I promise that this is it for me on this thread. I would like to thank
all that provided input concerning my post. You made me revisit my
resources and undergraduate references to form a reply that I hope
clears up the confusion concerning pinned vs fixed connections.
You must remember that the engineering models used by structural
engineers to define real world structures and their behavior under loads
is based on some basic assumptions concerning the members and how they
are connected to each other.
>From Laursen, Harold I., Structural Analysis, McGraw-Hill, 1969 New
a pinned connection is one where no moment is transferred from one
member to another.
What this says that for a connection to be fixed there has to be a
positive transfer of the moment from one to the other through the
connection. In reality a couple comprised of opposite forces separated
by a distance forms to react against the moment force.
In the case of the embedded tower section the legs are positively
connected and continuous into the base foundation. A force downward in
one or two of the legs plus the force upwards in the other form a couple
, forces in opposite directions separated by a distance, to transfer the
moment into the foundation. This meets the defination of a fixed
connection and has the ability to resist moment.
The flat plate, not tapered base pin, pier pin shown in Rohn catalogs
does not allow the transfer of moment into the foundation by the
formation of a "couple." This connection to the foundation concrete can
pass axial force from the compression load of the tower as a column, can
pass shear load due to the pin, the plate is positively connected to
the foundation for loads in the horizontal direction. It is not
connected to the foundation in such a way as to form a moment resisting
couple of forces... my friends it is by all structural modeling and
analysis viewpoints considered a pinned connection.
The concept of the torsion resistance of the plate due to friction of
the plate on concrete is valid up to a point. The static coeficient of
friction of steel to concrete is 0.4, Us=0.4.
The plate resistance to movement laterally, either sliding or rotating
is: F=Us x P. It is a direct function of the tower axial load on the
base plate. Based on 7000 lbs axial load in the tower, not uncommon a
70-90 foot 45g tower in 90 mph wind, there would be 7000 X.4 = 2800 lbs
of resistance to sliding, but the pier pin is there for that. The
torsional resistance is a little more complex to calculate but it is
approximately in the range of, with calculating, about 2000 ft lbs plus
or minus. This is a force times a distance so it can be considered
partially fixed or a spring. However it is not linear if the torque on
the tower exceeds the 2000 ft-lbs then all of a sudden there is no
torsional resistance and the tower base rotates also it is a function of
the downward load. As you can see this is not a simple concept, best to
minimize the torsion through the use of guys and not the tower or base.
The use of a UHMW plastic plate at the steel concrete interface is
something to consider to act as a low friction washer might be a good
idea if one really wants to allow for positive rotation of the base.
Enough of this, as you can see a pin is a pin and a fixed base is a
fixed base as all can plainly see.............
Hank / KR7X
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