Doug Renwick wrote:
> You guys get my goat. You can come up with all sorts of theories to not
> do something but you have no actual experience to support those
You didn't ask for experience. I was trying to point out why it's not a
> theories. Time to think outside that box. I have guyed self-supporting
> towers (5) for more than 30 years without incident or accident using
> 1/4" EHS ... towers that vary in height from 70' to 110'. We are not
> talking 300'+ towers here. You can theorize all you want, but my
> experience sends those theories down the drain.
Or proves you are lucky.
> I am also beginning to
> see that I am wasting my time trying to educate those who do not want to
> be educated.
Possibly those who have been working with this stuff for years feel the
"To me" there is nothing wrong with pushing the limits IF the one doing
the pushing knows the risks and consequences of what they are doing. I
would not want to encourage someone new, or who is not familiar with the
risks to do some of the things I've done in the past.
Oft times we can point to where something has worked for years, such as
the 50 or 60' 45G installed self supporting with a reasonable sized
antenna, or an American Steel TV tower on a dirt base holding a 5L KLM
20 meter monobander with only a house bracket at 14'. Some times things
work for years in spite of our ...transgressions against the laws of
physics and just down the road a properly engineered system goes down
when the first thunderstorm goes through, or the wind rips a TV antenna
and tripod that was lag bolted into the roof rafters while that 5L KLM
comes through unscathed. Like mounting the rotator at the bottom of a
tower. It's a good idea, it's convenient , and it works most of the
time. But there are risks to the entire system such as resonances that
come with such a simple change. It's quite common to see hams using the
old American Steel TV towers far taller than they were ever designed to
go. I've done it. I've also seen the bottom two or three sections swaged
into each other to the point where it took a jack to get them apart.
I've seen the philosophy where 3/16" guys are good so 1/4 inch should
allow me to go with a much larger antenna system. Over stressing towers
by over guying is quite common in our world.
As I said, I've seen powerlines downed due to relatively mild, but
steady winds of a speed to create oscillations. I had one guyed wood
mast destroy itself due to oscillations and I've removed a self
supporting Heights tower holding a 5L KLM 20 meter monobander from a
friends living room ceiling (after it went through the roof). I've been
to the top of my tower in winds of 30 MPH and I've been on much smaller
towers that got to scary to stay on at a bit over 20.
I've flown airplanes that had the well established airfoil of the wing
modified to symmetrical so he could use larger tires on the retractable
landing gear for landing on dirt strips. The outboard section had the
angle of incidence changed to compensate. Not a good idea but it
worked. I've also flown a well modified plane that had a large engine
and tiny tail. Stick forces were non existent making it feel like a
computer joystick with no springs. Neither were good ideas, or proper
engineering practice but they worked in spite of that. Although the
owner of the second plane killed himself in it just a couple weeks after
I flew it.
I use the aircraft analogy because it also applies to towers. There are
many times we get by in spite of what we do, rather than because of what
we do. How many times have we seen the guy wire wrapped around a tower
leg, or simply attached to a tower leg. The forces on a tower go up
rapidly with height. You can "get by" using fence posts for guy anchors,
Radio Shack guy wire, and a dirt base on a 40 footer with a small
tribander on top. I'd not recommend it, but I've done it and it worked.
The same would not not be close to safe on a 60 footer. So 70 to 120
feet is getting into the realm of requiring proper engineering. Here
you'd have to present the engineering papers for anything over 120
feet. The ROHN catalog is accepted as long as you don't deviate.
So, as I said, I have no problem pushing the limits, but I'd not
encourage any one else to do so and any time you do something outside
what the manufacturer says to do puts you into uncharted territory that
can be almost as dangerous as those modified airplanes.
> "Those Island days are always on my mind,
> Someday I'm going to leave it all behind."
> -----Original Message-----
> From: firstname.lastname@example.org
> [mailto:email@example.com] On Behalf Of Roger (K8RI)
> Sent: November 4, 2008 1:27 PM
> To: Doug Renwick
> Cc: firstname.lastname@example.org
> Subject: Re: [TowerTalk] Guying a self-supporter
> Doug Renwick wrote:
>> Where is it written that a self-supporting tower has to be guyed with
>> the same tension as a guyed tower? Why can't the guys on a
>> self-supporting tower have no to little tension? Think about it.
>> guys can be there only as a secondary method of supporting the tower.
> Guying is not a simple construct. The guys become part of the structure
> and add their own set of limitations along with the help they give.
> Guys at no tension will add practically nothing to the installation
> except weight and when using EHS that can be substantial. Actually with
> EHS it would be impossible to add them without a fair amount of tension
> just due to that weight and pulling them out to the anchors. They would
> also have a huge catenary which would make them subject to low frequency
> oscillations exerting substantial force even in low winds. In that case
> the tower would probably be far better off without them. Also the tower
> reinforcement must be in a manner to withstand side pull on the legs. A
> number of towers are not constructed with structural allowances in mind
> for a side pull.
> Guys have two modes of oscillation. A high frequency and low frequency
> mode called "galloping". "I think the high frequency mode is called
> singing or ringing. The high frequency mode is handled by "dampers"
> attached to each line. Again, IIRC the makers of Phillystran recommend
> dampers on all guy lines over 100' in length. The low frequency mode is
> IIRC handled by guy size and and tension. There is a LOT of give to
> lines with deep catenaries. The deeper the catenary the more give.
> Should lines start oscillating in phase this can exert thousands of
> pounds of side force. Enough to do a lot of damage. Out here in the
> flat lands I've seen large wood power poles snapped off by steady,
> sustained winds in the 30 MPH range when the lines hit the resonant
> point long enough to build in amplitude. Gusty winds didn't stay at the
> required speed long enough for the amplitude of the oscillation to build
> to the point of doing damage.
> Hopefully we have a structural engineer on here who can explain this
> Guy tension is set at 10% of the working strength, so going to the 1200#
> test Phillystran would give 120# per guy or 360# per level.
> Basically I'd bet that attaching guys to most self supporting towers
> would negate the insurance. They'd see it as trying to beef up an
> overloaded tower. Otherwise, why add the guys to an already sturdy and
> well designed structure.
> As an added note, most ham towers I see do not have the guys attached
> properly and some towers do not even make provisions for doing so. ROHN
> towers tackle this by putting a very sturdy bracket _around_ the tower.
> The guys attach to the bracket which absorbs ALL of the pull rather than
> the legs of the tower. So with the ROHN installation. The only force
> exerted on the tower from the guys is the downward component if all
> three guys are tensioned the same. (of if the guys do not get into the
> low frequency mode of oscillation)
> IF I were going to add guys, which I wouldn't, I'd use guys of low mass
> such as Phillystran, but this still puts the builder into uncharted and
> unengineered territory. In many areas you'd need those engineering
> specs and I doubt they'd be available. It'd sure be interesting and I'd
> like to see a set showing just how the addition of guys would affect the
> characteristics of the tower.
> Roger (K8RI)
TowerTalk mailing list