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Re: [TowerTalk] Rohn 25 & Mast lingth

To: jim@audiosystemsgroup.com, towertalk@contesting.com
Subject: Re: [TowerTalk] Rohn 25 & Mast lingth
From: Grant Saviers <grants2@pacbell.net>
Date: Tue, 18 Apr 2017 17:47:43 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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