[TowerTalk] Rohn 25 & Mast lingth

Grant Saviers grants2 at pacbell.net
Tue Apr 18 20:47:43 EDT 2017


I'm not an ME either, and took those same courses about the same time 
ago, and have done a fair amount of recent calcs with some help from 
online calculators.  The  structural data for 10' section of Rohn 25 is 
available and specifies the moment of inertia as 15.3 in^4.  2" x 0.25 
wall mast is a calculated 0.54 in^4 . Leeson in Mechanical Design of 
Yagis shows the the strengths add when the neutral axis is common such 
as with telescoping tubing (perhaps an approximation with a 3 leg tower, 
as I am not sure the moment is exactly the same in all directions).    
So the mast centered inside an R25 lattice tower section contributes 
very little strength to the tower (3%).    Which again illuminates that 
larger diameter masts always win vs more wall thickness.   The radius of 
gyration of a R25 tower section is 4.59" vs 0.625" for a 2" x 0.25 mast 
a factor of 7.3x.

As I understand the Rohn specs, they provide the PE with the parameters 
of an equivalent black box structure without the need to delve into the 
complexity of a complicated welded lattice structural member.  My read 
of a PE analysis I had done is that there are a lot more calculations re 
leg compression & tension, shear, buckling, etc that are usually done.  
However, I think the moment analysis is a first order approximation of a 
mast inside tower properties.  A definitive analysis would use a 
detailed finite element model with a details about brace to leg welds, 
top and rotor plate properties, etc.  Perhaps Rohn has done that.

So beyond distributing the mast load over some amount of tower length, 
more mast in this common example doesn't make the tower significantly 
stronger.  The proxy I used for the minimum amount of mast inside is the 
top plate to fixed rotor bracket as provided from the factory and that 
is about 3' to 4' in many towers.

Re wind loads, I think the mast strength calculators all treat the top 
of the tower as a fixed hard attachment and perform the appropriate 
cantilever beam calculations on the exposed mast.  Thus they ignore 
whatever is going on inside the tower. I'm not aware of any that model a 
mast with two supports (top bearing and rotator), but then what would 
the proper models for those pivot points?

There have been prior posts re this topic as well.  Of course a real 
structural engineer may have more to say or correct me where I have erred.

Grant KZ1W

On 4/18/2017 10:21 AM, 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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