[TowerTalk] Guyed self-supporters (was "Concrete suggestions")

[K3BU at aol.com]

In a message dated 4/18/03 1:09:17 PM Eastern Daylight Time, 
olinger at bellsouth.net writes:

> Seems to be an awful lot of air time trying to kill fleas on the elephant.
>  

If the fleas will not go away, what's elephant got to do?
Seems some reasons are presented, so I will try to engage one more time.
I hope it is shining some light on the problem and will save someone, 
sometime a tower or house.

>  An earlier post is correct that the tower flexibility necessary to engage 
> guy wires is quite missing on self supporting towers.
>  

Not entirely, there are "rigid" self-supporting towers and there are flexing 
self supporting towers (they all are flexing). Tubular towers like Big Bertha 
and even frame time designs deflect. 

>  I have an 80 foot Trylon. Even if I swing my 250 pounds back and forth at 
> the top, I can't get it to sway even a 1/4 inch. It is significantly RIGID.
>  

How did you measure that? When you up there you can't "feel" the swaying. I 
was once up on my Bertha in 60 mph wind, and "I didn't sway" until I looked 
down and realized that I am whipping around by almost 2 feet, the tower was 
CURVED underneath me! If your 80 footer didn't move 1/4", then it must be 
made out of carbide. Elementary engineering calculation will show you exactly 
how much deflection there is under certain loads conditions.

>  I have been at the top of guyed towers, and my own self-supporter in high 
> winds, and the movement in the wind at 100' on a guyed tower will take your 
> breath away. I never have felt that at the top of the Trylon. It simply 
does 
> not move like a guyed tower.
>  

Depends on the design and material. You can't make blank statement. 

>  It is true that there is a huge overturning moment at the base of a self-
> supporter, a compression and lifting in a heavy wind. But the tower and 
base 
> are engineered to handle that moment routinely. Personally I have never 
heard 
> of a case of a properly installed self supporter being OVERTURNED by a high 
> wind, hurricane or otherwise. 
>  

Proper base and properly designed tower should take the beating. Depends on 
the design, loading, deterioration where the weak points develop and failure 
happens. It could happen anywhere along the structure. You could have 
corroded member, bad weld, cracked leg from ice, etc.

>  Therefore adding guys to help keep a self-supporter from being OVERTURNED 
> may feel nice, but it's guarding a bank with no money inside.
>  

There was no question about overturning the tower, the question is about 
reinforcing the tower and if adding guys to self supporting tower will make 
it worse (collapsing the legs).

>  I have heard of **ONE** case of the top third of a Trylon being folded 
over 
> by two inches of radial ice followed by 50 mph winds. That's turning a 30 
sq 
> ft rated tower into a 500 sq ft sail. But that's FOLDED OVER, UP the tower, 
> not overturned.
>  

Who is arguing about overturning? Tower failed at the weakest point under 
specific load.

>  Given the rigidity of the tower, and the fact that a guyed tower HAS TO 
MOVE 
> to create the counter force opposite the wind, here is the question...
>  
>  ***Would a self supporting tower's movement in the wind reach a failure 
> point before the guy could provide enough counter force to prevent it?***
>  

Depends on the design, more rigid (self supporting) tower should have tighter 
guys. More flexible tower should have guys with some slack. This way the load 
is divided between them and by properly rating guy wire strength and anchor 
you can beef up loading characteristic for the best survivability.

>  Guying a self supporter may make you feel better, but if the self 
supporter 
> isn't designed for the load forget it. You can STILL lose the upper section 
> with guys on it. Or stated another way...
>  
>  That two-inch radial ice plus 50 mph would have ruined that Trylon, even 
> with guys on it, anyway. It just wouldn't have folded over the same way. If 
> it was guyed at the top, it would have folded in opposite beneath the 
rotator 
> instead. Once the bend damage establishes someplace, even just a little, 
the 
> rest is history.
>  

You are wrong. If you had it guyed at the top of the tower, it would have 
survived. Distribution of the bending moment along the tower is such that it 
is (exponentially?) decreasing with height (plus steps with any antennas 
mounted on the way). If you insert the guy wire, you will change that 
distribution to two "bellies" around the guy, you are removing momentum taken 
up by the guy wire, and I bet that tower would have survived. The same 
situation was with W2HCW and his 80m Yagi atop of Big Bertha. Bertha swings 
and that adds dynamic loads in addition to static load from blowing wind. It 
snapped at about 3/4 of the height. If he had the guy wire at the top that he 
could hook up to the anchor in the direction wind was blowing from, it would 
have survived.

>  I have a suspicion that adding guys at the top may actually weaken a self-
> supporting tower with a mast and top load by providing a fulcrum at the top 
> of the tower that is otherwise not in the equation.
>  

Not so, calculation can easily prove otherwise and some examples were already 
given.

>  This is because the tower is deliberately designed to flex more at the top 
> and flex gradually less as you go down. This has the effect of SPREADING 
the 
> flex moment along the entire height of the tower. 
>  
>  The safety of the tower depends on all the moment being evenly spread. 
>  

The safety of the tower depends on the ability of the material to handle the 
loads at any point in the structure. This can be enhanced by proper 
engineering (type of frame, or taper on tubular tower).

>  If the total moment in a wind is 5000 pounds compression toward the east, 
> that must be spread out over the tower, AND is a constant sum that is 
> maintained as long as the wind is steady. 
>  

Moment is in FtLbs, the rest is wrong. 

>  Now add top guys. Suppose that the windload on the antenna is 1000 pounds (
> 4000 on the rest of the tower).
>  
>  You now have added a 1000 pound force to the west down at the level of the 
> rotator. The wind is pushing the antenna at top of mast to the east. The 
> force is transmitted through the fulcrum at the tower top guy point. It now 
> appears down at the rotator point pushing opposite direction toward the 
west. 
> This means that there must be the effect of 6000 pounds to the east 
elsewhere,
>  extra compression on the downwind side, to maintain the overall sum of 
5000 
> pounds to the east. 
>  
>  The tower was not designed to have a fulcrum point at the top. It was 
> designed to be proportionately flexible at that point.
>  

I will not get into lessons here. I will do some calculations and 
illustration of the guyed vs. non-guyed self supporting tower with my Bertha, 
when I get time to do it. There will be graphic illustration of forces and 
deformations (flexing) under various loads. 

>  This stuff is NOT simple. It is NOT intuitive. It is deep doodoo PE stuff. 

Yep, I haven't seen one other PE to argue that I am wrong. 

>  Gets back to the prime directive. DO WHAT THE MANUFACTURER SAYS. Unless 
guys 
> are in the manufacturer's construction details, then DON'T.
>  

By all means, but don't say here that tower designed to be self supporting 
would become weaker when installing guy wires. In the nutshell, self 
supporting towers have stronger members, heavier construction than towers 
intended for guying. When you add guy wires (at the proper places) you can 
only make it stronger and safer. 
You want to play games with what manufacturer says and insurance agent 
insures, by all means. You can also add some wire (or philistran) and anchor, 
have it if needed and you can increase loading capability and survivability 
of the tower and sleep well.

>  73, Guy.
>  

Yuri, K3BU