It's not difficult to calculate the vertical load on a tower from the
initial guy tension. It's a lot more difficult to calculate how many
guys are needed and how they interact with the tower structurally, when
the wind blows.
The tension in the guy times the cosine of the guy angle with the tower
is the down force from each guy. A straight down (useless) guy has zero
angle with the tower, cos(0)=1 so all tension is downforce. A parallel
to the ground (best) guy angle is 90 deg so cos(90)= 0, no downforce .
For an 80' R25 with three sets of guys (a tower I put up), using the
geometry in the Rohn catalog (64' to guy anchor from tower base) ,
top guy angle 43.5 deg; mid guy 52.0 deg; bottom guy 68.7 deg; all
guys are 3/16" EHS, spec'd at 399# initial tension.
So the down force is 3*399*cos(43.5) + 3*399*cos(52) + 3*399*cos(68.7)
= 2040# (3 sets of 3 guys = 9 guys creating compression load)
If we use Gerald's (Texas RF) 7000# compression load rating for R25,
that is 29% of 7000#.
Re the 7000# max load value: What really matters I think for most
situations is not the static dead load on the tower but the maximum
tension or compression of a single leg when the tower leans from wind
load. The Rohn R25 mechanical spec has some properties so the tower PE
can treat the lattice structure as simple structure. The compression
load capacity is not specified for the tower section. Each leg
compression and tension capacity is specified as approximately 8300#.
The total area of the leg steel is 0.726 sq in which would have a simple
yield point of about 24,000#. However, the tower is a slender column
and thus subject to buckling. There are a number of calculations done
to insure none of the several failure mode thresholds are exceeded. e.g.
see https://en.wikipedia.org/wiki/Buckling for how slender columns can fail.
The total tower dead plus dynamic load does affect the base design.
Consider the base foundation ground load for that 80' tower which is
specified as 6.25 sq ft (2.5'x2.5'x4' H). 2000#/sq ft is a commonly
used acceptable load for soils (many are better) , so that base area is
ok for a 12,500# load, which includes about 1 yard of concrete for the
base at 4000#, plus 8 tower sections (328#) and some allowance for
antennas (200#?), etc. So 7000# seems ok as what the Rohn engineers
expect as a maximum compression load in at the base. However, R25 can
go to 190' with 6 levels of 3/16 EHS guys each with the same 399#
pretension. The base area for 190' is specified at 9 sq ft, or 18,000#
allowable minus 6000# of concrete in the base, 780# of tower, about 360#
of guys (~3000') or about 11,000# allowable compression load. All of
course only with the guy designs specified by Rohn. Yet another cross
check would be to add up all the initial tension loads on the 190'
tower. A wet stamp analysis of a R25 tower would cover all the bases.
What is more relevant for single tube mast is where guys should be
placed and what guy pretension is appropriate where there isn't the
stiffness of a lattice Z brace structure to prevent the small diameter
leg tube from buckling. Think of Z bracing as a way to make a large
diameter (stiff) tube out of 3 small diameter ones.
One approach is to make the guy baseline as long as possible to reduce
the static vertical column load. Rohn consistently uses 80% of height
as the baseline for R25 (same thru R65). My tower PE said that was a
"rule of thumb" and that other choices are equally valid with the
supporting engineering. However, the shorter that distance, the more
static plus dynamic compression load, and likely the more guy levels are
needed to prevent buckling. Also consider that 100' of 3/16" guy is 1.6
sq ft of area, so guys can contribute significantly to wind loads. Long
guys also add more guy weight as dead load to the tower. Then the
catenary sag is also increased. So there are a number of variables that
all interact.
With very low elasticity high strength guys e.g. Kevlar, dyneema, or
vectran polymer lines, which can be small diameter and thus low mass per
foot, less pretension should be needed to keep the mast straight/in
column. Dacron and dacron/polyester blends are what I use for temporary
masts in benign weather as the cost is a small fraction of the hi-tech
lines. There are also polyester lines such as New England Ropes StaSet
which has been in use for more than 35 years for low stretch lines, with
about 3% stretch at 30% load of break strength vs 1% for the exotic fibers.
YMMV.
Grant KZ1W
On 2/7/2016 6:44 AM, TexasRF--- via TowerTalk wrote:
Duane, the guy tension is only a fraction of the downward force created by
a 90 MPH wind storm blowing on the structure and antenna load. 25G sections
are rated for about 7,000 pounds vertical load.
Still though, adding too much guy tension is not a good thing. The tension
can be estimated by formula based on guy length and sag if one doesn't wish
to invest in a tension measuring device. Google will turn up information
on this.
Yes, 390 pounds is the correct tension for 3/16"EHS cable,
73,
Gerald K5GW
In a message dated 2/6/2016 9:42:04 A.M. Central Standard Time,
bw_dw@fastmail.fm writes:
I believe it the case that tower manufactures recommend guy-wire tension
to be 10% of the lines breaking strength?
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?
And there is some concern about tower failure due to downward
compression from over-tightening?
I believe the AB-621 instructions specify 200lbs as the guy line
tension.
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
--
Bw_dw@fastmail.net
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