[TowerTalk] FW: Guy article in CQ

[Hank Lonberg]

This is a somewhat long and maybe winded response and violates
the concept of snipping the previous posts. If you are not
interested you should hit delete now.

Tom, good call, as this is the foundation of vector static
analysis of physical systems. However, the appropriate Laws
are Newton’s First and Third Laws of Motion (see below).

>WC1M said:
><snip>
>I agree that the tension must be the same in all three
directions, but not
>only in magnitude. It must also be the same for the
horizontal and vertical
>vector components of the force along each guy wire.
<Snip> 

>The horizontal force components of a stationary tower guy
point where the three guys intersect is always zero. If they
were not, the tower would be moving. F=ma 


.









.
>Tom - N1MM


Newton's First Law of Motion:
I. Every object in a state of uniform motion tends to remain
in that state of motion unless an external force is applied to
it.  

Newton's Second Law of Motion:
II. The relationship between an object's mass m, its
acceleration a, and the applied force F is F = ma. 
Acceleration and force are vectors (as indicated by their
symbols being displayed in slant bold font); 
in this law the direction of the force vector is the same as
the direction of the acceleration vector.

Newton's Third Law of Motion:
III. For every action there is an equal and opposite reaction.



>Roger K8RI said:

Roger: No personal affront intended but there are some
confusing concepts at work
..

>


.
>Guy wires serve one purpose. That is to keep the tower
vertical, 
>although you could within the same vein say they also prevent

>oscillations and need to be spaced such as to prevent said
oscillations.

True guys hold the tower up and transfer the lateral loading
subjected to the tower to the ground.
They have little to do with preventing oscillation in the
tower structure. The maximum guy spacing on triangular trussed
towers
is based on full tower section Euler buckling not on tower
resonance.

>To operate properly the wire angles should be such that "all
in any 
>particular tier" have the same vertical and horizontal vector
tension.

As per Tom’s comment above what needs to occur at the tier or
common point is that the summation of the horizontal vector 
components of the guy tensions equal zero or the common point
is going to be moving in the lateral direction, not a good
thing. 
The sum of the vertical vector components is directed into and
resisted by the tower legs.
 
>With EHS wire which is *heavy* length is also important. Even
with 
>Phillystran the length does not want to be long enough that
the natural 
>resonance of the guy is any where near a resonant point in
the tower. As 
>the tower is guyed at several heights with variable
spacing/distance 
>between the tiers the tower will have several resonant
points. These 
>points will not only depend on the natural resonances in the
tower using 
>the guy points and base as pivot points for the bending
moments. However 
>the ability of any of these points to move due to both the
elasticity 
>and catenary of the guys will add (or subtract) from or to
these natural 
>resonant frequencies of the tower sections.

You are getting yourself confused. The guys will not force
vibration or excite the natural frequency of the tower
sections. 
Calling “EHS” heavy is a bit of a stretch. ¼” EHS weighs 0.100
lbs per foot, 3/8” EHS weighs  0.237 lbs per foot. 
Rohn 25 tower sections weigh 4 lbs per foot. Phillystran even
weighs less. The weight of the guys are negligible in
contributing to the forces 
in the tower in the normal range of height of amateur towers.
TIA-222-G doesn’t even concern itself with potential guy
vibrations in guyed 
towers under 1200 feet in height. Don’t be fixated on guy
vibration and apparent mass.

>Based on the mechanics involved I see little problem of using
longer 
>guys when using Phillystran, at least to a point. IOW I see
no problem 
>of using 45 degrees as the guy angle for each level. Of
course this 
>means a guy anchor on the ground for each guy which could be
a bit 
>inconvenient.

This is personal preference and makes little difference to the
system behavior other than adding to the cost of the system
installation. 
Phillystran is a direct replacement for comparable strength
EHS;  with the added benefit of transparency to RF which
minimizes the 
disruption of the radiation pattern of the antenna system.
Important for vertically stacked antenna systems on a tower.

>Usually the top guy in any installation is on the order of 60
degrees. 
>Changing that to 45 degrees should reduce the stress on the
tower as the 
>translation from horizontal wind load to pul on the guy is
considerable 
>as well as to the load on the tower in the vertical axis.
That would 
>make the center tier much less of an angle and as long as
there isn't 
>too much mass in the catenary to create a resonant problem it
should 
>work, but in this case the weight of the guy line translates
to a 
>lateral (sideways) pull on the tower. Hence a good idea to
use the guy 
>bracket system that puts the tension on the mount and not the
tower as 
>the tower more or less sets inside the bracket. With a 3 tier

>arrangement this makes for a very low angle on the bottom
tier. Again 
>if using a guy bracket this puts little stress on the tower,
but guy 
>resonances should be a concern if of EHS.

