On Oct 14, 2019, at 8:13 PM, Tom Hellem <tom.hellem@gmail.com> wrote:
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their resistance to being withdrawn from the wood that they are screwed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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