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

[Grant Saviers]

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!
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> 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.
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> Gary
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> 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.
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> John KK9ATo: <towertalk at contesting.com>Subject: Re: [TowerTalk] Rotating tower guying questionFrom: "StellarCAT" rxdesign at ssvecnet.com
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> 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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