[TowerTalk] adding up wind loads

[Jim Lux]

At 10:47 AM 1/31/2007, Jeff Kinzli wrote:
>So I know that wind load at the top of the tower is much different
>than wind load 10 feet up from the top of the tower, and that towers
>are wind load rated at the top of the tower.
>
>I'm trying to understand how this fits together in terms of stacking
>yagis. My plan is to use a 4el Steppir and a 2 el Cushcraft 40M about
>10 feet above that. With the roughly 10sqft for the Steppir, and 6
>sqft for the Cushcraft, that's 16 sqft total, but I'm sure it's quite
>a bit more than that given the stack, and ratings for towers looking
>at the wind load at the top of the tower rather than 10' above the
>tower.

In short strokes...  as far as the bending moment of the mast goes, 
there's a huge difference, because the "pivot point" is at the top of 
the tower.. you're looking at a load, say, 12 ft from the pivot vs 2 
ft from the pivot, and the bending moment is a torque (force times 
distance). In this example, the load is 5 times greater (10/2).

  As far as the bending moment for the tower (or guy loads and 
downforce, for a guyed tower), not as big a difference, because the 
pivot point is at the bottom of the tower.  You're looking at the 
difference between a load at, say, 50 ft from the bottom and one 60 
ft from the bottom.  in this case the load is 20% greater (60/50)

guyed towers are a bit trickier, because there's really two or more 
pivot points (the base and whereever the guys attach), and not only 
that, but the system is overdetermined (meaning that there are more 
constraints than degrees of freedom) so you can't just assume ideal 
components (you have to model things as springs).

Fortunately, unless you're running at the ragged edge of strength, 
you can make simple and conservative approximations.



>Are there calculations that I can do to figure out how much tower I
>need to support this?

yes. download the spreadsheet and article from ARRL.  It wil get you 
started so you understand what's going on, even if find you need to 
hire someone else to do the detailed calcs.



>Or would that be something a civil engineer
>would do (or my architect) before I get the permit for the tower (and
>buy the tower).

Depends on your area and confidence in skills.  Some might want a 
professional's analysis, others might not.  As far as selecting the 
tower goes, it's more a matter of finding the models that exceed your 
requirement.

Jim, W6RMK