[TowerTalk] Rohn 25 & Mast lingth

Steve Maki lists at oakcom.org
Tue Apr 18 22:17:34 EDT 2017

Both of you are way more ME than I am, but I saw (& was hired to remove) 
a bent (crumpled right between the support tube and rotator) pointy top 
R25 top section with a tall heavy duty mast inserted only 3' to the 
standard R25 rotator position. To my way of thinking, that mast exerted 
a way different force vector on the tower than it would have if extended 
deeper into the tower.

I'm guessing it depends on how stiff the mast is. If it is perfectly 
stiff, the force at the top is mostly horizontal if the mast is deep 
into the tower.

Note: the pointy top R25 section is missing diagonals where the rotator 
is inserted.

-Steve K8LX

On 4/18/2017 20:47 PM, Grant Saviers wrote:

> Jim,
> 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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