>I have a crank-up tower rated at 30 sq. ft. for a 50 mph wind. I have 20
>ft. of chrome-molly mast with 4 ft. in the tower. I want to de-rate the
>wind load for 70 mph. How do I do this and what effect does the mast have
>on the wind loading limits of the tower? I want to put 8 sq.ft. near the
>top of the mast and 8.5 sq. ft. at the top of the tower. Can I do this, or
>am I dancing with disaster?
>
>tnx, Henry KB7RTA
>
>
>
>henry gillow-wiles
>henry@rio.com
Hi Henry,
Let me think "outloud" about this for a minute or two.
First, when the wind goes from 50 to 70 mph, the forces on objects (towers
and beams) DOUBLES since it goes up with the square of the wind speed.
Second, the tower itself has windload. Just for discussion purposes, let's
assume the tower equals 20 square feet of load by itself. I have never seen
a crankup tower manufacturer ever mention this point.
This means that the tower plus the beams on top represent a load of 50
square feet and the tower is designed to withstand that at 50 mph. Since at
70 mph it will only withstand a total of 12.5 square feet, we are in serious
trouble, because the tower with NOTHING on it is still 20 square feet . . .
You get the idea. This is precisely WHY your tower was rated at 50 mph. At
70 mph, I would expect it to fold with nothing on it . . .
Sticking a mast out the top with some of the load up there makes the problem
significantly worse . . . and yes, the mast also has windload which has to
be added to the total.
Get the PE that someone else suggested. Don't you find it strange that
derating information of this type is not published for crankups? Certainly
you are not the first guy who asked this very question. The reason is
simple. Publishing that data would "unsell" a LOT of crankups . . .
Stan w7ni@teleport.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|