>My question is this... rather than move the guys and brackets up to the top
>of the section (where it would make it difficult to haul up and take down
>yagis because it's in the way) should I add a "third" guy of Phillystran on
>the ears of the top flat section? The other guys are EHS, and I've heard
>arguments against adding extra EHS guys. Is that strictly because of the
>weight of the EHS or the downward force any additional guy tension may have
>on the tower? With this "extra" set of guys, I could drop them when
>adding/removing antennas while keeping the two sets of EHS in place. I
>really don't think it would be a good idea to move the current top set of
>guys without some sort of temporary guying... particularly with all that
>stuff up there. And if I'm going to install temporary guying I may as well
>put up the permanent stuff. Will cost another $400 or so, but that's small
>change compared to what I'm trying to protect.
>
>If this idea is not so far fetched would you use the 4000# or the 6700#
>Phillystran given the top EHS guy is only a few feet below the top of the
>tower.
>
>Larry WI5A
Hi Larry,
It sounds to me like you have overloaded the tower in that configuration BIG
TIME. You must have over 20 square feet of antenna on it and you really
need to include the area of the mast as well and that is over 3 square feet.
I believe the Rohn number for windload assumes all of the load is
concentrated right at the top of the tower and much of yours must be
substantially above the top of the tower, which makes the situation MUCH
worse. To back up my statement about the tower load being specified right
at the top and not above the top, take a close look at the guying diagrams
in your Rohn catalog. Right next to the wind load numbers you will see an
arrow pointing RIGHT AT the tower top. This means this is where the load
must be . . . not higher than this. You could probably get by putting a
smaller load up farther like on your mast, but it would have to be a lot
smaller if you put it up very high. You need a PE for this advice.
If it were my tower, I would have four sets of guys on it, one set every 20
feet with the top set right at the top of the tower.
I know there are no Rohn designs in the book that call for four sets of guys
on an 80 foot tower. If you have read some of my posts in the past, you
have heard me say that the designs in the Rohn catalog are the few dozen
examples that Rohn has chosen to publish for free. There are thousands of
different safe ways to put up these towers. That is what we have
professional engineers for . . . to tell you other safe ways to put up your
tower.
Think about the guy situation for a moment. In an early Rohn catalog that I
have which is dated September 1969, Rohn specifies the Gross allowable load
on the bottom section of 45G to be 19,440 pounds! I know they don't publish
this number or any other number telling you the Gross allowable load on the
bottom section in the current catalogs. I also don't think the materials or
design of 45G tower sections have changed either. So if you tensioned all 6
of your guys to 10 percent of their breaking strength, added the weight of
the tower, beams, rotator, guys, coax, etc, how much is it? About 4000
pounds, maybe. I would not worry for an instant that I was putting too much
weight on the bottom section, even by adding two more sets of guys.
Take a look at the Rohn guying chart for a 300 foot 45G tower. You will see
SEVEN sets of guys on that one. If that one can stand 7 sets, certainly
yours can stand 4 sets.
Personally, I am absolutely convinced these towers can take MUCH more load
than Rohn shows in their free examples in the Rohn catalog. You just have
to be sure and guy them properly. This is where the professional engineer
gets in the act. He can tell you what is proper guying for your situation.
If you decide to guy your tower the way Rohn shows in their examples, I
would certainly not put more on it than what Rohn recommends, including not
putting any load more than a foot or so above the top of the tower. If you
really do not want to deviate from anything Rohn shows in their catalog,
then you had better not use Phyllystran since I have never seen any mention
of it in any Rohn book I have ever seen . . .
All that load you describe way above the top set of guys scares the Heck out
of me . . .
Stan w7ni@teleport.com
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