[TowerTalk] Fwd: Guy Wire Tension Calculation

[Bob Shohet, KQ2M]

Tnx Scott!

73

Bob KQ2M


From: K9MA 
Sent: Saturday, September 21, 2019 5:19 PM
To: towertalk at contesting.com 
Subject: Re: [TowerTalk] Fwd: Guy Wire Tension Calculation

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 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

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