[TowerTalk] Windloading - Chrome-moly VS: steel

[Jim Lux]

At 07:46 AM 7/14/2006, kb0fhp at comcast.net 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
80 16.4
90 20.7
100 25.6

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.

Jim