[TowerTalk] Weak Link in Guy System?

[Signal Hill Ranch]

At 05:55 PM 2000-11-17, you wrote:
>  The 1/4" EHS is rated at 6400# and the 3/16 at 3900# break strength.
>The Rod 303 allowable load is only 7,330 #.

Ray, the key element is that the guy wire is rated in BREAK STRENGTH
and the rods are rated in ALLOWABLE LOAD.  These are NOT the
SAME.

I am not sure of Rohn's safety (conversion) factor, but it is probably
something like 3 to 1.  So the allowable load of the rod at 7,330
pounds is probably the equivalent of about 22,000 pounds of
break strength.

Cross sectional area of a 5/8 inch rod is 0.3068 sq inches.  Assuming
my estimate of 22,000 pounds tensile of the rod results in a steel tensile 
strength rating of about 72,000 pounds per square inch.  Rohn does not
specify what steel they use but this value is consistent with the types of
steels normally used as structural elements.

So I believe that the guy rod is adequate for the guys you will be
using.  The only time you can get into trouble is if the force (tension)
on the guys is not along the axis of the rod and there is something
that forces the rod to bend sharply.  This can cause the rod to
fail.  But this is normally not a problem since Rohn specifies
their concrete bases  to be well below the surface of the ground
and the dirt keeps the anchors from bending sharply.

Where one can get into trouble is if the concrete base comes
to the surface and the anchor rod comes out of the concrete at
some angle to the direction of the force.  This could result in
failure in the future.  Fortunately the rods are of a relatively
soft steel and can deform SOME without danger of cracking
to which a hardened steel rod would be more subject.

I'd trust Rohn and follow their advice.  Most of these Rohn tower
installations fail not from guy wire or guy anchor failure but
from twisting the tower due to severe overload and torque
forces from BIG Yagis.  Actually ,when I think back about the
ham failures that I have seen, they have been due to really stupid
installation short cuts.  Follow the directions--it is YOUR life and
property!

John   W0UN


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