[TowerTalk] [Bulk] Re: Rotating tower guying question

Tue May 5 02:30:34 EDT 2015

On 5/5/2015 12:22 AM, Grant Saviers wrote:

My 45G is only 100 feet, but with the big array on top, I went up there 
on some pretty windy days, where by the time I headed down I questioned 
the wisdom of  having made the climb.  Under those conditions with that 
large array on top, I could detect no movement, or vibration of the 
tower, let alone lean.

73

Roger  (K8RI)

> The stretch of Philly is dependent on the cross section of the fiber 
> bundle, the cable construction, and the applied force.  Same for EHS.  
> The modulus of Kevlar 49 used in Philly is 16.3E6 psi per the DuPont 
> documentation and the modulus of high strength steel is 30E6 psi.  
> However, 7 strand guy wire rope has several modes of stretch so the 
> net modulus us about 20E6 psi (USS Wire Rope Handbook).  So any same 
> diameter strand of Kevlar 49 will elongate 1.3x the same EHS 
> diameter.  I think this is less than shown in Kurt's analysis, but 
> haven't done the calculations.  However, if the kevlar is larger in 
> diameter than the EHS then they should stretch the same amount per 
> pound of tension.  So as Kurt suggests in his paper, using higher 
> strength rated Kevlar can have the same stretch and thus same tower 
> deflection as EHS.  Of course, as Kurt notes, a pier pin base mostly 
> eliminates overstressing the tower due to lean.
>
> In my test stand I validated that the stretch of "old" Phillystran is 
> as predicted by the modulus, there appears no "cable lay" 
> degradation.  Perhaps this is because "old" Philly has no stranding or 
> twist, all fibers are laid along the length of the cable.  New Philly 
> does have stranding and twist, but I have not tested it to determine 
> the net modulus.
>
> One thing Kurt doesn't mention or seem to consider is that the safe 
> working load (SWL) for EHS and Kevlar is 30 to 40% of breaking 
> strength.  Hence when his analysis shows a guy safety factor of less 
> than 2.8 the load is too high for the guy size selected.  This is the 
> case for several of the "tower leans too much" cases he evaluated.  
> Thus, if those guys were up sized to SWL, the tower would lean less as 
> a larger diameter guy at a lower stress level will stretch less.  
> Kevlar guys are also rated about the same percent of breaking strength 
> SWL.
>
> The USS handbook states that the yield point for wire ropes is reached 
> at about 44% of breaking strength.  Thus any guy stressed to more than 
> that value will permanently stretch, obviously a severe problem for 
> tower guys.
>
> Kurt did terrific work in calculating what goes on with a guyed tower 
> and the principles are very helpful in understanding what is happening 
> in a guyed structure.  We have seen a substantial beefing-up of most 
> factory designed ham radio self supporting tower bases and guy systems 
> in recent years, perhaps driven by code changes, and perhaps because 
> of better tools to analyze the structures.
>
> Grant KZ1W
>
>
> On 5/4/2015 9:57 AM, StellarCAT wrote:
>> R65 is VERY heavy duty stuff ... coupled with 11200# philly I’d 
>> suspect that it would indeed feel very strong! My point was only IF 
>> K7NV’s analysis is correct and I’m surely not in a position to 
>> dispute it – and IF one has their bottom section in concrete (and 
>> important consideration for the analysis) .... then the conclusion 
>> would be that EHS is stronger than the equivalent philly. As simple 
>> as that. So if a tower has survived with the philly that would only 
>> suggest that it would have had more margin if it had been EHS. The 
>> fact that a tower has survived thus doesn’t prove that philly is as 
>> good – it only proves that it is good enough for what THAT tower has 
>> seen since installation. Which I’m sure is indeed ‘good enough’ .... 
>> but it doesn’t imply it is equivalent!
>>
>> At least that is how I’d interpret his analysis.
>>
>> as for life expectancy – if salt air is an issue I can see how this 
>> might affect the decision – but for most it isn’t a concern. There 
>> are just as many if not many more anecdotal stories of both ham and 
>> commercial towers with steel that have been up for 40 or more years 
>> and still doing fine.
>>
>> Gary
>>
>>
>>
>> I have a 150' Rohn 65G tower guyed with 11,200 pound Phillystran and 
>> Ihave no concerns about guy stretching. The tower feels solid and it 
>> has been up for years at two different QTH's with no issues.I used 
>> Phillystran at my Aruba station. Steel guy cables and grips 
>> neededreplacement every four years and the Phillystran was still in 
>> excellentshape when I dismantled the station.
>>
>> John KK9ATo: <towertalk at contesting.com>Subject: Re: [TowerTalk] 
>> Rotating tower guying questionFrom: "StellarCAT" rxdesign at ssvecnet.com
>>
>>
>>
>>
>>
>> Date: Mon, 4 May 2015 08:57:34 -0400Just as a reference:I had a 141? 
>> rotating R45 tower in AZ ... the guys were attached at about120?(top) 
>> and out ~105?. I used 1/4? throughout. I used insulators throughout 
>> aswell using the values provided in the handbook.thoughts:-I stayed 
>> with EHS because of the article written by K7NV regarding 
>> towersandguys and the fact that philly stretches so much more than 
>> EHS. That concernedme greatly.-Although there is extra labor in 
>> adding the insulators I?d say it mightbe, asit was for me at the 
>> time, a non-issue as it is a one time adder and not THATtime 
>> consuming. Its not as if you?re getting paid for the hour if 
>> youDIDN?T doit!-The values in the handbook... the one thing they 
>> don?t seem to take intoaccount is the shortest length going to the 
>> tower ? I believe mine waslike 6?or something similar ? but it is 
>> that x2 (or x3 considering all three) ANDthewidth of the tower so for 
>> me it seriously interacted with 10 meters, a bit
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>>
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-- 

73

Roger (K8RI)

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