[TowerTalk] Calculating load when raising tower

Thu May 13 22:13:41 PDT 2010

It's easy or complicated, depending on your point of view. Here's a quickee estimate.

You can sum up the point loads of the antenna, rotor, mast, and multiply that by 35/30 to get the 
vertical lifting force needed for that stuff at the attach point. (note it's more than their weight 
because of the relative position of them, attach and pivot point.

The weight of the tower is small (100 lbs I'm guessing?) but by lifting at the end, you can say 
that's a point load at the halfway point of the tower (think levers...it's all about making moments 
match at the pivot point at the base). If you didn't lift at the end, you'd break it into two point 
loads.

So your total vertical lifting force is maybe 200 lbs required? But your cable is at a shallow 
angle. think of a triangle and pythagoras for resolving the force vectors. with the house 30 more 
feet away, and 15 feet high, the cable will need see about 4x that 200 lbs in it..i.e 800 lbs. 

But the pulley is seeing 2x that, since the cable is going back to the tower: the pulley sees both 
the tower and the winch forces. I'm assuming the extra pulley at the tower isn't providing much of 
an extra compound pulley effect since it goes to the winch at the base, so I'm ignoring that 
benefit.

So the house pulley and attach will see 1600 lbs pullout force, which can be decomposed into 
horizontal and vertical components. (which might be interesting to understand, depending on how you 
attach to the house)

You can reduce this a lot by lifting the tower some other way from perfectly horizontal for a 
couple of feet. You won't be fully horizontal anyhow because the antenna is on there.

If the forces are too high, you need to raise the height of the cable where it crosses the pivot 
point. You can do this by attaching at the house higher, or some other way.

But compared to a steel crankup, your forces aren't that high. However: it calls into question what 
you consider an adequate attach at the 15 ft point at the house.

Note also the tower attach sees more than just the vertical force, it sees the force the cable 
sees. 

So there's no one equation or number. You have to analyze all the components separately. Changing 
distributed loads (tower) to point loads (before and after the attach separately if necessary, then 
using equal moments to figure out resulting forces. Then working out how the angles affect the 
force vectors

And don't forget some safety factor. Also, it's easy to overestimate how strong a wall attachment 
to a house is. (plus you have to worry about leaks/wood damage)

Separate analysis would be questions like: are you going to bend the tower or base attach? You can 
imagine that at some very low angle the answer is yes (probably not for you). You can get that by 
figuring out the horizontal component of the cable forces on the tower.

-kevin
ad6z
------- Original Message -------
>From    : Alfred Frugoli[mailto:ke1fo at arrl.net]
Sent    : 5/13/2010 12:10:18 PM
To      : towertalk at contesting.com
Cc      : 
Subject : RE: [TowerTalk] Calculating load when raising tower

 Hello Everyone,

Is there a simple equation for figuring out the forces that would be
experienced in tilting up a tower?  I'm buliding a raising fixture, and want
to make sure that pulleys and the winch can handle the associated loads.
The tower is a 30 foot tower, and will be raised by a cable attached near
the top of the tower, that will run through a pulley 15 feet above ground
about 30 feet away from the tower (on an appropriately reinforced point on
the house), then back to a pulley near the top of the tower then to a winch
mounted at an appropriate height at the tower base.  The tower is a 30 foot
aluminum tower with a rotor, 5 foot aluminum mast, and a small tribander
just above the top of the tower.

A 2nd question is would it be better to have the cable routed in a different
way, or attach at a lower point on the tower (level with the raising point
at ~15 feet)?

73 de Al, KE1FO

K3 #3055
K3 #4094
-----
Check out my Amateur Radio Contesting blog at ke1fo.wordpress.com.
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