To all;
It seems this came up a year or so ago. I any event I dug out my typical
tower simple FEA Model and revisited the results. The model assumes a 70
foot guyed tower. It is guyed at 35' and 70' with a 10' cantilever. I
used a uniform load of 25 pounds per foot along the total length for a
simulated wind load on the tower section and a 300 pound antenna load at
the end of the cantilever.
This is a very simple model as the guy points are fixed, that is the
tower at the guy points can't move but can rotate. In the real world the
movement of the guy points reduces the moment that is generated in the
tower structure. This simplifies the model, but the model indicates what
happens when the base is fixed ( embedded) or is pinned ( pier pin).
With the pier pin base there is no moment developed at the base. With
the fixed base, moment can be developed. The fixed base reduces the
maximum moment in the tower sections above the base due to its ability
to develope a moment. This holds true except for the top cantilever
above the top guy point where in either case the moment is the same.
When the base is pinned, the moment above it in the tower increases. In
this model the increase is about 175 percent. Don't dispare the actual
magnitudes are not anywhere near the capacity of the assumed tower
section. The pier pin base having zero moment allows for a higher
allowable axial load at the base. However this higher allowable axial
load is based on the local buckling of an individual leg member. If one
looks at the tower as a column between the base the the first guy, etc.,
the allowable axial load is less than that of an individual leg due to
the longer unsupported length. For all practical purposes, for this
model there is no difference in the maximum allowable loading for either
the pier-pin or fixed base.
I see no difficulty in plumbing an embedded section, just plumb with a
level and use diagonal braces to stakes to keep it in position, like you
do when you set fence posts in concrete. As far as getting a level
surface for a pier pin base, use grout to set it level after the
concrete has set.
I agree with Steve on the practical installation issues with a pier pin
and the need for temporary guys. It is easier set sections to the first
guy point with an embedded base.
The movement of the pier-pin base with torque is what you want to see if
your tower is developing torque and wasn't designed for it. That is one
of the main reasons for a pier pin base. The torque developed is
translated into forces in the guys and not in to tower section where it
could add to the existing loads and cause local failure of a diagonal
member in the trussed type tower.
As I think I said sometime back, you can use either in most amateur type
installations. I have engineered both types, it really depends on the
the tower owner's preference.
The issue of side mounts and the torque they develop is quite real. Dick
Weber, K5IU, has written a paper that will be published soon that
discusses this very item. In short if you have side mounted antennas you
should think about adding, if you haven't already, the Star or double
guy bracket system that has two guys per guy point at the level of the
bottom of the side mount. This allows most of the torque developed by
the side mount to go into the guys and not into the tower section and
possibly causing a local buckling failure in the tower diagonals.
As far as the pier pin allowing for the equalization of the guy tension
is concerned, I don't see how this is true. The pier pin will allow the
tower to rotated due to misalignment of the guys from the 120 degree
plan layout but will not equalize the tensions. The only way the
tensions in all the guys at the same guy level can be equal is that they
are all aligned in plan and in elevation the same and have be
pretensioned the same. I have engineered installations where the guy
points are at different distances from the base but aligned in plan at
120 degrees and they never will have the same tension in them. The
system will be in equilibrium but the guys will have different tensions.
The initial prestress will be different since you also want the tower to
be plumb.
At this point you all are most likely bored this thread, so I cut it off
here.
Remember you can do either, just make sure you know what the forces are,
and what forces the tower system can take.
73
Hank Lonberg, P.E. / KR7X / Member of IH9P - Pantelleria Contest Team /
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