[Top] [All Lists]

[TowerTalk] 2" masts.. an eye opener !

To: <>
Subject: [TowerTalk] 2" masts.. an eye opener !
From: "Jim Thomson" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 17 Mar 2010 23:04:17 -0700
List-post: <">>
Date: Wed, 17 Mar 2010 18:42:35 EDT
Subject: Re: [TowerTalk] 2" O.D. Pipe???

hi Ted, you can find it at _www.texastowers.com_ 
( .
Click on the on line catalog button and select steel masts.
Gerald K5GW

##  I used the  arrl  mast software, available free online at

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  
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 
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

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>