[TowerTalk] Complex Guy Question

[Dick Green WC1M]

Regarding the pier pin base, I agree that in theory it's the way to go, and
that's what I have on my 110' Rohn 55. However, I've often wondered if the
Rohn flat-plate pier pin base used by most hams who install pier pin bases
really does what a pier pin base is supposed to do, namely provide some
"give" in response to torquing forces on the tower. Seems to me that the
friction between the base plate and concrete is substantial, especially
given the weight of the tower/antennas/rotors/cables and the downward force
of the guys. Does the plate really move? How much wind would it take to move
it? I've never detected any evidence that mine has moved. Commercial pier
pin bases are tapered and, if I'm not mistaken, the pin fits into a bearing
of some sort (probably a thrust bearing) attached to the concrete.

73, Dick WC1M

-----Original Message-----
From: Wilson Lamb [mailto:infomet at embarqmail.com] 
Sent: Tuesday, May 10, 2011 8:51 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Complex Guy Question

No expert here, but...
The K7NV analysis is magnificent and really gives some basic understanding! 
I wish Rohn would publish some of the same stuff.  I also wish they would 
make the pier pin option more obvious, since it is such a great tower saver!

It obviously makes getting up to the first set of guys more difficult, but 
30' can be manhandled without much trouble.  It's my gut feeling that the 
monster bases recommended by Rohn do more harm than good, by making the base

section so stiff.  The base of a guyed tower does not need to keep the tower

from leaning.

I've been head doodling pin bases and wondering if an automotive type 
universal joint wouldn't make a good pinned base.  I doubt if they have 
sufficient axial load capacity to carry expected compression, however.  My 
next wild idea involves a bowling ball.

The part of the K7NV analysis that speaks to your question is the discussion

of Aramid elasticity.  It shows that towers can experience large top 
deflection and excess bending stress in conditions far below what they 
"should" be able to handle.  The pin takes care of the bending part, but you

still see large deflections.

It seems to me that the stronger top guys should be tighter than one would 
run if they were steel.  That won't change their elasticity, but will 
flatten out their catenary "some" and increase the system stiffness, which 
would reduce your deflection.  Of course it will also reduce the load and 
bending
at the middle guy location, which is good.  The middle set being stiff glass

and steel will cause them to pick up load rapidly as the top deflects. 
Letting my imagination run a moment, I think that in a severe environment, 
with large top deflection, it might be better to loosen the middle set, and 
maybe the bottom too, to let the tower "lean" more and bend less.  Let the 
experts jump in on this!

73,
Wilson
W4BOH