First I am not suggesting any one violate the prime directive (Follow
the manufacturers directions) but:
Following the ROHN figures gives a set of standard figures for a
compromise on a flat surface in standard soil conditions that you can
present to a zoning board, or another engineer with confidence and they
are repeatable. Whether the zoning board will accept them is something
else. If you deviate from those figures, the calculations need to be
redone if the strength listed needs to be maintained or increased.
Yes, you can create a stronger installation than what you end up with
using the ROHN figures, BUT only if you know what you are doing, or the
engineer you hire knows what he is doing. You can easily increase the
ability for the top of the tower to handle higher wind loading by moving
the guy anchors for the top tier out farther, but you are adding weight
with a longer guy(s) with a deeper catenary and more of a tendency to
oscillate. Everything in a tower is a compromise. Any change and I do
mean any change right down to changing the bolts in the tower legs,
using a anti-seize compound, or changing guy structure.Never use a
drill, or turn the bolt into a hold that is too tight for it. Use a
taper pin punch so enlarge the hole until the bolt fits. the punch also
makes a good alignment tool.
Looking at the books the distance between guy tiers is different. For
instance if the tower between the middle and top tier deflects in the
wind (and it does) the section below the middle tier will deflect in the
opposite direction. If they are both the same length an oscillation
could develop under the right conditions while the pier pin base serves
as another pivot point. Having the spacing between the middle and top
tier different than the middle and bottom *tends* to dampen this
out...BUT Guy tension also has an effect on this as it allows the pivot
point (guy tier anchor point on the tower) to move laterally. This can
have dramatic effects not only on tower oscillation, but loading
capacity as well. At some tensions, loading, and guy spacing it *may*
dampen oscillations that would be developed at given wind speeds and at
others it would accentuate them. Wind loads change with tower height.
The structure taken as a whole has to be designed to handle the varying
winds from the ground to the top and that includes antenna loads. Side
mounted antennas add torsional forces that also can cause oscillations
and these too are affected by guy tension, mass/material, rigidity,
deepness or catenary, and cross sectional area. Like the tower sections
the guy tension is likely to have an unknown effect on torsional moments
created by side mounted antennas and the rigidity of those antennas. A
21 foot tall, side mounted, duo band vertical can really whip around.
Adding a stabilizer from the tower to the antenna at *roughly* 2/3 to
3/4 of it's length can help stabilize an antenna, or if the antenna is
flexible enough it might induce higher frequency oscillations.
A tower has to be designed for multiple dynamic loads to handle
twisting/torsional, expansion, and compression for values that are
almost always changing and they are different at different points on the
tower. Wind is a wide variable as it normally gets stronger as you go
up in height. While it might be 5 mph at shoulder level it may be 15 to
20, or even 30 at 100 feet. If you've ever been on a tall tower when
the wind came up this holds a special meaning.
I have a bit of a problem with some of the calculations showing tower
deviation with specific guys as I've been "on top in some pretty strong
winds and could feel no movement. OTOH I've been on far shorter towers
in less wind where the movement was quite evident of the kind that
causes finger prints in the steel.
As I said earlier, tower designs are a set of compromises. I've heard
airplanes described as a set of compromises flying in formation. A
tower is very much like the airplane and if either fails while you are
in or on it the results can be pretty much the same for you or people on
the ground.
This means a great many things were taken into consideration when that
tower was designed and those figures are reflected in the ROHN handbook
which strictly applies ONLY to those models in the handbook. That many
of these specifications *MIGHT* be applicable to other towers is of
little concern as we can not show they do in most cases. At least if you
do not have the background, or help to do the new calculations.
The following is not an endorsement or encouragement to do things other
than in "the book", go outside the figures and you are literally on your
own.
1. Does following the ROHN specifications for guys give you the
strongest installation? No, but it gives you an engineered and
documented set of specifications.
Moving the anchors out for the top guys which reduces the included
angle can enhance the tower's ability to handle wind loads, BUT I'd
recommend against it unless you are an engineer or have one available to
do the calculations. It's not quite as simple as it sounds as the
longer the guy the more likely you'll end up with a resonance and an EHS
guy 130 feet long resonating can develop tremendous forces. Put two more
the same length at the same level and they can tear a tower apart and
particularly if they are not tight enough. But it's rare for such a
thing to happen. However unless you follow "the book", or have a set of
engineering figures the insurance company might not be all that happy
even though your figures show the longer guy with the lower included
angle *should* make the installation stronger. Unfortunately, few of us
have the tools to figure out what it'd take for a guy line of any
particular mass at any given tension under any given wind load to
resonant as part of the system.
