[TowerTalk] Rohn 25 & Mast lingth

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Wed Apr 19 05:35:47 EDT 2017

Another item to add to the equation is strength and rigidity of the 
tower, not just the stated wind load capability.
IF the mast is strong, rigid, and extends through two guy points, that 
section(s) of the tower become much stronger, but this can be easily 
carried too far.

I had a local ask me to take a look at his installation. He had mounted 
the rotator one rung above the concrete on a 100' 25G. The mast was 2" 
chrome Moly with 0.5" wall. I could not lift one end of a section.He 
knew the torsion bar effect so he wanted "rigid", but that left the 
bottom 16 inches (give or take) of the tower taking the entire torque in 
shear mode as the tower could no longer twist to take up any twisting 
motion. It also put the entire weight of that extremely heavy mast on 
the base. I suggested he put the rotator on an independent support which 
would remove the weight and torque from the tower base.

No, I'm not an engineer either, but I have had the basic courses. I 
would think all you need is the formulas as a EE certainly could do the 
math.  Even with a math minor, I'd have to do a lot of reviewing. I 
earned my degree in 90 and the only math I've had to use since then has 
been Trig, Boolean Algebra, and Algebra.  I haven't done a derivative or 
integral since I was working on my masters.  Don't think I could do one 
now to save my life.

73, Roger (K8RI)

On 4/18/2017 1:21 PM, Jim Brown wrote:
> I'm not a structural or mechanical engineer, but from my courses at 
> the beginner level in those disciplines 50+ years ago, it seems to me 
> that how much mast is needed within the tower depends on what is above 
> the tower on that mast, how long that mast is, and the wind loads.
> If you're an ME and have done the calcs, I'll certainly defer to your 
> advice.
> 73, Jim K9YC
> On Mon,4/17/2017 2:26 PM, Grant Saviers wrote:
>> There is no structural advantage to more mast inside the tower beyond 
>> 3 ft or so.  More doesn't matter either way. 
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Roger (K8RI)

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