On 10/15/2014 1:28 AM, Richard (Rick) Karlquist wrote:
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.
This is not a fallacy! Originally towers were designed to be strong
enough that this would not be a problem. If properly guyed, it's still
not a problem. With the proper guys there is very little bending moment
at the base.
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.
Most pier pin bases as installed, address the torsional forces. To
address the bending moment they have to be mounted on an elevated
point. A that flat base, with a pin through the center does a poor job
of addressing the bending moment as the up wind side must lift off the
concrete. With the thousands of pounds of guy force pushing down on the
base, in general the base is unable to do this.The base of the tower
moves very little in the angular motion. Properly guyed, it's less than
a couple of degrees. Torsional forces OTOH can be substantial if the
tower is not properly guyed. Star guying is the answer to minimizing
the torsional forces
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.
That is a complicated issue, but the guys will even out the forces on
the base from wind, but they will INCREASE the load on the base. because
of their tension and wind load.
In general, self supporting towers should not be guyed. Guying reduces
the design load limits of the self supporting tower. Crank up towers
should not be guyed as it can greatly increase the load on the tower cables.
However, it will increase the downward force on the upper sections,
If you increase the load on the top you increase the load on the bottom.
which don't experience much force normally, and it could cause
them to buckle. If the tower is of uniform cross section up
Guys should NOT attach directly to the tower legs! This weakens the
tower, by applying a lateral force to only one leg. ROHN has a guy
attachment saddle that fits AROUND the tower. The saddle absorbs
lateral forces, while vertical forces are transferred to the tower. Look
at the ROHN site for proper guy attachment.
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.
Again, there is relatively little bending force applied to the base.
Torsional forces from the antennas and rotator can be substantial unless
mitigated with star guying.
Guy forces add with the top sections receiving the least force. They
may receive the most wind, but the guys absorb that force by a
combination of themselves AND translating to a downward force on the
base. There is a tiny tilting motion, but the guys keep that to a very
small number, Simple trig will show a substantial down force added both
to the up and down wind sides. A tower guyed at 3 levels will have all
the guy tension in the downward direction added. Additional force from
the wind will also add to that load on the base.
The tension of the guys adds to the vertical load/.
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.
Is the tower base designed to support the additional weight? Guying the
base is the same as adding additional weight to the tower.
When changing the installation of a tower to something different than
the manufacturers recommendations, it's wise to seek that companies
input as well as a certified engineer.
73
Roger (K8RI)
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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73
Roger (K8RI)
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