[TowerTalk] AB-105 Safety Question

[Dave Hawes]

When I installed my government surplus AB-105 tower 20 years ago, I
had a machine shop fabricate the guy wire attachment plates, since
the tower sections that I acquired did not come with them.  They were
made of carbon steel, and although I primed and painted them well,
they have become quite rusty over the years.  In addition, the 
Preformed Dead End Guy Grips pass through countersunk holes in the 
plates, with no thimbles.  Although inspections reveal no evidence of 
failure of the grip at the stress point at the plate's hole, I plan 
to replace the grips with new ones, this time using a clevis and 
thimble for attachment.  Unfortunately, the holes in the plates are 
not large enough to pass the bolt from the clevis, so merely 
repainting the existing plates is not an option.

Recently, I acquired a complete new set of plates from Norm's Rotor
Service (301-874-5885) in Frederick, MD, and am faced with the task
of replacing the old with the new.  To complete the description of
the current installation, the tower is standard duty AB-105, 117
feet total height, with three-way 5/16" galvanized steel guys
attached at 47, 77, and 107 feet, with a 4 element 20 and 2 element
Cushcraft 40.

The issue of safety in this project is two-fold:

1.  Detaching the existing guy wires, one a at a time, to do the 
work on the attachment plates, and
2.  Unbolting the plates and replacing them.

As to Issue #1, I believe that the AB-105 tower is robust enough to
withstand this kind of maneuver.  In fact, I have detached one top
guy wire many times to allow my large yagis to pivot down to the
tower using a "PVRC (tm) Yagi Mount."  I see no reason why removal
of intermediate guy wires, one at a time, should present any
significant danger, but am interested in opinions.  Of course, the
work would be done in calm wind conditions to minimize the potential
for buckling forces.

Regarding Issue #2, it is important to note that guy attachment 
plates mount on top of the splice plates at each tower juntion.  
Replacement of the plate requires unbolting half of the bolts at the 
splice!

Concern:  Structural stability of the tower.  Probably not a worry, 
since the splice plate has six more bolts holding the joint stable.

Concern:  Settling of the tower section.  This is my real fear.  I'm
not so sure that the tower sections are actually butting.  I think
that the bolts at the splice plate are in shear and taking most or
all of the dead weight load of the tower.  If I take the bolts out,
even of just half of the splice plate, especially at the 47 foot
level, and the tower settles, I might not be able to get the holes
lined up again.  In that event, I can visualize (in my nightmares)
having to jury rig some kind of jack to separate the tower sections
far enough to get the bolt holes lined up.  (I'm pretty sure a drift
pin would not have the mechanical advantage needed for the job, but
then again it might.) This would not be able to be done right away,
and the tower could be left in a dangerous condition for some period
of time while a workaround is developed.  Not a happy thought.

I would appreciate any insights other owners of or maintainers of
AB-105 tower might be able to provide on this subject.  I'm hoping
my fears as described above are merely overreaction, but better safe
than sorry.  I fully realize that this is a very, very narrowly
focused question, and apologize for the bandwidth.  However, this
reflector is a great resource, and I figured it was worth a shot,
especially since this is essentially a safety related issue.

TIA.

73 - Dave N3RD
n3rd@ix.netcom.com
http://www.QTH.com/topten

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