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 101520 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 HG72HD stress analysis that shows the max bending moment for
> the top section joint is 7368 ftlbs, 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@surfnetusa.com>
> Subject: [TowerTalk] comparing top section of crankup to unsupported
> 25g (and effects of long mast)
> To: Tower Talk List <towertalk@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 ftlbs 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 ftlbs/18 ft = 373 lbs (horizontal) allowed 1 ft above the top section.
> For comparison, on my HG72HD 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 ftlb 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 ftlbs/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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