At 05:36 PM 5/26/98 -0400, Fred Hopengarten wrote:
>I have in my head that all Rohn catalog designs include a safety factor
>of 3, but at the moment I cannot find any note in the catalog about this.
>
>Does anyone know what safety factor Rohn uses for 25G, 45G and 55G? Can
>you refer me to the drawing or page that says it?
Sorry, but the safety factor is not 3:1. I wish it was.
The EIA-222 standard requires a 2:1 safety factor on the guy wires.
I have done comparison calculations and I come up with about the same thing
they have in their literature. Everyone should do a sanity check calc like
this
to make sure they are doing it right.
The tower buckling load has a safety factor on it that varies with height.
As an example, drawing CB70488 R1 shows a 190' tower in a 90 mph wind zone.
The base load for this tower is 9,870 lbs. The same drawing shows the base
load
for the 70' tower at 3,010 lbs. Now this is the same section of tower but
the applied
load is different while the guy spacing is almost the same (31' vs. 32').
So I guess
you could say that it has a 3:1 or better in this particular case.
In both cases the limiting factor is the guy strength available in the top
3/16" guy wire.
Now if you would use 1/4" guy wire (6700#) versus 3/16" guy wire (4000#) on
a 70'
tower, you won't get anywhere near the 9,870 lbs of base load so you won't
over load
the tower section, but you should be able to get about 60% increase in
antenna wind load
available. This is provided the guy anchors don't pull out of the ground
and you use the
GA25GD guy bracket to get the load properly distributed into the tower -
especially
for the top guy. The second set can still be 3/16" and looped around a
tower leg.
Oops, I just gave away one of my tower engineering secrets.
de n0yvy steve
Steven H. Sawyers PE
ARRL Volunteer Consulting Engineer.
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|