[TowerTalk] comparing top section of crankup to

[Steve, W3AHL]

Kevin,

Based upon the info you supplied previously, any length of mast you add, while staying within the allowable base moment loading, will not exceed the top section's max of 7638 ft-lbs.  

Rather than continue this on the reflector, if you can scan and e-mail to me the stress analysis document for your tower, along with a description of what antenna configuration you would like to install, I'll try to find some time to explain the process using your real data, instead of using a few numbers out of context.

I am not a P.E., so it would only be an example.  Otherwise, you should find a ham-friendly P.E. in CA to determine if what you want to do is feasible. 

If you can forward any pictures or descriptions of telescoping towers that have folded at the top section's overlap, I would be interested in them.  I have only seen pictures of the mast bent over, resulting in the booms preventing the tower from being lowered.  And lift cables that have snapped, nesting all of the sections and making a real mess.

Steve, W3AHL

Date: Sun, 21 Mar 2010 08:22:31 -0700
From: Kevin Normoyle <knormoyle at surfnetusa.com>
Subject: Re: [TowerTalk] comparing top section of crankup to
To: "Tower and HF antenna construction topics."
<towertalk at contesting.com>

Steve, W3AHL wrote:
>
> It appears that the tower's wind load is limited by the moment at the base, if the base F.S.=0.98 and top F.S. is <0.5.  Therefore using the Equivalent Moment method of determining what the allowable antenna projected area is on a mast extending above the top of the tower stub is appropriate.  This is consistent with Travanty's article and the ARRL Antenna book methods in chapter 22.
>   

I looked at the Equivalent Moment description on page 22-21 of the 20th 
edition of the ARRL Antenna book.
The graphic, Fig 32, shows a uniformly designed 70-foot lattice tower. 
The section design is the same from top to bottom. That is not what a 
crankup is.

I guess we'll have to leave it this as I just don't understand.

I agree with equivalent moments, but believe they need to be done at 
each joint. The effect of a long mast is not the same on each joint, 
with the largest relative increased effect on the last section.

Here's a fer-instance that should make my thinking more clear.

Imagine that the top section was designed, so that the antenna load 1 ft 
above the top, caused F.S of 1.0 at the same time as FS of 1.0 at the 
base. Now make the mast 10' with the antenna at the top. Do you think 
the top section and base now have the same F.S? It sure seems to me that 
the answer is no. So it doesn't matter what joint has the worst F.S  
when you do one analysis, for nonuniform sections. You need to know the 
max moment allowed at each joint, and compare to the final situation.

I may be just wrong, and we can leave it at that. I believe that 
anecdotal info on towers folding on top sections more than bottom 
sections, supports me.

Here's another fer-instance.  If I don't have to do per-section 
analysis, why do I care about the mast strength...I'll put another mast 
on top of the mast, am I'm good to go! It's not a long mast..it's a 
short mast on top of 5 more sections!

-kevin
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