Another point is 60', 80', 100' and taller towers are different animals
than the typical 40 foot tower on a fold-over dirt base. What doesn't
even merit consideration with the 40 foot tower can quickly become a
critical item as we increase height. With the proper soil a dirt base
will work as well with even *some* 60 footers as a concrete base, but I
sure wouldn't want to push my luck beyond that. The base on a guyed
tower only serves to keep the tower from sinking into the ground as that
makes keeping the guys tight a real problem and it prevents the base of
the tower from moving sideways. The first depends on the soil and base
design, but the latter takes very little to keep the base from moving
sideways. So a dirt base will work just fine in the proper soil ..."up
to a point". ROHN used to make one, but I have an idea some customers
pushed them beyond "that point"
Elevated guys are another issue. Sure you can do it, BUT a 16' steel
pipe set 6' into the ground into a 4 or 5 yard block of concrete has a
tremendous over turning moment applied to it and it will do albeit, very
slowly if not back guyed. When set in clay they seem rock solid, but
clay flows like water. Its just it has a much higher viscosity.
Remember even window glass appears as a liquid.
But except where required by code few of the average ham antennas are
properly engineered, or is thought even given to the capabilities of
said tower. Just remember that every change affects the dynamics of the
entire tower and antenna system. I once had a ham ask me to take a look
at what he was planning for his VHF installation with a base mounted
rotator and large array at the top. The "shaft" he was going to use was
2" chrome moly with a 3/4" wall inside a 120', 25G. Now at the time I
weighed only 165, but was doing a lot of body building workouts and
weight lifting. When I went to pick up one of the shaft sections I could
not lift the one end of it. Just supporting that at the base was
exceeding the tower's ratings for vertical load. The torsional forces
would have been tremendous, particularly if the brake was engaged before
rotation stopped. No way would I make fun of or criticize him as he has
the common sense to ask about it.
Most ham towers are roughly 40 feet and at 40 feet the usual question is
"what do I need", but that question can eventually be rephrased into,
(and often is) "what can I get away with" although that though seldom
enters the builders mind.
I've seem 5L KLM 20 meter beams and 6L 15 beams with 42' booms on 40' TV
towers, (not the same towers) guyed with the old, open wire galvanized
steel telephone wire. (It's roughly 1/8" in diameter) The guy anchors
were nothing more than 9' steel fence posts driven into the ground at a
right angle to the guy wire. The guy wire was just wrapped around the
base of the fence post and folded back on the guy line. Now other than
the hazard of some one impaling themselves on the end of a rather frayed
out steel fence post (hitting them with a 12# sledge hammer tends to do
that) There were no real problems. Not only did they work, but they
stood for over 10 to 12 years. The also stood with no problems after we
had a major ice storm back around 76 that took out miles of power line,
and one wind storm that took the TV antenna right off the roof.
So, yes it can be done and I'd not hesitate to climb those 40 footers to
this day although they have long since been replaced. I'm familiar with
them as I put them up. Those were the materials available so they were
what was used.
Yes, if you know the risks, to climbers, to people and materials on the
ground if it comes down and what the installation can stand. You can
get away with a lot in spite of the installation or the lack of
knowledge which is often accompanied by a lot of luck. You can just
plain get away with a lot when using a relatively small antenna on a
relatively short tower, but don't accept that as a blank check for
anything taller or larger.
Those things that the average ham "just does" without a thought are not
things we can get away with when things get larger. Matter of fact the
rules get a lot tighter, very fast as the tower height increases. Just
50% higher at 60 feet and the builder better be paying attention to what
"the book says". But 60 feet is still not "critical" in most cases
unless the tower is small and the antennas are large. When you get to
100 feet and more there's not a whole lot of "wiggle room" left for
changes although we do still have choices such as regular guying, star
guying, and alternative materials for guy lines.
Yes, we can "get away" with a lot when using smaller tower and antenna
installations, but if it's not in "the book" like the airplane with
changes we have entered the experimental realm with no guiding numbers
to make zoning boards or our insurance companies happy. Nor are they
anything to give a tower climber a warm and fuzzy feeling.
Actually I've not only seen them, I put them up. They were mine and BTW
I used the same technique for a 7L Wilson 10 meter Yagi on a 39' boom.
Would I do it today? Probably in the same location if those were all the
materials I had available although I'd just as soon be 50 years younger
too as I was back then. However if I had the access I have now I'd
install them the way I do now, not then. Would I deviate from the "Prime
directive"? Only if it seemed advisable and you can be sure the stresses
would be calculated. In any "non standard" installation there are too
many unknowns to catch them all, so the tower and antennas would have to
be in the clean where they could not fall on any one or any thing
important. In computers those unknowns that would likely pop up are
called side effects. IE, unexpected results from changes.
73
Roger (K8RI)
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