Tower Guy Wires Question
John Cramond
Looking at various rigging arrangements on professional tower drawings,
three top guys spaced 120 degrees apart, run to the ground forming an
angle of 35 degrees to the tower.
Does anyone know how important the 35 degree angle is ?
At my location, to clear obstructions, the guy anchor points will have
to be further from the base of the tower, such that the wires will run
at 47 degrees to the tower. I'm obviously hoping that this will not
adversely affect the strength of the rigging.
--
John Cramond - GM4NHI
<jo...@gm4nhi.demon.co.uk>
Ron Schwartz
John, the 35 deg angle at the top guy station is close to the _minimum_
typically recommended, which is about 60% guying (guy anchor is at a
distance from the tower base of 60% the tower's height). 100% guying,
which would be 45 degs, is about at the outer limit of where you'd
typically want to go. As you go further out, compression loads due to
wind on the tower are lower, however the guys take more of the load and
can be a bit more finicky to pre-load properly while eliminating sag.
It's a trade-off that depends on the tower, guy station, and guy wire.
In general you should be ok.
But if that's the top guy station, what about the rest lower down?
Obviously, if you use the same anchor the guying at the lower levels
will be over 100%. Ideally you'd want another anchor closer in if you
have more than 2 guy stations (maybe this will be inside of your
obstructions?). Best advice I can suggest here is to check with the
manufacturer's recommendations on guying.
Are the obstructions trees? Getting the guy wire clear of the trees is
an inconvenience, as you'd find when you try it. Also the trees could
grow into the guys in later years, which you should absolutely avoid.
Hope some of this helps.
VE3VN
Gary Coffman
On Sat, 2 May 1998 16:19:24 +0100, John Cramond <jo...@gm4nhi.demon.co.uk> wrote:
>Looking at various rigging arrangements on professional tower drawings,
>three top guys spaced 120 degrees apart, run to the ground forming an
>angle of 35 degrees to the tower.
>Does anyone know how important the 35 degree angle is ?
>At my location, to clear obstructions, the guy anchor points will have
>to be further from the base of the tower, such that the wires will run
>at 47 degrees to the tower. I'm obviously hoping that this will not
>adversely affect the strength of the rigging.
There is nothing magic about 55 degrees (35 in your reference
frame). It just tends to minimize the amount of property required
at the tower site to host the guy anchors while still staying in the
"reasonable" range for static loading.
Ideally, the angle from the horizon to the guy should fall between
45 and 60 degrees (45 and 30 degrees in your frame of reference).
The steeper the guy, the more static load it puts on the tower. Some
static loading is a good thing because it tends to stablize the tower
joints, too much isn't because it puts too much static load on the
tower base.
The shallower the guy angle, the more side force it puts on the tower,
some is good because it increases tower stability in the wind, too much
isn't because it can rip the section where the guys are attached apart.
Exactly where you fall in the range depends on site factors. As long as
you stay in the range, and calculate your guy tensions properly for that
angle, everything will be fine.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
kf...@juno.com
John,
VE3VN's advice was good.
If you are using a ROHN tower, their catalog with guying is available from
Tower Tech for $5.00, which is a "must" for any tower owner.
Rohn uses 80% distances from tower base to guy anchor points.
73-Jim,KF4HK
In article <354BC719...@magma.ca>#1/1,
Ron Schwartz <v...@magma.ca> wrote:
>
> John Cramond wrote:
> >
> > Looking at various rigging arrangements on professional tower drawings,
> > three top guys spaced 120 degrees apart, run to the ground forming an
> > angle of 35 degrees to the tower.
> > Does anyone know how important the 35 degree angle is ?
> > At my location, to clear obstructions, the guy anchor points will have
> > to be further from the base of the tower, such that the wires will run
> > at 47 degrees to the tower. I'm obviously hoping that this will not
> > adversely affect the strength of the rigging.
>
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Ian White, G3SEK
John Cramond wrote:
>Looking at various rigging arrangements on professional tower drawings,
>three top guys spaced 120 degrees apart, run to the ground forming an
>angle of 35 degrees to the tower.
>Does anyone know how important the 35 degree angle is ?
>At my location, to clear obstructions, the guy anchor points will have
>to be further from the base of the tower, such that the wires will run
>at 47 degrees to the tower. I'm obviously hoping that this will not
>adversely affect the strength of the rigging.
A good resource for this kind of information is the TowerTalk list.
There was a short discussion a few weeks ago about guy angles on sloping
sites, so it will be in the archives at http://www.contesting.com/km9p/
search There were also some discussions on raised guy anchors which
may be relevant.
