On 12/6/11 2:23 PM, TexasRF@aol.com wrote:
> Hi Gene and Mike, my recollection is there were no 90 mph calculations for
> the BX series towers.
>
> The conversion from 70 to 90 mph is not simple or straightforward. The
> problem is that the tower itself represents a large windload and when that is
> taken into account, there is nothing left for an antenna load.
>
>
Gerald is right, here..
BUT, the scaling approach you've identified *might* actually work for a
*limited* set of circumstances.. Here's why...
Let's say that a 100 ft tower can tolerate a uniform horizontal load of,
say, 1000 pounds, plus 100 pounds at the very top. And those limits were
developed based on, say, 10 pounds/square foot, with 100 square feet of
tower plus 10 square feet of antenna. (10 lb/sf is about 60 mi/hr)..
So that would get published as "up to 10 square feet of antenna at 60
mi/hr" in the tower specs.
Now, what happens if we go to 70 mi/hr.. We're at about 12.5 lb/sf. So
the uniform load is 1250 lb. The tower was able to withstand a bending
moment of 1000*50+100*100 = 60,000 lb-ft for 60 mi/hr. Just the tower
1250*50 =62500 lb-ft (for 70 mi/hr) is more than the total load was for
60 mi/hr, so it's not going to work.
However, for a different tower and different ratios between antenna area
and tower area, it might work. It just depends on the tower design.
If, instead, it was more like 400 pound load on the tower and the same
100 pounds for the antenna at 60 mi/hr...(total moment of 20k+10k = 30k
lbft) that's like 40 sq ft of tower and 10 sq ft of antenna.
Bump up to 70 mi/hr and the load on the tower (alone) is 500 lb (or a
moment of 25,000 lbft) leaving 5000 lbft for your antnena, so you could
have a drag of 50 lbs or 4 square feet at 70 mi/hr.
The problem with going from 70 to 90 is that it's an increase in drag of
65%. In order to have ANY antenna on the top at 90, the drag area of
the tower would have to be less than about 60% of the whole assembly.
A BX or HDBX which has pretty limited capability to start with, and has
a LOT of structural stuff in the air, it doesn't look good.
(and this doesn't even get into changes in drag coefficient with
velocity, or the fact that "the weak link" in the tower might not be the
bending moment at the base, but somewhere else)
>
>
> In a message dated 12/6/2011 3:43:51 P.M. Central Standard Time,
> w2lu@rochester.rr.com writes:
>
>
> As I recall wind pressure "on a flat surface" at 70 mph is 20.6 lbs/sq
> ft.,
> so if the tower is rated for
> 18 sq ft at 70 mph, would it be too much of a stretch to say to the zoning
> board that since wind pressure at 90 mph is 34 lbs/sq ft, by simple math,
> the tower should be good for 10.9 sq ft at 90 mph ?
> Gene / W2LU
>
>
>
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