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).
73, Mike W4EF...........
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