[ML-Hams] new mast

[Kris Kirby]

On Mon, 22 Aug 2016, Jeff Cotten wrote:
> I decided to not get an guy rings cause we inspected the area around 
> the shop a few times and I think we decided guy wires weren't an 
> option for us.  The only thing I could see us using one for is if we 
> had a guyring at the very top of the mast and ran a guy wire to the 
> roof of the building to help reduce sway from wind.

There are a few ways to guy towers; both of them you covered in 
pre-calculus. The standard way is a 45-45-90 triangle, the 90-degree 
angle being the ground-to-tower base angle. This is how WHNT's 900 ft 
tower is guyed, which results in guy wires coming down on adjacent 
properties to the tower base. 

Another way is the 30-60-90 triangle, which results in a different 
hypotenuse / guy wire length, but in order to provide the required 
rigidity, means that the guy wires have more stress on them, which 
translates into more vertical force pushing against the tower base, 
effectively lowering the tower "payload" due to additional stresses on 
the structure. This is how WAAY's 1000 ft / 900 ft tower(s) were 
constructed, where less land was required to fit the entire tower and 
guy anchors. The tower and bases fit inside the same property. 

It's important to point out that broadcast towers are often designed to 
carry 100,000 lbs of ice in addition to the 60-80,000 lbs they may weigh 
to being with -- and the pylon antenna can be 20-40,000 lbs itself. 

Another factor is that the tower represents a long moment arm, where a 
one pound force applied to the tip of the tower translates into a 
length*force force at the bottom, or 50-ft lbs. Windload is given as 
equivalent flat-plane area, which is what size in sq ft that the 
structure equates to, if you have a flat plate of metal in the wind 
instead of the antenna. 

http://k7nv.com/notebook/topics/windload.html

The ARRL has an old QST article that's useful for this, but I don't have 
it handy. 

Poles and towers generally translate well, but towers absolutely cannot 
handle lateral loads in the middle. Most microwave towers you'll see out 
there with more than a single dish on them are actually very capable and 
heavy-duty towers. But that's a different ballgame since you get into 
torque arms and sometimes six guy anchors, in addition to multi-level 
guying. 

Also remember that you need to do something for a ground at the top of 
the pole, and make an effort to bond all nearby metal so that if hit, it 
has a quick place to go and not get there by going through equipment. 

WSM's guy wires on the Blaw-Knox are tensioned at 20,000 lbs each, all 
eight of them.

OTOH, the big towers come down to a small point to concentrate the 
forces on a single point, because a foot out from the base is a pulling 
moment on the side the wind is blowing from, and a pushing moment on the 
side the wind is blowing to. +/- 1 ft from center of the base on a 900 
ft tower with a few tens or hundreds of feet of windload above is a 
whole of force at the ground!

--
Kris Kirby, KE4AHR
Disinformation Architect, Systems Mangler, & Network Mismanager