[TowerTalk] Tower Guy Calculations

[K9MA]

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.
>>>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk


-- 
Scott  K9MA

k9ma at sdellington.us