Tom,
There is not a single thing about this installation that I like or would do if
it were me.
Stan, K5GO
> 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 their resistance to being withdrawn from the wood that they are screwed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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