[TowerTalk] Following The Prime directive

[Roger (K8RI)]

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)