[TowerTalk] Philly installation

[Michael Tope]

jimlux wrote:

>K7LXC at aol.com wrote:
>  
>
>> 
>>In a message dated 8/31/2009 6:00:48 P.M. Pacific Daylight Time,  
>>towertalk-request at 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...........