Correction:
T = horizontal component of _*guy tension*_ at guy point
F = wind force at guy point
a = angle between wind and guy wire
On 9/21/2019 15:20, K9MA wrote:
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 _*guy tension*_ 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@sdellington.us>
*Sent:* Saturday, September 21, 2019 12:57 PM
*To:* towertalk@contesting.com <mailto:towertalk@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@sdellington.us>
> To: towertalk@contesting.com <towertalk@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@sdellington.us
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Scott K9MA
k9ma@sdellington.us
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