One quotation that comes up regulatorly is the "K7LXC Prime Directive":
ALWAYS DO WHAT THE MANUFACTURER SAYS. To learn what Rohn says, you can
get a copy of their catalog from K7LXC (a Rohn dealer, professional
tower rigger and all-round knowledgeable guy) - e-mail to
UpThe...@aol.com
(John: Steve will airmail you a copy - it's well worth having, even in a
Rohn-free country like ours.)
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
Reg Edwards
Gary, if a tower or mast is slender, what is the optimum height to attach,
say, just one set of guys to break up flexural oscillations due to wind ?
It may depend on whether the bottom end is directional-fixed or hinged. Or
doesn't this failure mode matter enough to be concerned with ?
--
Reg G4FGQ
http://www.btinternet.com/~g4fgq.regp
Gary Coffman
On 5 May 1998 02:37:05 GMT, "Reg Edwards" <g4fgq...@btinternet.com> wrote:
>Gary, if a tower or mast is slender, what is the optimum height to attach,
>say, just one set of guys to break up flexural oscillations due to wind ?
>It may depend on whether the bottom end is directional-fixed or hinged. Or
>doesn't this failure mode matter enough to be concerned with ?
>--
>Reg G4FGQ
I wouldn't use just one set of guys for anything but a very short tower.
In that case, guying about 2/3rds of the way up gives about the best
compromise in dealing with wind flex (but if there is significant wind
load at the top due to an array, I might consider moving the guys up
closer to the top). Normally, I would use at least two sets of guys,
one set very near the top to carry the array loads, and a second set
about halfway down to resist buckling moments. Taller towers need
guys at more levels to resist buckling.
Now that's just rule of thumb generality. I'd strongly recomend having
a competent analysis done by a structural engineer for any tower
installation beyond the most trivial. Falling towers are dangerous,
and can open you to substantial liabillity. Rohn publishes generally
trustworthy guides in their engineering selection guide. In the absence
of an analysis by a structural engineer, I'd at least follow their
guidelines.
Ron Schwartz
Reg Edwards wrote:
>
> Gary, if a tower or mast is slender, what is the optimum height to attach,
> say, just one set of guys to break up flexural oscillations due to wind ?
> It may depend on whether the bottom end is directional-fixed or hinged. Or
> doesn't this failure mode matter enough to be concerned with ?
Reg, you addressed this personally but posted in the newsgroup so not
quite sure if you opened this up to general comment. Anyway...
First question is, where's the load placed? A typical ham 'tower' tends
to put all the antenna loading at the top (e.g. yagi). A 'mast' (in the
terminology I use) more likely just supports wires and/or is part of the
antenna, in which case the load is mainly the mast itself.
For example, I once put up a 45' mast (made up of boom and element
leftovers from some junked Hygain 20m beams) to support a 40m delta
loop. I chose to guy it about 1/3 from the top since, it seemed to me,
that with one guy station no part of the mast was greater than 1/3 of
the mast length away from an anchor point (ground and guy station). I
can't rigourously claim this is optimum but it seemed sensible. This
system sustained one major wind storm (>100 km/h) and the top of the
mast swung about quite severely and the mast below the guy station
flexed quite a lot as well. Interestingly the curve of the bending mast
was quite smooth going across the guy station. To my surprise it all
survived quite nicely. This is purely anecdotal of course, so I can't
say this is generally what to expect.
More generally, with regard to oscillations/vibrations, there are 2 main
modes I've noted (others might want to jump in if you have more specific
insights). First, masts and antenna elements will move at a fairly low
frequency in the wind, say 1 to 5 Hz, but this may not be a true
oscillation but simply a response to turbulence. Second, rigid
structural members can vibrate at much higher, even audible,
frequencies. You can notice this in "singing" of undamped yagi elements
or by feeling the sub-audible vibrations of a large tower subjected to
wind. Element fatigue due to singing is pretty common and is often
fixed by damping the vibrations by threading rope inside the tubing.
The phillystran guying in one 140' tower I helped put up sang quite
severely. After consulting with someone in the business, the tower
owner acquired special-purpose damping devices which were affixed to the
guys both near the ground and near the tower, at specified points. The
damper was basically a helicall-wound plastic tube several feet long,
with the guy running thru the centre of the helix.
Does any of this help with your questions, Reg?
VE3VN
w...@world.std.com
Gary ke4zv wrote:
> Ideally, the angle from the horizon to the guy should fall between
> 45 and 60 degrees (45 and 30 degrees in your frame of reference).
One extra disadvantage of guys anchored more than about 45 degrees out
from the top (in questioner's frame, since that is relevant here) is
they will "rise" up out of the lightning shadow cone of the tower.
It'd be silly to have the lightning strike the guy line part way down
and have it part and topple the tower. (Then it sidestrikes to the
tower or to the ground past the insulator...)
--
Bill Ricker N1VUX w...@world.std.com "The freedom of the press belongs
http://world.std.com/~wdr to those who own one."--A.J.Liebling