Kevin,
Most of what you are asking is covered in a July/August 2001 QEX article by
W9JCC and the accompanying spreadsheet:
http://www.arrl.org/qex/1123.pdf
http://www.arrl.org/qexfiles/Travanty.zip
Steve, W3AHL
Date: Fri, 19 Mar 2010 00:08:30 -0700
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Subject: Re: [TowerTalk] 2" OD pipe.. again
To: "Tower and HF antenna construction topics."
<towertalk@contesting.com>
Message-ID: <4BA322EE.2020209@surfnetusa.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Question for the people who are sleeping at night:
If people create an antenna load that requires such a strong mast to
resist bending, then that force is transferred to the top section of a
crankup.
How do people calculate the bending moment a top section can resist?
It's difficult because most tower vendors spec a wind load close to the
top of the tower right? And you don't know if the limit is because of a
moment limit at the base, or a moment limit of the bottom of the top
section. (unless you can analyze a lattice tower yourself).
If the moment limit is caused by the bottom of the top section
(narrowest), then having 10' of mast past the top of the tower, can be
be say a 50% increase in the moment the bottom of the top section has to
deal with?
Or are people just worrying about cases with the tower cranked down. If
so, isn't the wind going to be less then, since it's closer to the ground?
Is the real problem here ice loading? I'm wondering if people with
anecdotal experience about mast failures are mostly in ice areas.
How do people calculate what a crankup tower, extended, can resist with
10' of mast extending from the top, with ice loaded antennas?
If people are using spreadsheets, how come we don't have a spreadsheet
that has "close enough" analysis for bending moments typical crankup
tower sections can resist?
I have some big masts I've decided not to use, just because they're too
damn heavy. Heavy on top is not a good thing. I've replaced rotor plates
with 1/4" steel to boost vertical limits, but you hit the horizontal
load limits of rotators etc.
Anyone have numbers/equations?
-kevin
ad6z
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