The top guy angle (measured from the horizontal to be clear)
can be just about anything but horizontal (0) or vertical
(90). 
The change in guy angle from 60 to 45 would increase the
horizontal component and reduce the vertical component. 
This would, all things being equal, decrease the downward
force in the tower section at that point but would increase
the tensions in the all guys, 
at that tier, due to equilibrium requirements. The mass of the
catenary and resonant concepts do not apply, 
and only confuse how the tower support system behaves. Guy
brackets are always good advice. 
They provide a mechanism that applies the vertical guy forces
to the tower section in a more uniform way. 
I am thinking of the practice of looping the guy around the
leg at a horizontal brace versus the bracket. 
The guy bracket also allows for the uniform distribution of
the lateral forces (remember equilibrium) to the other guys
sharing the lateral force component. 

>There is a lot of lattitude available when it comes to the
distance from 
>the tower to the guy anchor base, BUT depending on the mass
of the 
>guy/material I'd not go far without doing a stress analysis
on the 
>tower. Why I favor the Phillystran is the light mass puts the
resonance 
>for even relatively long runs well above any resonances in
the tower 
>itself of the kind that are likely to do damage. Longer runs
of EHS OTOH 
>are massive and likely to develop resonances that can be
destructive by 
>themselves. I've seen power and telephone lines oscillate to
the point 
>of snapping off the power poles.

I reiterate that the mass (weight) of the guys do not
determine anything. The use of EHS or Phillystran, 
Fiberglass rod or whatever should be based on the breaking
strength, elastic creep, transparency to RF, safety to
abrasion, vandalism and cost. 
I do agree that for non standard installations a full system
engineering analysis should be done. 

>Just East of Alma MI on the North side of Lincon road is a
long run of 
>telephone and cable. Quite a few years back (just shortly
after being 
>installed) we had some strong winds that caused the lines to
oscillate. 
>I saw them moving over 10 feet between the poles. Later that
day they 
>actually busted off a number of the poles. When you go
through there 
>now there are some triangular fittings or wings attached and
the 
>resonance has changed enough the cables move very little
although it's 
>still noticeable.I would expect the same phenomena with a
tower guying 
>system. 

The phenomena that you witnessed is called "Galloping" in the
power transmission industry. This is characterized by low
frequency-large amplitude displacement. 
It really only occurs in areas with icing and it is a function
of the span length and diameter of the line. 
It also requires that the line be fairly horizontal. The ice
builds up the diameter of the line and due to its long length
to diameter (including ice) results in 
classic vortex shedding and the large displacements
perpendicular to the direction of the wind.
The other phenomena that occurs on long span to small diameter
lines is Aeolian Vibration which is characterized by high
frequency low amplitude vibration ( think humming). 
This vibration usually occurs at relatively low wind
velocities. Both of these are usually mitigated with line
dampers,either mass or length change, which detune, if you
will, the line. 
Amateur tower guy lengths are not usually in the range of span
lengths were galloping occurs. You may hear “humming” which is
indicative of Aeolian vibration 
but is not of any real concern unless the noise becomes an
issue.

>So, although I don't see any problem going longer with guy
lines within 
>reason, as soon as you get outside the manufacturers specs
it's time to 
>do, or have an analysis done on the system which is not at
all simple. 
>On aircraft with smaller parts we can do what is called a
vibrational 
>analysis and come up with a dynamic model that will give an
upper speed 
>limit for a particular design as configured. It also lets us
rebalance 
>control surfaces to eliminate resonances which show up as
flutter. (a 
>control surface vibration, or oscillation with positive
feedback - not a 
>good thing)

If you vary too far from the published specifications of the
manufacture (LXC prime directive), then competent analysis is
a prudent idea. 
Aircraft surfaces and aircraft themselves operate in “Reynolds
Number” regions far larger than tower guys. The velocities are
significantly greater 
and the phenomena that occur (flutter for one) are not at all
applicable to amateur tower support system design.

>73

>Roger (K8RI)

This is not a personal attack on Roger or anyone else who has
posted on this subject but is an attempt to educate and
increase the knowledge and confidence of those individuals who
are building or thinking of building cost effective and safe
amateur tower installations.

I hope I have cleared up some miss-conceptions and added to
the knowledge of those on this reflector.

Regards
Lonberg Design Group, Ltd.
H.S. Lonberg, P.E.,S.E. / KR7X
President