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[TowerTalk] Pier pin vs. burying in concrete

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Subject: [TowerTalk] Pier pin vs. burying in concrete
From: (Kurt Andress)
Date: Fri, 01 Jun 2001 01:46:19 -0700
Hi All,

Just to clarify a few things....

alsopb wrote:
> I have been following this thread and see only tangential references
> to the type of pier pin installations I've seen.
> First, they use a flat bottom plate on the concrete.  I doubt that
> these rotate at all to relieve the moment at the bottom.  Kurt's
> analysis looked at this and concluded it almost is equivalent to a
> burried installation.

I think there may be potential for a bit of confusion about what various
people mean when refering to "a pier pin" base.
The text referred to here, was in the context of comparing the difference
between a base buried in the footing and the "pier pin" type of tower base
that uses a full sized tower section footprint on the pier (a full sized
section with a big flat plate sitting on the pier). I do not see that there
is any appreciable ability of that base to free itself of the base bending
moment by leaning over, as it is being held fairly firmly against the
footing by the axial tower load. If it actually did lean over at the base,
it would most likely overload a leg. This type of "pier pin" base
connection is the only one available for Rohn 25G, and is one of two types
available for Rohn 45G. 

The other base termination available for 45G (and larger sizes) probably
should referred to it as a "tapered pier pin" base. The 45G part no. is
45TG. The difference is that the bottom of that section looks like an
down pointed top section. The legs come together at the bottom and are
terminated on a small diameter plate with a hole in the center to sit over
"the pier pin." (the steel pin embedded in the concrete pier). This base
more freedom to rotate axially and torsionally than the other type. You can
find examples of this type of section in the Rohn commercial catalogue.


Stan Stockton wrote:
> On Kurt's website he makes the following observation referring to a tower
> that is mounted on a pier with the equivalent of a ball socket mount:
> "Putting the tower on a free base connection has allowed us to increase the
> antenna loads by 49%."

Be careful....
It is important to look at what's in the discussion in the full perspective
of what is being looked at and what is not. The cited statement means that
it took 49% more antenna load to generate the same combined stress (as
defined) at the base. That is not all that is going on. 
Some things not being considered there were the resulting loads in the
individual tower section members (legs & braces), or the global buckling
status of the tower spans.

The intent of the study was to look at relative behavior, not absolute
design, to show some basic things like the relative effect (and importance)
of guy selection (and its associated elongation), load distribution, and
base constraints. Think of it as taking a look at what the tower under
consideration does when the changes are made, not what you can do with any
old tower.

I apologize if that has caused confusion, it was not intended to do so, but
rather get people thinking about some things that had previously been
absent from the general discussion here. It's simply a "please think about
this" suggestion.


There are differences between what Hank posted today about his model and
what is found in the tower bizzo on my website. There are some substantial
differences between the load cases in the two, so they should not be
expected to say the same thing.

The tower in Hank's example appears to be limited by column buckling in the
lower span between the base and lower guy connection. When that is the
case, the potential benefit from simply freeing up the base connection
can't be realized. If the bending loads at the tower base are not the
limiting factor, improving them won't help. The tower span buckling problem
can be changed to make something else the limiting factor, and there are a
lot of other things to try.


Sometimes, trying to say anything about this stuff is a recipe for
disaster, because it is impossible to say everything, or enough to not
cause confusion. That's probably why we go so long between these things.
Hank has found that there's no difference between a pier pin base and a
buried one, so everyone should go with that.

73, Kurt, K7NV

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