How does the presence of a 1/2 to 1" thick coating of ice on the tower
base and plate affect the ability of the plate to rotate?
It seems that kind of condition is exactly when the added safety
factor is needed.
It isn't a rare event in some parts of the country.
73 de Brian/K3KO
Hank Lonberg wrote:
>
> All:
>
> 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
> York, p.20
>
> 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.............
>
> Ciao
>
> Hank / KR7X
>
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for information on our fabulous Trylon Titan self-supporting towers - up to
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