If each guy had a different tension, then the tower/guy system would not have a
clean natural frequency
to which it could respond/oscillate...?? (probably quit shaking a lot quicker
if you ran into it with a tractor...)
----- Original Message -----
From: Roger (K8RI)<mailto:K8RI-on-TowerTalk@tm.net>
Cc: TOWERTALK@contesting.com<mailto:TOWERTALK@contesting.com>
Sent: Tuesday, September 01, 2009 10:31 AM
Subject: Re: [TowerTalk] Philly installation
jimlux wrote:
> Roger (K8RI) wrote:
>>
>> Cqtestk4xs@aol.com<mailto:Cqtestk4xs@aol.com> wrote:
>>>
>>> In a message dated 8/31/2009 10:17:31 PM Greenwich Standard Time,
>>> jimlux@earthlink.net<mailto:jimlux@earthlink.net> writes:
>>>
>>> Nothing says you have to tension the cable to a particular fraction
>>> of it's breaking strength.
>> Except both the tower makers and the guy makers. Both say to tension
>> them to 10% of breaking strength. They do that for a reason. If you
>> do not follow the manufacturer's specifications you open up for a
>> number of side effects such as resonances and a tower not as "firm"
>> when you are up there. Will the "Big Grip" ends establish as good a
>> grip at less initial pull? ROHN specifies 3/16" tensioned to 400#,
>> so the equivalent Phillystran would be the 4000# test cable.
>
>
> Resonance? That's almost entirely a function of the ratio of the
> tension to the mass per unit length. EHS and regular cable are almost
> exactly the same mass/unit length, as I recall. the tighter tension
> proposed here for the EHS (e.g. 10% of breaking strength) will raise
> the resonant frequency.
There are different modes of resonance, two of which are of concern in
this case.
If you hit a guy line hard enough to displace it the resonance can be
seen as a very low frequency gallop and you can watch it move up and
down the guy. Compared to that Phillystran is a much higher frequency
that dampens in a fraction of the time taken for the EHS with the deep
catenary. Then there is the classic resonance, where you hit the guy
line and it rings which is due to the mass and tension. I've been on
larger towers with large guys that would ring just like a tuning fork
when stuck and it was a nice clean tone.
>
> I think that tower "stiffness" as a structure is almost independent of
> the kind of steel cable, but almost entirely due to the tension in the
> cable, so that would say use the same tension regardless of whether
> the guy is EHS or not.
>
Agreed.
> If you want the same mechanical resonance, then I can see increasing
> the tension in proportion to the cross sectional area (since the
> resonant frequency is proportional to sqrt(tension/(mass/length)).
> And for a fixed "kind" of steel (e.g. EHS or not), a "percentage of
> breaking strength" might make sense.
>
> OTOH, I don't think mechanical vibrations drive tower guying design.
> All the usual literature concentrates on having sufficient strength to
> turn the side load from the wind into down force AND not having the
> guy on the downwind side go (too) slack.
>
Agreed.
>>
>> If you are putting up a little 40 footer with a small tribander of
>> vertical on top, you can get away with ignoring a lot of the rules
>> and quite often without problems, but the larger the antenna, the
>> taller the tower, and the more expense that *might* be required of
>> your insurance company the less you can ignore the manufacturers specs.
>
>
> This, to me, is a perfectly valid reason to do something that
> otherwise doesn't make sense from a mechanical standpoint.
>
>
>> On top of this is liability which I suppose I should put in all caps,
>> but
>>
>>> It's more a matter of the desired tension for appropriate
>>> stability for the structure. You could tension million pound
>>> breaking strength cable to the same few hundred pounds as the the
>>> regular non-EHS cable.
>>>
>>>
>> Probably not as the tower most likely couldn't support it hanging
>> straight down.
>
> My magic million pound cable is infinitely thin (!)..
>
I'll take two. <:-))
>
>
>>> I've found that unless you tighten to specs, the tower is
>>> "sloppier" and has more play.
>> As have I.
>>> The sloppiness allows wind gusts to have more of an effect on the
>>> tower.
>
> Yes.. but if I have two cables, one that has 4000 pound breaking
> strength and one that has 6000 pound breaking strength, both of which
> are "acceptable" for use in guying, I don't see that the mechanical
> properties of the system are much different if I tension BOTH to 400
> pounds.
>
>
Probably not unless they are EHS. I can carry all of the Phillystran
guy lines for my tower on one arm. A 100 feet of EHS takes two hands
just to drag across the floor.
With EHS both tensioned to 400# the 6000# test would likely have a
noticeably deeper catenary.
There are a number of variables, not the least of which is the material
used for the guys.
73
Roger (K8RI)
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