Kevin,
Without seeing the full report, I can't answer your specific issues, but I
would accept the analysis as valid for the standards used at the time.
It appears that the tower's wind load is limited by the moment at the base, if
the base F.S.=0.98 and top F.S. is <0.5. Therefore using the Equivalent Moment
method of determining what the allowable antenna projected area is on a mast
extending above the top of the tower stub is appropriate. This is consistent
with Travanty's article and the ARRL Antenna book methods in chapter 22.
Regarding analyzing allowable mast loads using a spreadsheet, you misinterpret
what my point was. There is nothing wrong with electrical resistance welding
of seams in pipe or tubing, if the process & material complies with the
specification required for the application. The only testing done on A53 Type
E Grade B 2" pipe is typically a hydrostatic pressure test, which is not
adequate for structural use. There is no test to determine if the weld seam
yield strength is the same as the rest of the pipe. Larger sizes of A53
require NDT and ductility testing to verify the weld seam is as strong as the
base pipe material. Therefore standard stress analysis formulas are valid for
predicting how the material will respond to structural loading.
I haven't found a good reference book that would serve as a tutorial on tower
design and stress analysis for those without mechanical engineering degrees.
There are many good engineering and materials texts at university libraries,
but none that detail the specific issues with antenna supports. The TIA
RS-222-C, -F and -G standards are worth reading. The current -G version is
less straightforward than earlier versions and costs $600. Check your nearest
university engineering library for a copy.
Steve, W3AHL
Date: Sat, 20 Mar 2010 19:02:58 -0700
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Subject: Re: [TowerTalk] comparing top section of crankup to unsupported
To: "Tower and HF antenna construction topics."
<towertalk@contesting.com>
..snip...
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)
..snip..
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?)
..snip..
-kevin
AD6Z
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