[TowerTalk] Rohn 25

[Joe Subich, K4IK]

Hank, 

> Using TIA/EIA 222-F wind loading criteria and 70 mph wind 
> velocity (min. recommended) a 40 foot section of Rohn, 25g 
> un-guyed and cantilevered will develop a moment at the base 
> of 7200 ft-lbs at the base. This is without an antenna load,
> just the tower itself. The published allowable moment for 25g
> ,by Rohn, with a 1/3 increase for wind loading duration is 
> 6,720 ft-lbs. At 40 feet it is clearly overstressed. How do 
> you say that this is ok and that Rohn fully supports this?

"Self-Supporting Towers - Allowable Antenna Areas (Sq. Ft.)
Drawing A871266RI  dated 9/24/1987 and found in the Rohn 
Commercial Products Catalog (mine is copyright 1993).  

For 70 MPH w/no ice the maximums are: 

#25  40' @  1.5 sq. ft. (1 x 5/8" line per face) 
#45  45' @  2.3 sq. ft. (1 x 7/8" & 1 x 1/" line per face)
#55  60' @  0.8 sq. ft. (2 x 7/8" lines per face) 
 "   55' @  4.0 sq. ft. (2 x 7/8" lines per face) 
#65  80' @  0.9 sq. ft. (2 x 7/8" lines per face) 
 "   70' @  8.7 sq. ft. (2 x 7/8" lines per face) 

The chart also shows self-supporting heights for 80 and 90 MPH 
as well as the data for the same heights with 1/2" of ice. 
Surface areas assume round antenna members.  

73, 

   ... Joe, K4IK