[TowerTalk] 2" OD pipe.. again

[Steve, W3AHL]

Kevin,

The three crank-up towers I've seen a stress analysis for (MA-550, MA770 & TX-455) all have the greatest F.S. ( 0.48 - 0.56) for the top section and the least margin (FS= 0.90-0.93) in the next-to-bottom section (at the bottom of that section), not the base.  US Towers does show the M/EI for top, CG and bottom of each section and there is enough detail to create a stress analysis model on your own and compare the intermediate results to the manufactdurer's.   So Travanty's method is probably close enough in many cases, but isn't a substitute for a full stress analysis if a really long mast was being used.  But nobody should be using a long mast on a self-supporting crank-up anyway.

Lift cable failure due to poor lubrication and inspection is probably a more common cause of failure of crank-ups than top section overstress in my limited circle of hams.  But most lower their tower when high wind is forecasted.   Which is a good thing, since many self-supporting crank-ups are overloaded as the antenna farm grows.  Which you can get away with, as long as you remember to crank it down.   But then all that raising and lowering is what causes the unlubricated cable to fail  -- especially because of the small diameter pulley used at the bottom of tubulars, which stresses the cable.  

GRAPE is a fairly easy to use and capable stress analysis program.  It keeps reminding you to register and pay for it, but it keeps on running without registration.  Once you get familiar with GRAPE, you can build a section of lattice tower in about 20 minutes, of any length.  Of course for a telescoping tower, you have to build the model for each section.  So with a few trips to a university library to learn structural design and stress analysis, it is possible to do meaningful evaluation of tower design issues.  K7NV's Guyed Tower Study at:

http://wiki.contesting.com/index.php/Guyed_tower_study  used an early version of GRAPE, as an example.

It won't do a dynamic load analysis for oscillations.  I don't believe it does p-delta calcuations for large deflections, either, so you have to allow extra design margin similar to what the manufacturer's analysis shows.   But it is a useful learning tool to go beyond what a spreadsheet allows.   But given the inherent safety issues with towers, only a P.E. that knows towers should approve design "extensions".  

In my opinion, there is no way to do any stress analysis of pipe without a material testing lab, and then the results only apply to the specific batch of pipe sampled, at best.  Pipe is not a structural element.  Even if you know the yield strength of the steel, the manufacturing process does not result in a predictable structural component.  Ask a P.E. to certify a mast using "pipe"....  Hundreds of hams use pipe successfully for years.  But it isn't a sound practice that I would ever recommend to others as "safe enough, most of the time".

But that is just one side of the eternal pipe debate....

Steve, W3AHL

Date: Fri, 19 Mar 2010 18:17:37 -0700
From: Kevin Normoyle <knormoyle at surfnetusa.com>
Subject: Re: [TowerTalk] 2" OD pipe.. again
To: "Tower and HF antenna construction topics."
<towertalk at contesting.com>

Hi Steve.
I had read Travanty's article. Travanty assumed the moment limit implied 
by manufacturer's wind load restrictions, is because of a max moment 
limit at the base of the tower. On page 5 he says

"Wind loading specification from the tower manufacturer. In my case, the 
tower was rated for 23.3 ft2 of antenna located one foot above the topof 
the tower. This is a 350-lb wind load at that point, or 19,600 ft-lb, 
referred to the *base* of the tower."
and pm page 6
"The spreadsheet then returns the mast yield, total moment at the tower 
*base* for the complete system and the moment of the tower without 
antennas."


Travanty does no analysis of top sections. Please correct me if I'm 
wrong. Like I said, I do have a set of engineering calcs on a 
per-section basis for my hold HG-72HD. What surprised me is I think that 
analysis is "light"..i.e. too low antenna load. (also old UBC wind analysis)

I, along with lots of you folks, know that crankup towers don't fold at 
the base. There are anecdotal reports of top sections folding more than 
bottom sections right?  Travanty didn't have more data than I have. You 
have to analyze each section separately. The manufacturers don't give us 
the data to do that.

But you see my main point. A claim that anyone sleeps better than 
someone else, because he ran a spreadsheet on a mast, and "that's 
it"..well there's a bit of head-in-the-sand aspect to that, and doesn't 
deserve a fingerpointing at Joe Ham?

Any time people load up something, you gotta understand how that load 
gets transferred to the ground. And as someone pointed out, it's not 
just a wind horizontal loading problem. Columnar buckling, dynamic 
oscillations etc.

It would be really, really funny, if the added weight up top, adds 
oscillation modes that make things worse!

If people want to berate Joe Ham (it's funny, I actually just did put in 
1.9" A53 water pipe in a triex w-51, because last time I used it, I was 
getting too much binding in the top with a 2" mast. Am I comfortable 
with the loads it's going to see? Yes.

Now with respect to berating the use of fence post, I think that's 
another thing that's wrong.
Wheatland and Allied both sell quality 1.9" O.D. Fence post with .120" 
wall (down from the normal .200" wall of schedule 40 water pipe). BUT: 
it's 50k psi yield (although Home Depot might not have "the good 
stuff"..thinner wall and 30k psi is sometimes used too).

This SS40 and WT40 fence post, is STRONGER than schedule 40 A53 30k psi, 
even though it has a thinner wall.

I've tried fence stuff for no-big-deal things, but didn't like it 
because the walls were too easy to compress for the stuff I was using.
So it's fair to question local buckling issues (which is a whole nother 
analysis!) with certain wall thicknesses.

So, like people say: It all depends on your situation. What we lack are 
the tools for fully analyzing towers. We have a mixture of good and bad 
data and models. Let's explore making that better, not making fun of 
each other like this is actually fully scientific. It ain't.
(I notice there are good tower modelling programs available today. But 
they're not free. Too bad. )

-kevin
AD6Z
..snip..