Michael Tope wrote:
> jimlux wrote:
>> K7LXC@aol.com wrote:
>>
>>>
>>> In a message dated 8/31/2009 6:00:48 P.M. Pacific Daylight Time,
>>> towertalk-request@contesting.com writes:
>>>
>>>
>>>> Nothing says you have to tension the cable to a particular fraction of
>>>>
>>> it's breaking strength. 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.
>>>
>>> Are you saying that there is no particular tension specification for a
>>> guy cable? Please clarify. Since you're an experienced tower owner, I'd be
>>> shocked if your tower guys were tensioned at some random tension.
>>>
>>
>>
>> Not at all.. just that the tension spec is derived from the expected
>> loads, not the breaking strength of what you're tensioning. Obviously,
>> there's not much to be gained by using a very low strength member
>> (because you want the static tension to be a small fraction of the
>> dynamic loads..)
>>
>> But, if you have a given tower, with given loads, the expected loads on
>> the guys would be independent of what the guys happen to be made of or
>> how strong they are.
>>
>> There are some second order effects because stronger materials tend to
>> have higher stiffness, but I suspect that's not a big deal.
>>
>>
> Hi Jim,
>
> I think what you are dismissing as a "second order effect" is the reason
> that the tension matters. If I have two identical towers each guyed
> with a different material, one very stiff and one very elastic both with
> equal breaking strength and both tensioned to the same static load (say
> 400lbs), the dynamic stress during high winds in the members of the
> actual towers sections will be very different for each of the two
> towers. I believe the tensioning specification is designed to reduce the
> guy catenary to the point where the guy material stiffness dominates the
> lateral displacement of the tower under dynamic load. The tension
> required reach this point is going to depend on the strength to weight
> ratio of the cable as well as the stiffness of the cable (Young's
> modulus). A really heavy cable may still have a significant catenary at
> 400lbs. Seems like that heavier cable (if only tensioned to 400lbs)
> would allow the tower to displace more under a heavy wind gust causing
> greater stress in the tower members that might not otherwise be there if
> a lighter cable were used.
>
> Of course, determining where it would be appropriate to dismiss the
> differences in guy material stiffness and where it would not requires
> some real analysis. Presumeably if you follow the "prime directive" you
> are taking advantage of the analysis already performed by the
> manufacturer's engineering department (with review by the legal deparment).
I agree, where there is big difference in the mass of the cable.. but
when you're talking about tensioning either a 4000 lb breaking strength
or a 6000 lb breaking strength cable to 400 lbs, I don't know that the
difference is that big.
Certainly, K7NV's analysis shows potentially BIG problems from combining
very stretchy and not so stretchy (e.g. phillystran on the top guys,
steel on the bottom two sets), but there, you're talking about a bigger
than 4:1 difference in the stretchyness, and the problems are more a
matter of mismatch between materials than the inherent properties.
The old mechanics problem where you have a 100 ft 2" rope and a 100ft
piano wire, both with a 1000 lb breaking strength, supporting a 1500
pound load, comes to mind.
However, I still wonder whether or not the 10% of breaking strength was
just a "traditional installation guideline" that then got sanctified by
analysis as being "ok", and doesn't necessarily contemplate the
existence of cables that are basically the same diameter and mass, but
very different strengths and stiffnesses.
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