Here is what "cheap mast" pipe did to me when the eye of Charlie came over..
http://www.n4zkf.com/files/Tower.jpg
I have since went to steel and it's done well.
The TV antenna mounted to the power weather head in the foreground, it's me
but a moron neighbor. FPL
has since made him remove it.
73 Dave n4zkf
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Thursday, March 18, 2010 2:04 AM
To: towertalk@contesting.com
Subject: [TowerTalk] 2" masts.. an eye opener !
Date: Wed, 17 Mar 2010 18:42:35 EDT
From: TexasRF@aol.com
Subject: Re: [TowerTalk] 2" O.D. Pipe???
hi Ted, you can find it at _www.texastowers.com_
(http://www.texastowers.com) .
Click on the on line catalog button and select steel masts.
73,
Gerald K5GW
## I used the arrl mast software, available free online at
http://thebont.com/spreadsheets/AnalysisOfAntennaMastStrength.htm
Scroll down , till u get to 'download here' . This is from the 2004 ch-22
of the arrl ant book.
I got a REAL eye opener, when plugging stuff into it ! I'm stuck using a
2" mast on the HDX-689 I just bought, since conversion to 3" is a
horror
show. The PPM, bearings, etc, holes in plates, etc, everything is 2".
That being the case, and wanting to install 3 x yagi's on the mast + a 80m
rotary dipole, the 2" mast demands are enormous.
I can get, locally, at any of our 4 x steel supplier's... 1026 DOM.. in any
diam
up to 12". All their 1026 DOM is the same... 75 ksi. AS sells 1026
masts in 87 ksi.
4130 chromoly can be had in 104 ksi from AS...25" thick. 4130 chromoly in
106 ksi
can be obtained from US Tower corp.... .375" thick.
Check out the below calcs.
1.9" x .145" wall x 35 ksi = 42 mph [sched 40]
1.9" x .200" wall x 35 ksi = 47 mph [sched 80]
1.9" x .950" wall x 35 ksi = 60 mph [SOLID ! ]
2" x .25" wall x 75 ksi = 78 mph
2" x .25" wall x 87 ksi = 84 mph
2" x .375" wall x 75 ksi = 87 mph
2" x .25" wall x 104 ksi = 92 mph
2" x.375" wall x 106 ksi = 104 mph
104 mph / 92 mph = 1.13 1.13 x 1.13 = 1.28
The .375" thick chromoly is 28% stronger, than the .25" thick chromoly.
Pressure goes up to the SQUARE of the wind speed, regardless of whose
formulae
you use.
Here in Canada, all new tower /mast specs/ ant specs are done in pressure
[metric as well]
Most are converted to mph as well.... so we can relate to it... [ and lbs
per ft, etc]
I just got yet 3 x more pix via e-mail... of masts folded right
over...in the last 3 x weeks !
Several more install's I have seen... are just an accident waiting to
happen.
This has been going on since 1972, when I got my ham ticket. Nobody seems
to... 'get it'.
No rocket science here. Yield strength x section modulus = max bending
moment of a mast.
B4 pc's... I used a calculator by hand.. and the old EIA/rohn rs-222-C spec
"70 mph = 19.6 lbs per foot"
Calculate the Effective wind load of each yagi in flat plate eq. [
projected area x .67]
Effective Wind load x pressure = total force on yagi.
Total force x height of yagi up the mast = bending moment at base of mast.
Add in the bending moment of the mast itself.
These days it's easier with the various spreadsheets. Now try and
explain to joe ham the above,
and they still won't believe it. Joe ham doesn't have a clue what the
windload on his huge quad
actually is. Any els that are either vertically polarized, or on an
angle... have a huge windload.
Point a vertically polarized yagi in ANY direction.... and the els are
ALWAYS broadside to the wind !
Same deal with a quad. Now put the boom of the quad broadside to the
wind... yikes look out.
Mast selection is not to be trifled with. Removing a bent mast is extremely
dangerous... esp with
ants still on it. In some cases a crane can't be brought in.. due to
power lines, etc, etc. You take
a heavy mast, loaded with ants... and it snaps off.... it can easily slice
polygon rod + Phillystran ..
then down comes the entire tower!
later......... Jim VE7RF
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