[TowerTalk] comparing top section of crankup to unsupported

[Steve, W3AHL]

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..