Roger (K8RI) wrote:
>
> Cqtestk4xs@aol.com wrote:
>>
>> In a message dated 8/31/2009 10:17:31 PM Greenwich Standard Time,
>> 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.
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.
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.
>
> 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'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.
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