On Wed, 19 Aug 1998 03:23:36 -0400 (EDT), you wrote:
>
>Towertalkians,
>
>Rohn offers a service (for a LOT of money) to analyze a NON-CATALOG tower.
>This means there are a LOT of ways to use Rohn towers NOT shown in the
>catalog. Rohn has fully engineered and calculated the few dozen or so
>configurations you see in their catalog. There are literally thousands of
>possible configurations not shown there. You are not resticted to using
>only the examples they show. If you choose another design, just make sure
>it was done by a competent PE and you should be OK.
>
... snip ....
>Stan w7ni@teleport.com
Many thanks for several very forthright posts.
Along this line, take a look in your favorite Rohn catalog. Count the
number of designs which have the guy wires going to the *TOP* of the
tower. If you come up with zero, you win the prize. A lot of the 55g
designs have 15 feet free and clear.
Why is that? Easy...these are designs for commercial installations.
Sidearms, microwave dishes (steel golf umbrellas if you read my
earlier post). Why such low ratings?
Well, for starters, consider 15 feet above the guys with dishes.
These things oscillate in the wind, if not *really* pinned down. A
thirty foot boom and a blizzard of elements would seem far less likely
to oscillate than a microwave dish because of the enormous damped
moment on the boom. How much less likely than that if the guys are at
the top.
In terms of pure windload, with guys at the top, the load goes
straight to the guys and tower downward compression. With the
commercial 15'-in-the-clear design, that top 15 is a lever with it's
own windload that gets to counterflex the tower under the top set of
guys, and requires a derating.
I could go on.
It appears that there is *not even one* ham design in the catalog.
Just a bunch of commercial designs that they copied over. It's not
that they haven't thought about it, they sell a top plate with the guy
connections right there. Just no tower designs with the plate and the
wind loads to match.
We really need ham designs that assume top guying, and dampened
twisting moments on top. And an optional derating factor for offcenter
down-tower mounting (ring-rotor, etc).
I second the call for software. We need a "Towerstress" program with
Rohn, US tower, Trylon, etc, in it.
73, y'all. Guy.
--
Guy L. Olinger
k2av@qsl.net
Apex, NC, USA
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