I think the idea in different tensions vs height is to generate the same
horizontal restraining force since that changes with the cosine of the
guy angle with the ground. i.e a horizontal guy (zero deg) provides
100% of its tension horizontally to the tower, and a vertical guy (90
deg to ground) zero force.
To stay < 10% of break strength, larger guy diameters are used for upper
guys and need to be stronger since it takes more ultimate strength to
restrain the tower from sway. The amount of strain (stretch) with high
wind forces increases the sway so larger guys stretch less per pound of
tension. (see the K7NV finite element analysis) btw EHS from what I've
read, even though very high yield strength, does start to stretch
permanently at about 60% of break strength. So that is the "normal"
limit of allowable tension. While stretch is only a few percent of
length when stressed that high, that can change the initial tension.
After a max wind "event" it is probably smart to check tensions.
So with that in mind, I think the correct tension is what is specified
for each guy level by the tower manufacturer, independent of the
strength of the guy. My considerably oversized guys (1/2" Phillystran,
25k# break) were specified by the PE at near normal Rohn pretension
levels. Another limiting factor is the compression yield strength of
the tower sections. The bottom section has the weight of all the metal
above plus the vertical component of the sum of all guy forces from
every guy level.
So if your polyrod break specs are the same or better than 5/16 EHS all
is well, follow what Rohn specifies for pretension. One small
difference might be the pretension stretch vs EHS, which is very low.
Anyway, by tensioning to the Rohn spec that initial stretch is removed.
Since you have a dynamometer, leave the Loos gauge in the toolbox.
Grant KZ1W
On 5/15/2018 9:20 AM, john@kk9a.com wrote:
What is the breaking strength of the rod? 5/16 EHS is 11200 pounds and
typically you tension it at 10% of breaking strength. I am not sure what
diameter Polyrod is. If it is large, measuring its tension with a Loos
gauge may be a challenge.
John KK9A
Ed K5OF wrote:
Question: For those that have used what is commonly referred to
depending on discussion “poly rod”, “fiber glass rod”, “Fiber protrusion
rod”, etc. in lieu of ehs or phillystran what is the recommended tension
of said rod for 5/16” rod?
I have done quite amount of research both on line and contacting
manufacturers and have found nothing that I can use for a reference or
make any sense due to the engineering specs the manufacturers provide me.
I am not an engineer and cannot figure out what the recommendation would be.
I have my current tower project spec’d on ROHN 55G 130 mph at 130 feet
100 % guying (130 feet) and it calls for the lower guys if using EHS
spec ¼” at 665 lbs tension per guy and 5/16” at 1120 lbs tension per guy
for the upper guys. (3 sets of guys, the two lower ¼”, the final upper
at 5/16”)
If I am correct if I was using phillystran the tension would be the same
based on the same equivalency of that material used to the EHS guy specs.
Since I am using this 5/16” “rod" on all three guy levels I want to make
sure I do not under or over tension.
Rod provided was second hand (no issues, brand new) and was a great
deal! I am all set to go up with tower but want to get this right before
proceeding further. I have Loos gauges and Dynaometer at my disposal
along with everything to get this project up as described above. Hope to
get going when I get back from Dayton. From what I have determined so
far it is best to tension everything up of course beginning with bottom
set of guys to roughly 250 lbs + or -, get plumb and go from there with
turnbuckle adjustment and get somewhere in the ball park of a safe and
secure tension on final product as we go up to the final set of guys
(#3) at 130 feet. 250 lbs is far off from 665 and 1120 but because of
the material possibly those tension specs are not in the ball park of
EHS or Phillystran? What say the group with any knowledge of using this
type of guy material? When I get back from Dayton I am working on
getting the tower up with a successful and safe out come? Thanks in
advance , if you see me at the Crowne or the venue stop and say hi! 73!
Ed K5OF
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