pin footing. The fixed base is easy, it is what it
is.
In the wind bisecting 2 guy sets load case, the base moment could cause
the base to roll over and put the lions share of the compression on the
single downwind leg.
This is a real bear to analyze. I haven't spent enough time to make
friends with it yet.
My current opinion (just that), is that there may be enough pure
compression in the tower to make the flat base stay planted on the
footing, causing it to act like the fixed base.
If the two upwind legs start to lift off of the footing, then the down
wind leg goes away pretty quickly.
It's on my list, just like straightening all of the bracing in the pile
of sections in the back lot pile, digging holes, rebuilding prop
pitches,
etc, ad nauseum!
The best solution is a tapered top section on the bottom, sitting on a
ball socket!
Don't know if you caught my comments about the bracing configuration in
the Rohn towers, somewhere in the recent past.
I made detailed models of 10' 25G and 45G sections, including all the
bracing. Then I ran some tower models and got the loads at the top of
the
bottom section. Took these loads and used them as input into the
detailed
section models.
WOW! When you put a bunch of compression into a Rohn
tower section, the thing winds up like a corkscrew!
This is caused by the fact that they build the sections with all of the
diagonal bracing angled in the same direction. No mystery why the tower
wants to wind up. The Triex sections, with "W" bracing don't do this,
but
they are a bitch to climb!
I think Rohn knows this, and recommends the pier pin footing to relieve
the torsion.
I often wondered, why don't they reverse the direction on one face to
reduce the torque deflections by 33%.
Happy to talk with you more about these things. BTW, thanks for many of
your comments that have made me stop and think, and gain a better
understanding of the electrical side of life. Not necessarily my strong
suit!
73, Kurt
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