[TowerTalk] Guy wire / Phillystran vibration

[K7LXC at aol.com]

In a message dated 1/5/04 6:26:16 AM Pacific Standard Time, w4pa at yahoo.com 
writes:

> Question about guy wire vibration.  K4JNY and I were doing tower work
>  at his place all day Sunday and have noticed that on our 120’ Rohn 45
>  that we are experiencing quite a bit of vibration to the tower and guys
>  that we believe is being transmitted down the tower by the 4 el 40m
>  beam at the top.   This vibration occurs at wind speeds of 10 mph or so
>  or higher.  
>  
>  Jeff’s first thought was that guy wire tension was too high, but
>  checking with the tension gauge resulted in only 2 of the 9 being too
>  tight – loosening them didn’t seem to mute the vibration on them much. 
>  
>  
>  The guys are EHS at the 40’ level, and EHS for the bottom third and
>  Phillystran top two thirds for the guys at 80’ and 120’.  
>  
>  Our concern is that the Phillystran is not going to respond well,
>  long-term, to vibration.   Can excessive vibration be an early failure
>  problem for Philly guys?  

    Interesting question. I would say that the wind induced harmonic 
vibrations wouldn't do much if anything to the Phillystran but would play havoc with 
the attendant hardware - i.e. shackles, tower guy attachment, rotator hardware, 
etc. 

    This is a common problem with utility lines - especially if they're long 
span. What you need to do is to change the resonant frequency of the guy. 
Typical fixes add some sort of weight to the cable or have a device in the middle 
of the span that does the same thing. I'm sure you've been driving along some 
power lines and have seen those little trianglular devices hanging on the 
wire. 

    Preformed Line Products makes a Air Flow Spoiler that wraps around the 
cable. Here's what they say about them:

>>  Air Flow Spoilers are designed to counteract the damaging effects of 
galloping.

Galloping or dancing is a wind-related phenomenon when low frequency, high 
amplitude wind-induced motion can cause trip-outs, service interruptions, cable 
damage, damage to supporting structures, and damage to support hardware at 
their point of connection. High winds combined with cold weather and ice 
accumulation are the main contributors to galloping on regular cables. The 
accompanying ice build-up and wind can cause cable failure and structural damage. 

Air Flow Spoilers maintain aerodynamic stability by continually changing the 
profile of the cable to the wind. The one-piece helically formed rod, made of 
rigid, non-corrosive, non-metallic plastic, controls motion problems that can 
damage lines. The three-section structure of the Air Flow Spoiler enables it 
to grip the cable or conductor firmly.

Several Air Flow Spoilers are required in each span to offset the aerodynamic 
lift forces which cause galloping.

The number and placement of Air Flow Spoilers in each span are determined by 
a computer program which considers the results of on-going field and 
laboratory research. An Air Flow Spoiler Placement Request should be completed and 
returned to Preformed Line Products Company prior to installation of Air Flow 
Spoilers. A placement scheme will be returned to determine placement of Air Flow 
Spoilers in each span. Please advise whether the cables contain optical fibers. 
This may influence Air Flow Spoiler Selection.

Our Air Flow Spoilers are made of a rigid, non-metallic, non-corrosive 
thermal plastic. The unique design of the PREFORMED Air Flow Spoiler maintains 
aerodynamic stability be providing a continually changing profile to the wind. This 
helps to dramatically reduce the incidence of galloping on lines. >>
>  
>  Any other concerns with this situation we should be aware of?   Aside
>  from the loosening of connections, antenna hardware, etc. –  our
>  concern is for the tower and the guys itself.

    Since longterm vibration can cause fatal tower problems (and we all know 
what it can do to antenna elements!), you need to figure out some way to 
dampen the vibrations. Doing with the guys already installed makes it somewhat 
problematic. I'd live with it for the short-term and come up with some plan to fix 
it this summer. 

    I couldn't find anything specifically in the Phillystran info I have but 
you might want to talk to Ken Knight, the Phillystran sales guy that's at 
Dayton every year. He's at kknight at phillystran.com - tell him I sent you.  

Cheers & GL,
Steve     K7LXC
TOWER TECH -
Professional tower services for commercial and amateur
888-833-3104