I did a static analysis of my 70 foot Rohn 25 tower by assuming the
tower behaved like it was perfectly rigid, but "hinged" at the base and
lower guy point. I believe that's close to a worst case situation.
Because my guy anchors are closer that the standard design, the guy
tensions are higher, so I took that into account. I allowed a safety
factor of close to 2:1 for everything except the bending moment at the
upper guy point, as I figured that would not cause a tower collapse. The
anchors are a Rohn design, with lots of buried concrete and rebar. It
has survived nearly 30 years so far.
This is all Engineering Mechanics 101. I don't know how one could do a
much more accurate analysis short of Finite Element Analysis, which is
beyond my capabilities.
When I see some of the poorly designed towers some hams put up, and the
fact that some of them stay up at all, I'm pretty confident the my
design is more than adequate. How may hams even do that much?
73,
Scott K9MA
On 10/14/2019 08:35, jimlux wrote:
On 10/13/19 10:46 PM, Grant Saviers wrote:
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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Scott K9MA
k9ma@sdellington.us
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