Steve:
>
>
> 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?
Because it's hard as hell to interpret an engineering report, and be
sure what I think I'm reading is what it says!
I'll walk thru the details, and you'll see why I can't be sure what an
allowed max moment might be, in modern analysis, for each section.
Also: I've never seen anyone describe how to migrate a manufacturers'
spec on windload at 1' above the tower, to something distributed across
a 10' mast, given moment limits for each section.
People have pointed to the Travanty article, but as I say, I don't think
that's exactly right.
I have a "Load and Stress Analysis, Telex/Hygain Product 131, Steel
Crank-Up Tower" by Morris Stover, March 10, 1983 for my HG-70HD. The
section face widths (tube o.c.) are 12.75", 15.188", 18.033", 21.125".
The manual mentions 16 sq ft antenna load limit, and 60 mph wind
survival with max load.
The use of 12.5" top sections still exists, for example, in the US
towers TX-472. I don't know what kind of steel they use in their tubes
though, nor diameter or wall thickness. However: The TX-472 has a 21.6"
bottom section so the taper schedule for the 4 sections must be similar.
The TX-472 ships as 1040 lbs. The shipping weight for a HG-70HD was
supposedly 1160 lbs, so similar. They rate a TX-472 for 10.3 sq ft at 70
mph, 35 sq ft at 50 mph. I mention this for comparison.
From my report: Wind loads were analyzed with UBC 30. 40lbs per sq ft
of wind area for the antenna mast and top section.
But I could swear it's using *1 sq ft* for the antenna in this
analysisThis is then used (along with other section analysis) to create
a moment curve and shear curve, from base to top.
The moment shown at the first joint below the top is 2959 ft-lbs with
this analysis.
allowable stresses for legs, single and double diagonals are shown as
from AISC Allowable Stress for Compressive Members 1.5.1.3.1 or 1.5.1.3.2
These calculated "allowable" stresses appear to be increased by 1.33
(from Fa to F'a) "by UBC". Don't know why.
Then the calculated moment at each joint, is used used to create a
stress number to compare to allowable stresses. This is a section
"Actual Stresses & Compressive Loads-Legs". It seems to combine the dead
load due to pulley reactions and the wind induced moment.
For the top section this results in F'a/fa = 2.49
My reading is that means a 2.49 safety factor, for that assumed antenna
load.
The same safety factor appears to be only 0.98 (which they take to be
one. ...i.e. they see fa of 33,350 psi in the bottommost base legs (and
F'a allowed of 32,530 psi)
So if a moment of 2959 lbs gives a 2.49 safety factor, I'm reading the
max allowed as 2.49 * 2959 = 7368 ft lbs (at that joint)
Compressive loads on the diagonal braces has a higher safety factor, so
I ignore that analysis. (not a limiter)
However, who knows if the allowed stress has changed since 1983, or the
1.33 factor allowance?
Interestingly, it seems to say it's using 45ksi Fy (yield strength) for
the legs, and 36ksi Fy for the single and double diagonals.
On the mast thing: it sounds like the spreadsheet stuff is worthless,
because we can't just analyze using yield strength. The manufacturing
process is important (although I've never seen a photo of a split weld).
The mast spreadsheets then are only good for some set of manufacturing
processes. (and it's not just about electric resistance welding or
not...I thought a lot of DOM starts out as ERW?)
Personally, I think people focus on masts, just because it's easy to.
If we had tools to analyze the whole system ...whoooie! people would be
stuck trying to justify their 20' masts on crankups. :)
-kevin
AD6Z
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