[TowerTalk] Re: Guying question

[Tom Rauch]

Hi Tom,

You wrote:
> I'm not referring to  the pretensioning  of the guys either, although
> their tension plus the windload force must be taken into consideration.  I
> know that Rohn specifies, typically, pretensions of 800 to 1200 pounds
> (not psi). Ok, let's look at vectors.  As I said earlier, a horizontal
> force applied at the top of the tower will result in the same force in the
> guy cable in static/steady state condition.  Moving the anchor position
> i.e. "wider is better" will affect the horizontal and vertical force
> components, but the resultant force will always be the same.

You have the triangle upside down. 

> Example:
> A 100lbs. of horizontal force restricted by a 45 degree guy arrangement
> would result in horiz. & vert. force components of 70.7lbs each.  The same
> loading with a 22.5 degree guy arrangement would result in a horizontal
> component of 38.3lbs and a vertical component of 92.4lbs.  The vert. comp
> keeps going up as you approach the tower, but you'll never exceed the 100
> lbs. of force put in the horizontal axis, agreed?? And, at this juncture,
> I kinda forgot what I was arguing about! Hi! So I'll drop it at this
> point! 7 3 Tom K3GM

No way! If that was true, we'd all be using self-support towers with 
tiny base piers for TV broadcast towers!! Our uniform cross section 
towers would stand up hundreds of feet without any strain, because 
the vertical load would never exceed 100 pounds and the horizontal 
load 100 pounds.  

The vertical load in the tower due to side force  = side force / tan of 
angle 

The load in the guy line due to side force = side force / sin of the 
guy angle

Watch how this makes sense......

With a 90 degree angle the vertical tower load is 100/ infinity or 
zero pounds.

The guy line load is 100/ 1 or 100 lbs. Tower load is zero.

With a 45 degree angle, the vertical tower load due to the 
horizontal force is 100/ 1= 100 lbs
The load in the guy line is 100/ .707 = 141 lbs

With a 22.5 degree angle the tower load is 100/.4142 = 241.4 lbs
The guyline load is 100/.3827 = 261.3 lbs

With a 5 degree angle the tower load is 100/.0875 = 1143 lbs
The guyline load is 100/.087= 1147 lbs

The closer the guyline moves to the tower, the higher both loads 
become and the closer they are to the same value. 

This is the reason why self-support towers have to have a wide 
base, and LOT's of concrete just to support a tiny load at the top. 
This is also the reason we can not short guy towers, and why 
some sag is necessary in a horizontal cable or it will break.

If you stretch a guy line out with very little sag, and I throw my 
weight on the guy line at right angles, I can stretch it or pull the 
anchor up. I can also move a 4000 lb car with the brakes on, if I 
push sideways on a tight cable between the car and a fixed 
support.

I regularly stretch number eight and larger wire by tying it to two 
fixed supports and pushing sideways on the wire.  


73, Tom W8JI
w8ji@contesting.com


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