On Fri, 16 Jan 1998 13:57:09 EST UpTheTower <UpTheTower@aol.com> writes:
>Steve:
>
>I was browsing thru tower talk archives, I'm not subscribed and ran
>across the attached message concerning derating a crank-up tower. To
>properly derate the tower an engineer needs to look at this case.
>Consider the following per the 1994 UBC
>
>the basic stagnation wind pressure formula is Q= 0.00256 * V*V
>where Q is in psf and V is in mph.
>
>This is the basic wind pressure that is modifiec for height, exposure,
>gust, importance and
>geometric coefficients.
>
>Basically if increase from 50 mph to 80 mph then the increase is:
>
>(80/50)(80/50) = 2.56 due to the fact that it is a function of the
>velocity squared.
>
>Hope this helps this gentleman and others.
>
>KR7X / Hank Lonberg P.E. (structural engineer and ARRL VCE)
The formula that is usually used is Q = .04 V*V
so:
50 mph = 10 psf
70 mph = 20 psf
80 mph = 25.6 psf
It is correct to say that that at 80 mph the horizontal
wind force on the overall tower structure is 2.56 times
what it is at 50 mph.
It is NOT correct to say that, since a tower can carry
30 sq ft of antennas at 50 mph, it can carry 30/2.56
or 11.7 sq ft at 80 mph. You have to hire the PE to
refigure the whole tower and find which structural
member fails as the wind speed increases.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
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