[TowerTalk] Guyed + self supporting /2 ??

[Roger (K8RI) on TT]

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)