[TowerTalk] Rohn 25G or 45G?

[Stan Griffiths]

>In a message dated 98-07-14 13:14:15 EDT, you write:
>
><<  For example,
> what kind of load could you put on a 25G, 120 feet high, guyed every 20 feet
> with six sets of guys?  I certainly don't know, but I'll bet its a lot more
> than Rohn says you can put on 120 feet of 25G with the guying plans they
> show in the catalog.   >>
>
>       Yes - this person should proceed with caution. 
>
>       But I don't think that adding more guys has any influence on the tower
>capacity. In fact it may be detrimental as you're putting more preload
>compression on the legs. It's the leg compressive capacity that is the
>limiting factor of these towers - not necessarily the guy wires.
>
>Cheers,  Steve   K7LXC

I do think adding more guys has influence on the tower capactity . . . to a
point.  There are official Rohn guying diagrams showing 6 sets of guys and
we know from earlier Rohn Catalogs that 25G, for example, is speced at more
than 15,000 pounds of vertical load.  If you tensioned all 18 of these guys
to 400 pounds each they would impart only 7200 pounds if all the tension
were directed straight down, which it is not, so the total downward pull
from the guys would be considerably LESS than 7200 pounds.  The weight of 12
sections of 25G is only another 480 pounds.  This is no where near the
capacity of the tower under compression.  Of course, you need more capacity
since the wind will tension the guys more and there will be more downward
forces acting on the tower because of that.  So I disagree that compressive
capacity is the normal limiting factor of these towers.  I can't think of
one case I have ever heard of where the failure mode was compressing the
bottom section of an otherwise undamaged Rohn 25, 45, 55, or 65 tower.  If
the bottom section is not in good shape due to rust or a split from water
freezing inside it or some other mechanical damage, all bets are off.

But this is why we have PEs and why we should use them for figuring stuff
like this out for us.

Stan  w7ni@teleport.com


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