comments in line
On 2/6/2016 7:41 AM, dw wrote:
I believe it the case that tower manufactures recommend guy-wire tension
to be 10% of the lines breaking strength?
That is what is in the Rohn manuals. Also, what my PE set as the
"maximum initial tension". Why that number? I have found that EHS guy
wire has essentially no initial "set" per the Engineering Mgr of one EHS
maker. However, there will be some number of thimble loops, insulator
loops, etc that will stretch out under an initial tension. Since a
tower and antenna have mass, they can cause the tower to lean so some
initial tension is needed even in the absence of wind.
In that case, assuming that 3/16th line is common for Rohn 25 towers
etc, and has a BS of 3900lbs, then one would set the guy tension to
390lbs?
This is what Rohn specifies (399#) for towers with 3/16" EHS. Note that
the safe working load for EHS is about 60% of break strength. Even
though EHS has a very high modulus (low stretch) and yield stress point
(strong) it will permanently stretch well before the break strength.
And there is some concern about tower failure due to downward
compression from over-tightening?
Yes. Two of the simple failure modes are exceeding the compression
strength of the tower leg and column buckling of a thin slender column.
The initial tension is added to the tension created from wind forces so
does reduce the maximum load permitted from wind. Towers have to lean a
bit for the tension in the guy to increase, which offsets the wind
load. The tension in the guy generates a horizontal force to oppose the
wind load and a vertical force which increases the tower compression
loads. Simple geometry can be used to calculate those forces. Keeping
towers and masts straight ("in column") is what multiple guys levels do,
otherwise they would buckle with just a top guy. Guys are also elastic,
a pathological case is using nylon rope which will stretch almost 50%
before breaking. There are pictures of towers fallen over on the ground
with the nylon guy lines still intact. NEVER use nylon for guys, only
EHS, fiberglass rod, Phillystran, or for temporary masts, very low
stretch line (dacron, Kevlar, etc).
I believe the AB-621 instructions specify 200lbs as the guy line
tension.
I think that might depend a lot on the type of guy line and how much it
stretches.
I'm wondering if those who have setup the military masts like the AB-621
follow this or some other rule?
I want to make sure I've got guy line tension well established.
Thanks,
Duane
Never had a AB-621, but I have put up many temporary masts and Rohn 25s
to 35' with small tribanders using dacron guy lines. 200# is more
tension than we used, but then they were up for a couple of days with
pleasant weather expected.
Grant KZ1W
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