Barry --
You are looking at a very challenging mechanical engineering situation.
As others have pointed out, a sag limit of 50 ft will require
substantial tension in the line.
Any post-type anchor must be back-guyed. Two back-guys at different
angles (away and the to 'front' and 'rear' of the wire span) would be
wise, since wind will introduce side loads on the wire which much be
restrained by the anchors. You will want the wire to fail before the
anchors pull out.
You will also need to consider additional loads:
-- wind load: not necessarily trivial on long runs. #10 AWG is
about 0.1 in diameter. For 800 ft of wire this represents about 6.7 sq
ft of flat projected surface area. As a cylindrical object this can be
reduced by a third to 4.5 sq ft. For a 60 lb/sq ft wind load, an
additional 270 lb will be added to the tension. Using #10 Copperweld
(breaking strength 1200 lb) and a 10% pre-tension (120 lb, which will
not meet your sag goal), you are now up to 390 lb. You are now above
the safe working limit of 340 lb (see below).
-- ice load: this is very serious, even for small loads. Even a
1/8" radial ice load takes that wire from a 0.1" diameter smooth surface
to 0.35" rough surface -- more than tripling the wind load. In the
rough calculations above the wire span would fail. Ice also adds
substantial weight to the system in addition to the wind load area.
Check with your local power utility to see what ice they allow for
in their engineering for their long haul lines. In my very limited
experience the power utility picks a number in the once-every-30 or -40
year range, accepting that it will be cheaper to rebuild for less
frequent events. Many times utilities will assume the wind loading
while wires are ice-loaded is smaller. But I've been through one ice
storm where 2" radial ice were deposited under calm conditions, followed
by a 60 knot storm. The results were not pretty!
A safe way of maintaining tension in the system is through
counterweights. Counterweights will lift when wind or ice loads
increase, increasing the wire sag temporarily but preventing failure.
W0UN has mentioned before the "Lineman's and Cableman's Handbook"
and the "Handbook of Rigging for Construction and Industrial
Operations". Chapter 21 & 22 of the former will give you some
approaches to stringing your wire safely, although much of the material
is oriented to the power line industry. Chapter 7 of the later
discusses care and handling of wire rope; also take note of chapter 10
on blocks (pulleys) if you decide to use counterweights. For guys, the
"Handbook of Rigging" says the minimum safety factor is 3.5; i.e., that
1200 lb breaking strength for #10 AWG Copperweld is around 340 lb safe
working load when you've added up all the loads.
If you use counterweights to tension the system, you can multiply
the counterweight's force through an appropriate 2:1 or 3:1 system of
blocks. Thus a 120 lb pre-tension can be generated by a 50 lb chunk of
concrete with a 3:1 system (allowing for some friction in the system).
A come-along can be used to attach the counterweight. But wire blocks
that handle these forces and have a long outdoor life cost a bit.
There isn't anything terribly difficult about what you want to do.
It just takes a bit of research and care.
-- Eric K3NA
on 05 Oct 10 18:32 Barry Merrill said the following:
> Run these specs through your nomographs...
> 12 ga insulated copperweld at 22 lbs per 1000' with a breaking strength of
> 900lbs
> ==> Tension would be 220 pounds; how the heck can I haul this up without
> come-alongs,etc?
>
> or 10 ga insulated copperweld at 35 lbs per 1000' with a breaking strength of
> 1231lbs
> ==> Tension would be 325 pounds; even heavier load.
>
> and in both cases, that 24% and 26% of breaking strength.
>
> The book had 10%, and said the tension could be raised to 15%, but your
> suggesting that double that tension is still reasonable?
>
> But it's those big tension values that are going to play havoc
> with the four foot fence post on the other side of the bluff!
>
> Is there any wire thats small and strong, so I can have both
> acceptable sag and an antenna I can handle without winches?
>
> 73
>
> Barry, W5GN
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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