[TowerTalk] Rohn 55 vs. Rohn 45

[kr7x at comcast.net]

Joe:

Rohn 55g legs are 0.5203 sq in and 45g legs are 0.3043 sq in each. That is 71% larger, this is significant. Give the leg clear spacing (between the horizontals) and 50 ksi yield steel the allowable leg force for 55g is 14,000 lbs. vs 45g which is 7,950 lbs. This is based on the Kl/r of the legs. 55g has 76% more compressive capacity.

Based on the full section properties ( all 3 legs) and the maximum spacing for the guying arrangements shown on the Rohn drawings for 110 mph and 100 feet in height, the max unbraced length for the 55g is 33 feet and for 45g it is 34 feet.
Given this then the allowable section load for 55g is 36,220 lbs. and for 45g it is 20,740 lbs. 55g has 75% more compressive capacity. By the way in this case the controlling value for 55g would be 36,220 lbs as it is less than 3X 14,000 = 42,000 lbs. For 45g it would be 20,740 lbs as it is less than 3X 7,950 = 23,850 lbs.

The guy systems are sized to develop the required horizontal load capacity and the tower section is sized to develop the vertical component of the guy forces. In a multi guyed tower the vertical components of all guys are added to get the total compressive force.

Your comment about what users are getting away with has no relevance to the physics of the system. 

The upshot is that the capacity is directly related to the " .. modestly larger side rails. ..." and the spacing between the guy attachment locations on the tower.

As to the added cost, only the user can decide that if the increase in strength (allowable antenna area) is worth the increase in cost of not only the tower section and foundations but all the other appurtances necessary.

Regards
Lonberg Design Group, Ltd.

Hank Lonberg, P.E.,S.E. / KR7X
President

-------------- Original message -------------- 

> 
> OK, here's a toss up that should generate some real discussion: 
> 
> Rohn 45 and Rohn 55 have essentially the same face width and 
> the same cross bracing. The Rohn 55 has slightly thicker wall 
> side rails -- the center to center spacing of the side rails 
> is only 1/4" more (about 1.5% larger) than that of Rohn 45. 
> 
> I am confused when comparing the Rohn 45 and 55 catalog specs 
> for 100' at 110 MPH. The 45 design uses 1/4" EHS at the top and 
> 3/16" for the intermediate guys ... it is rated for 7.9 sq. ft. 
> The 55 design uses 5/16" EHS at the top, 1/4" EHS in the middle 
> and 3/16" EHS at the bottom ... it is rated for 15 sq. ft. The 
> guy anchors for the 55 design have about 25% more concrete (.84 
> cu yd vs. .67 cu yd). 
> 
> I can't imagine the difference in rated capacity is explained by 
> the modestly larger side rails. Does this indicate that Rohn 45 
> could safely be loaded more heavily if the guy wires were upgraded 
> (e.g., to the Rohn 55 design or by using TA45 "star" guy brackets 
> and double guys at each level)? I also know that many users have 
> significantly overloaded towers when compared to Rohn's catalog 
> designs and gotten away with it. 
> 
> Is the added cost (about 25%) for Rohn 55 "worthwhile" -- is it 
> really that much stronger or is Rohn 45 "as good" when guyed the 
> same way as 55? 
> 
> 73, 
> 
> ... Joe, K4IK 
> 
> 
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