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@contesting.com>Subject: Re: [TowerTalk]
Rotating tower guying questionFrom: "StellarCAT" rxdesign@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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73
Roger (K8RI)
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