[TowerTalk] Fwd: Guy Wire Tension Calculation

[K9MA]

Hi Bob,

Here's the equation for guy wire tension as a function of wind force and 
direction:

T = F*(cos(a)+sin(a)/2*sin(120))   or approximately,   T = 
F*(cos(a)+0.58*sin(a))
where:

T = horizontal component of force at guy point
F = wind force at guy point
a = angle between wind and guy wire

(all angles in degrees)

This, of course, is for a symmetric three wire system. The easiest way 
I've found to work it out is to line one guy wire up with the X axis, 
and another at 120 degrees. A little trigonometry and algebra yields the 
above formula. If you differentiate it, and set the derivative to zero, 
you get:

tan(a) = 1/(2sin(120)) and a = 30 degrees. That yields T = 1.15F.

The formula also tells us that at a = 60 degrees, T = F. So, as wind 
direction varies, T varies from F to 1.15F.

One thing I overlooked on my earlier tower loading calculations, is that 
with the wind at 60 degrees, right between two guys, the downward force 
on the tower is twice what it is when the wind is aligned with a guy. 
The huge safety factor I thought I had there isn't quite so huge, but 
still adequate for 70 feet of Rohn 25 at 100 mph.

73,
Scott K9MA



On 9/21/2019 14:05, Bob Shohet, KQ2M wrote:
> Hi Scott,
> Yes, please share.  I am very interested.
> Tnx & 73
> Bob  KQ2M
> *From:* K9MA <mailto:k9ma at sdellington.us>
> *Sent:* Saturday, September 21, 2019 12:57 PM
> *To:* towertalk at contesting.com <mailto:towertalk at contesting.com>
> *Subject:* Re: [TowerTalk] Fwd: Guy Wire Tension Calculation
> Hi Hans,
>
> By coincidence, I just finished doing the math, recalled from my statics
> course some 50 years ago. 1.15 at 30 degrees is indeed the worst-case
> factor. It's really a simple calculation, but I'm a bit rusty on this
> stuff. I'll share the details, if anyone is interested.
>
> 73,
> Scott K9MA
>
>
> On 9/20/2019 23:57, Hans Hammarquist via TowerTalk wrote:
> >   Hi Scott,
> > I don't have the reference but I did some calculations myself some 
> years ago. I don't remember the exact numbers but the 1.15 factor is 
> right. If that happens at 30 degrees or if it was 15 I can't recall, 
> the 1.15 factor is correct.
> > The explanation is that when the wind attacks the tower 30 degrees 
> off the guy wire facing the wind, one of the guy wires, on the other 
> side of the tower, will pull on the tower sidewise and will increase 
> the tension in the wire facing the wind.
> >
> > Now, the 15% increase in the guy wire tension isn't that much. You, 
> presumingly have a much larger safety margin, so an additional 15% 
> should not be the factor, collapsing the tower.
> > 73 de,
> > Hans - N2JFS
> >
> >
> > -----Original Message-----
> > From: K9MA <k9ma at sdellington.us>
> > To: towertalk at contesting.com <towertalk at contesting.com>
> > Sent: Fri, Sep 20, 2019 10:30 pm
> > Subject: [TowerTalk] Guy Wire Tension Calculation
> >
> > Does anyone have a reference for calculating worst-case guy wire tension
> > for a 3-wire system? The only thing I'm unsure of is the effect of wind
> > direction. One source states that worst case is for wind 30 degrees one
> > side of a guy, and is 1.15 times the force with the wind aligned with
> > the guy.  The 1.15 factor is easy to calculate, but is that really the
> > worst case?. It's also clear that, with the wind exactly between two
> > guys, the tension in each is the same as when the wind is aligned with
> > one guy. Whether 30 degrees really is the worst case direction seems a
> > non-trivial calculation.
> >
> > Note that I'm not asking about calculating the effect of vertical angle,
> > etc., as that's straight-forward. I'm just interested in the worst case
> > horizontal component of guy wire tension as a function of wind 
> direction.
> >
> > 73,
> >
> > Scott K9MA
> >
>
> -- 
> Scott  K9MA
>
> k9ma at sdellington.us
>
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-- 
Scott  K9MA

k9ma at sdellington.us