Towertalk
[Top] [All Lists]

Re: [TowerTalk] Wire size for 800 meter long wire

To: barry@mxg.com
Subject: Re: [TowerTalk] Wire size for 800 meter long wire
From: Eric Scace K3NA <eric@k3na.org>
Reply-to: eric@k3na.org
Date: Tue, 11 Oct 2005 16:38:47 -0400
List-post: <mailto:towertalk@contesting.com>
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
> 
> _______________________________________________
> 
> 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.
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> 
> 
_______________________________________________

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.

_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>