[TowerTalk] Tower Guy Calculations

Tue Oct 15 05:11:09 EDT 2019

The only thing missing from this plan is the use of a Yaesu G400 to turn 
the antenna and RG58 coax to feed it.

Now what was it Forrest Gump said?

Martin, HS0ZED

On 15/10/2019 14:40, Stan Stockton wrote:
> 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 at 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 their  resistance to being withdrawn from the wood that they are screwed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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