Kurt wrote:
>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%.
Indeed.
I think the answer is: When Rohn designed these towers they probably were
looking exclusively at the "two-way" market, which in those long-ago days
meant some kind of omni-whip or simple derivative.
I seriously doubt any design engineer at Rohn ever contemplated a big
ROTATING structure with significant distributed moment like a long-boom 20m
yagi, let alone a full-size 40 or even 80m yagi. When was Rohn 45 designed?
40 years ago? I doubt there were five full-size 40m yagis in the world then.
In 1974 I had discussions with experienced user/installer Ted Gillett, W6HX,
and Frank Clement, W6KPC, then Chairman of Tri-Ex. Their advice led me to
purchase Tri-Ex T-15 stacked tower instead of Rohn. Frank indicated that
although it had smaller face-width than Rohn 45 (14", instead of 18"), the
heavy gauge legs and W-bracing made it actually superior to the Rohn in
terms of torque loads. It was designed to compete in the stacked tower
two-way market, but also with REAL ham radio in mind. In addition, the
accessory plates, top plates, guy brackets and torque bars are more
versatile and rugged than the Rohn equivalents, so it is a superior system
in my estimation, even though it is a bit harder to stand on.
I have been on the top of many tall towers with big antennas, when their
rotators were activated. All the towers wind-up a bit, but for as big as it
looks, Rohn 45 seems pretty squishy to me.
The top section of an LM-470 crankup is the equivalent of T-15, made in the
same jigs. When Frank had them designed, he made sure a small prop-pitch
motor would fit inside, anywhere you want in the section (unlike Rohn 25).
Thanks, Frank!
73, Glenn K6NA
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