Tom,
Self-supporting crank-up towers should never be guyed. When the wind blows,
the lateral force is translated downward through the guy wires. This adds a
new load to the supporting cable that it was never designed for. Typically
the cable will break first, although sometimes one of the tower legs will
buckle first, as it is being stressed by the additional downward force
beyond its design as well. The weak anchors for the guy wires are totally
beside the point.
Never guy a self-supporting crank-up!
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Tom Hellem
Sent: Monday, October 14, 2019 6:13 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Tower Guy Calculations
Thanks to all who responded and for the link to K7NV's excellent work.
What prompted my original question was a visit to the home of a member of
our local ham population and upon learning of his proposed installation. A
description follows:
The tower appears to be one which was originally designed and sold as a self
supporting approx. 55' tower, consisting of two telescoping sections that
nest down into a fixed base section, all of steel.There is no "z"
bracing like what is found on a Rohn type tower, only horizontal slats every
14" or so. Raising and lowering is accomplished with a "boat winch"
type device, a few pulleys and what looks to be 1/8" aircraft cable. I do
not know who the manufacturer was. The op is using this tower in a guyed
configuration, with guys (1/8" aircraft cables) attached near the tops of
the 2 movable sections and the base section fixed near its top end to the
roof overhang of the house with a flimsy piece of 1-1/2" aluminum angle. 4
of the guys are anchored to 1/2" screw eyes that are threaded into various
framing members (of unknown configuration) of the house's roof structure.
The screw eyes are not the forged variety where the eye portion is
continuous, but the Home Depot variety where the eye portion is simply bent
into a loop. He is proposing to mount a rather large multi-monoband beam,
similar to C31-XR, onto a 2" mast about 4' above the top of the tower.
There are 13 elements on a 30' long, 3" diameter boom.
After looking at K7NV's info it seems safe to predict that the guy loads
might well approach 3000 lbs or even more on the top section. I am very
concerned with both the ability of the screw eyes to remain closed and with
their resistance to being withdrawn from the wood that they are screwed
into. I am also concerned with the strength of the cable that is used to
crank the tower sections up and down, given that is what holds the entire
assembly in the fully extended position and the fact that there is
considerable downward pressure on it when the wind is 60 degrees off the
line of a guy.
Does anyone else share my apprehension about this setup or am I overthinking
it? I'm trying not to sound like an old nag with this fellow and I think
he's getting tired of me preaching at him, but I sure don't want to see his
new beam laying in a crumpled heap on top of his roof, or worse.
Input and suggestions will be appreciated.
Tom
K0SN
> What really goes on in a guyed tower is pretty complex. A simple
> static analysis might be possible with a spreadsheet, but not a
> realistic analysis IMO.
One could probably get within 10% for a simple system, where you assume a
single guy, rigid (not flexible) bodies, equivalent flat plate areas for the
tower, and antenna. That's basically trig, with the complexity of 3 guys
(as the wind blows from the direction of a guy, the tension increases on one
and decreases on two)
Where it starts to get real tricky is when you have multiple guys attached
at different heights. And you're not going to get is a good model of the
flexing of the tower, the loads on the tower structural members, etc. AND
it's going depend a lot of some good quality estimates by the ham of drag
areas.
That might meet the OP's original request of "Does anybody know of a tool
for calculating the forces associated with sizing guy wires on a tower?"
>
> Kurt K7NV a long time ago did a Finite Element Analysis of a
> simplified tower structure using the standard Rohn section properties.
> His model is not a detailed model of the actual lattice construction,
> hence failure modes are coarse approximations. His website has that
> analysis last time I looked and it is quite instructive as to how a
> guyed tower behaves. k7nv.com
http://k7nv.com/notebook/towerstudy/towerstudy1.html
>
> Recall tower axiom #1: Follow the tower manufacturer's design unless a
> PE provides an analysis. If what is wanted is different than the
> catalog designs, then it is time to hire a PE. Many configurations are
> possible that are not in the catalogs.
>
> Unfortunately, two PE's I have used are refusing amateur radio tower
> analysis jobs because too many hams don't implement to the plan, or
> don't want to pay the fee, or want to argue with the numbers. The
> hassle, cost of the required software, and liability risk aren't worth it.
Interesting, but not surprising.
The PE has to worry about defending the lawsuit, even if the ham didn't
follow the plans, but used them to get the building permit, and then later
overloaded the tower. Your wet stamp is on the plans and that's the *first*
place they'll come to when something bad happens.
>
> Grant KZ1W
>
> On 10/13/2019 7:51 AM, Tom Hellem wrote:
>> Does anybody know of a tool for calculating the forces associated
>> with sizing guy wires on a tower? It feels to me that this would lend
>> itself rather easily to a spreadsheet where one could enter the
>> variables of his installation and the spreadsheet would spit out the
results.
>> I found a few rudimentary calculators on line but they don't seem to
>> quite take it all the way.
>>
>> Any engineers out there willing to share something like this? I think
>> it would be very useful to anybody who has or is contemplating the
>> construction of a guyed tower. I personally know of a few
>> installations that look like a catastrophe waiting to happen and not
>> being an engineer or tower erector I am having a tough time
>> convincing the owners of these installations that they should make
>> some improvements.
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