I think Dick has a good point. I've always assumed that it would not
move until the last 5 percent of wind loading, and maybe not even then,
given the rise in downward forces. I think if I had it to do over
again, I would put a sheet of Teflon or maybe even Lexan between the
plate at the bottom of the lowest section and the concrete base, in the
hope that it would promote rotation when really needed. I think that
K7NV's study assumed a near-zero-friction pier pin base, and the Rohn
version for these "little" towers certainly isn't.
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On 5/11/2011 10:45 AM, Dick Green WC1M wrote:
> 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@embarqmail.com]
> Sent: Tuesday, May 10, 2011 8:51 AM
> To: towertalk@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
>
>
>
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