[TowerTalk] Guy wire strength question

[K7LXC at aol.com]

In a message dated 5/19/2010 12:01:39 P.M. Pacific Daylight Time,  
towertalk-request at contesting.com writes:

>  Can anyone help me out? I am in the process of putting  up a 85' tower. 
I am presently looking into guy wires. The calculated maximum  force on the  
guy wires is 2990 lb. I decided that 5/16" EHS 1x7 wire  should be fine. 
Now when I receive quotes I find various grades (if the quoter  even knows 
what grade it is). Can somebody explain what the difference is? I  see that EHS 
is recommended by most tower manufacturer but  why?

>  The strength of the different grades are mind  boggling:

>  For 5/16" galvanized steel wire I have break force  for:

>  Common Grade             3200
Siemens Martin Grade   5350
High strength Grade   8000
Extra High  Strength Grade      11200
Utilities/Specification  Grade    18000


Howdy --


    I'll bite. What kind of tower is it? What does  the manufacturer say 
about it? Why did you decide on 5/16" guys? Are you an  engineer? 
 
>  One of my questions is:" Can I use 9/32" Utilities Grade  (11500 lb) 
instead of 5/16" EHS (11200 lb)? 11500 lb seems OK for a maximum  expected 
force of 2990 lb (120 mph wind)? What would happen if I use 5/8"  Common Grade 
(11600 lb)? 

>  Could I possible use 1/4"  Utilities Grade (6000 lb) or is that to be 
too close to the maximum force  (2990 lb)?
 
    These are engineering questions. If you're an  engineer, feel free to 
do the calcs yourself. If not, follow the LXC Prime  Directive to "DO what 
the manufacturer says" or get an engineer to run the  calcs. 
 
    Something you'll have to deal with is guy wire  terminations. Nowadays 
everyone uses Preformed Guy Grips and you'll have to  get the exact grip for 
the guy wire that you're using. You CANNOT interchange  Preforms on the 
same size guy wire material. 
 
    Why reinvent the wheel? Tower manufacturers  specify EHS for good 
engineering and industry practices (e.g. TIA-222 Tower  Standard). I think you're 
making this more difficult than it is. 
 
    Something else you have to account for is guy  tension. The bigger the 
guy, the bigger the tension. And the bigger the  tension, the bigger the 
compressive forces on the legs. With guy wires  that are too big, the initial 
tension may reduce the tower capacity  dramatically. The strength of a tower 
is leg capacity and it's got nothing to  do with what guy wire you use. 
 
    DO what the manufacturer says. Or have an  engineer calc it for you. In 
the absence of that, you can make some educated  inferences from similar 
Rohn towers and their specs. 
 
Cheers,
Steve   K7LXC
TOWER TECH -
Professional tower services for hams