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Re: [TowerTalk] comparing top section of crankup to unsupported 25g (and

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] comparing top section of crankup to unsupported 25g (and effects of long mast)
From: jimlux <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Sat, 20 Mar 2010 15:38:44 -0700
List-post: <">>
Kevin Normoyle wrote:
> Thanks for the thoughts on analysis. sounds like some folks were 
> interested in this, so here's what I was thinking when I said I wasn't 
> confident  analyzing the moment limit at the base was sufficient
> I can't help thinking that the top section of crankups is similar to 
> looking at unsupported 25g.
> I am no PE, just trying to understand what I should worry about and what 
> I shouldn't.
> If we just had a little database of max bending moment for different 
> crankup tower sections, we could expand spreadsheets like Travanty's to 
> do "per section analysis"
> Steve is probably right abuot which section typically has least 
> margin...but here's a quick swag just thinking about the top section, 
> and what happens if you put a long mast up.
> Rohn 25g is 12" o.c. tubes right? Very similar to many top sections of 
> crankups. (US tower is 13" o.c. tubes?)
> So a very rough estimate would be comparing a top section behavior, with 
> a long mast, to unsupported 25g with a long mast.

You need to know the diameter of the tubes and the wall thickness, too, 
because the failure mode is typically some member failing by buckling 
under compression load, rather than bending.   Length/diameter ratios 
are real important.

Don't forget the diagonal braces: They're typically longer and more 
slender, so they might actually be the strength limiting member.

If you have data on the whole section, as a unit, that makes life easier 
(accepting the limitations)

> <ssnip>

> 6720 ft-lbs/18 ft = 373 lbs (horizontal) allowed 1 ft above the top section.
> For comparison, on my HG-72HD analysis, it appears that if I take the 
> numbers on compressive load limits on the vertical tubes of the top 
> section, they imply a 7368 ft-lb limit for max moment at the 
> I'm in the right ballpark. Also it appears compressive load limits on 
> the vertical tubes might be the limiter? (there were other failure modes 
> analyzed).

That depends on the design.  There's probably a compressive load on the 
diagonal strut, too, and because more than one member takes the load, 
you have to know the distribution of loads, which in turn depends on the 
stiffness (in compression).. it's an overdetermined system.

You can make a worst case analysis by assuming that ALL the load is on 
each member in turn.

> So what's my point: Sure if you think you're cranking down your tower in 
> big winds, and that's how you get away with a big mast and big loads, 
> and you need the strong mast to survive when the tower is down.

Seems to me that someone who is depending on cranking down to take the 
loads is already taking some risk (e.g the wind comes up fast, and they 
don't get the tower down in time).

> Now on whether ASTM A53 Type E Grade A (30k yield) is no good for 
> structural (which is what my pipe is stamped)
> Type E is Electric Resistance Welded. ASTM A53 Type E Grade B is used 
> for structural stuff. Typical it has a higher 50k yield strength. The 
> composition of the steel is slightly different.
> But the only manufacturing difference I can find is that Grade B heat 
> treats the weld afterwards, while Grade A doesn't. Grade A is pressure 
> tested. Grade B isn't.

Pressure puts only a tension hoop load on the pipe, while structural 
applications put bending/twisting loads. The stress concentration and or 
different material properties in the weld region from the non-heat 
treated weld probably make a bigger difference in the structural 

> When people say the manufacturing process is no good: are they talking 
> about "not testing" or something specific like heat treatment of the weld?
> When people say "water pipe" are they saying ASTM A53 Type E Grade A 
> (galvanized) or ???

I think ??? is the real issue..  "pipe" covers a lot of ground from some 
ratty thinwall conduit to actual structural steel.

If you tell someone to use 1 1/2" pipe and they just whip on down to 
Home Depot and ask the guy in the orange smock: "give me 20 feet of 
pipe", it's hard to know what you're getting.

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