At 07:46 AM 7/14/2006, firstname.lastname@example.org wrote:
>Now for a 20 foot mast, the max load (located at the tippy-top of the
>mast) is about 2850 pounds for the 4130, and about 1700 pounds for the
>black steel A53 Shedule 80 pipe. If I use a safety factor of 2 - which I
>don't think is unreasonable, then the maximum loads are about 1425 pounds
>for AISI 4130, and about 850 pounds for the A53 Steel pipe.
>This is where I need help. Now assuming the codes, how do I calculate the
>windloading?. I know there was a long discussion on that a while ago -
>which I need to review. I really wish there was a chart - antenna loading
>in ft^2 vs wind speed......
Loading(Lbs/sqft) = windspeed (mi/hr)^2/391
( 391 takes into account all the unit conversions and the air density at
sea level standard conditions)
60 mi/hr = 9.2 lb/sf
70 mi/hr = 12.5 lb/sf
Actually, for rough and ready back of the envelope estimates, I use
(mi/hr)^2/400, so 20 mi/hr = 1 lb/sf, and then scale from there, since load
goes as the square of speed.. 40 mi/hr = 4, 60 mi/hr = 9, 80->16, 100->25, etc.
>.Does someone know of such a beast? SInce the worst case is a point load
>at the end of the mast - this could really help size the necessary mast sizes.
Don't neglect the drag on the mast itself. cross sectional area of mast,
load at half way up the mast.
>As with all calculations, it is best to verify the calculations, and if
>necessary contact the appropriate engineering or other cognizant authority.....
All a matter of risk posture. Back of the envelope, double the numbers,
put it up and hope it doesn't fall on anything important is just as valid
as a finite element model with 0.1" grid cells and statistical wind models.
"Analysis paralysis" is an ever present hazard.
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