[TowerTalk] comparing top section of crankup to unsupported

[Dan]

When I lived in Dallas some 15 years ago I was given a damaged TX472 
tower by a friend who was moving to another state. It was folded over in 
the 2nd section from the top. I believe it had supported a TH6 and a 
shorty forty. He said he had a small tornado come through doing the 
damage I  do not know the mast situation. He had hack sawed the tower 
off at the bent area. There was a very slight bend in one of the corner 
tubes just above the attachment point at the base. I ordered a new 
section 2 from UST and replaced the damaged one. I drove a steel bar a 
couple of feet long up into the bent leg. This repaired tower supported 
a 10-15-20 meter stack for 5 years with no problems. When I moved to 
Washington I brought the tower and it now supports a 4el 20/ 2el 40 
interlaced F12 yagi and has given no problems in the 10 years up here.

73, Dan, N5AR

Steve, W3AHL wrote:
> Kevin,
>
> Based upon your assumptions and rough estimates, you are correct that the top section in your example would be operating near its max stress limit with the rated load at the top of the tower and adding a mast load based upon an "Equivalent Moment" at the tower base  would result in failure of the top section.
>
> But without seeing the manufacturer's spec's & stress analysis or doing one from scratch, it's hard to know if your assumptions and estimates were valid.  
>
> If you have the HG-72HD stress analysis that shows the max bending moment for the top section joint is 7368 ft-lbs, why extrapolate anything from an uncertain 25G spec?
>
> A spreadsheet with "per section"  limits shouldn't be necessary if you have the manufacturer's stress analysis, since it should give the calculated combined stress / maximum allowable stress ratio for each section (F.S.).
>
> Steve, W3AHL
>
> Date: Sat, 20 Mar 2010 09:04:10 -0700
> From: Kevin Normoyle <knormoyle at surfnetusa.com>
> Subject: [TowerTalk] comparing top section of crankup to unsupported
> 25g (and effects of long mast)
> To: Tower Talk List <towertalk at contesting.com>
>
> 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.
>
> I found I think on towertalk, a claim that Rohn spec'd max bending 
> moment of 6720 ft-lbs for 25g.
>
> Let's look to see if the wind load due to the top section itself is 
> significant:
> If we say .29 sq ft of wind load per ft, and 17 feet of top section 
> above the last joint, then there's the equivalent of maybe 1.2 sq ft of 
> point windload at 1 ft above the top, just due to the wind on the top 
> section. (equation not shown). Seems small, so let's ignore for now.
>
> So taking the max moment 6270 and dividing by 17+1 feet, (since people 
> spec a wind load 1 ft above the top, and taking away the overlapped top 
> section (about 3') ) we get
>
> 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 joint..so 
> 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).
>
> At 60 mph...12 lbs/sq ft of wind load? (I may be off there)...so 373/30 
> -> it makes sense they spec 30' sq ft of wind load at 1' above the 
> tower, for a US Towers HDX say, with 13" o.c. top section ...I'm just 
> making rough estimates here.
>
>
> Now what happens if you put a infinitely strong mast, so that you put 
> that wind force 12' higher?
> 18 + 12 = 30 feet
> 6720 ft-lbs/30 feet = 224 lbs allowed. So at 60 mph, that's 18 sq ft of 
> antenna allowed (at the end of the mast)
>
> Adding 12 ft to a 72 ft tower, isn't much of an increment, if you're 
> calculating the moment at the bottom.
> Adding 12 ft to a 17 ft section (distance from the last joint), is 
> significant, if the last section is the limiter.
>
> 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.
>
> But if the tower is up, the top section is quickly at risk with a long mast.
> But if the tower is down, it's fair to look around at trees and house 
> and get a more reasonable peak wind estimate.
>
> ..snip..
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