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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|