On 10/14/2014 12:31 PM, Patrick Greenlee wrote:
We talk about guyed towers and their minimal needs for big heavy bases
vs. free standing towers with enormous bases. I follow these arguments
and basically agree with the "standard" prescriptions but wonder about,
as the thread subject sort of indicates, the possible benefits of a
hybrid design, i.e. splitting the difference.
If the tower in question was designed to be free standing but was guyed
at the traditional lower guy level wouldn't we get the following:
I have been doing a lot of modeling of this situation. Depending
on the details (as always YMMV), the wind loading of the tower itself
can cause it to try to tip the base because the part of the tower
below the guys bends in a strong wind. This is the fallacy involved
in the idea of using just enough base to keep it from sinking into
the ground. The fix for this is to go to a pier pin base. Then
when the tower bends, it just pivots. You still need enough base
to keep it from being pushed sideways.
The other issue applies to a tapered tower. If you add guys to
a self supporting tower, it will reduce the stress at the bottom.
However, it will increase the downward force on the upper sections,
which don't experience much force normally, and it could cause
them to buckle. If the tower is of uniform cross section up
to the guying point, then you will be fine as long as you spread
out the guy force over more than a single point on the leg by
using a stiffener over the leg where the guy attaches. In general,
at least with the tower sections I have, the smaller sizes aren't
worth fooling with in a guyed version, as they always become the
weak link. Or else, have a hybrid design that only tapers above
the guy attachment point.
On a crank up, you can guy just the first section and use a much
smaller base, but then you need really strong guy anchors, so
there is no free lunch. Also, the usual tilt over fixtures
won't work since the whole base, tower, and tilt fixture will
fall over as a unit.
So I will not make a blanket statement as we often hear
"never guy a self supporting tower". It's just that it may
not be a good "value proposition" as they say.
Rick N6RK
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