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.
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.
I've noticed that some crankup manufacturers use ASTM A53 Grade B for
building their towers, along with finding it on a tower plan for the
tubes. (there are some variant Type F and Type S for A53. I can't tell
for sure but am assuming it's Type E also).
People are claiming there are substantial differences in manufacturing
(I know there are higher grades of structural steel...but ASTM A53 Grade
B Type E "is" used for some structural stuff as far as I can tell.
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 ???
-kevin
AD6Z
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