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[TowerTalk] Clarification? Windloading - Chrome-moly VS: steel

Subject: [TowerTalk] Clarification? Windloading - Chrome-moly VS: steel
Date: Fri, 14 Jul 2006 21:44:47 EDT
List-post: <>
In reading through the exchanges below it appears that the term "Pipe" and  
"tubing" may be intermixed in the discussion.  I am sure most know that 2"  
tubing (with a specified wall thickness) and 2" pipe (with a wall schedule 80 
40 specified) are very different in outside diameter.  See the following  
chart -
Pipe Size    Schedule 40  **         Schedule 80  **
(in)Nom.OD(in)  ID(in)  Wall  Thick.(in)  ID(in)  Wall  Thick.(in)
1-1/2    1.900  1.610   0.145             1.500     0.200
2      2.375  2.067   0.154             1.939     0.218

It is possible that "Tubing" uses a "schedule" dimension for wall  thickness, 
but I am not aware of it.  
So the question is - was the correct material definitions used in the  
calculations e.g. pipe or tubing, and where the correct outer diameters and 
thickness's used?  
Regardless, I agree with the conclusion that chrome-moly 2" tubing is the  
better choice.
Norm W4QN

Message:  8
Date: Fri, 14 Jul 2006 14:46:08 +0000
Subject: Re: [TowerTalk] Windloading - Chrome-moly VS:  steel
To: Mike <>

Now for a 20 foot mast, the max load (located at  the tippy-top of the mast) 
is about 2850 pounds for the 4130, and about 1700  pounds for the black steel 
A53 Shedule 80 pipe.  If I use a safety factor  of 2 - which I don't think is 
unreasonable, then the maximum loads are about  1425 pounds for AISI 4130, and 
about 850 pounds for the A53 Steel  pipe.

This is where I need help.  Now assuming the codes, how do I  calculate the 
windloading?.  I know there was a long discussion on that a  while ago - which 
I need to review.  I really wish there was a chart -  antenna loading in ft^2 
vs wind speed.......Does someone know of such a  beast?  SInce the worst case 
is a point load at the end of the mast - this  could really help size the 
necessary mast sizes.

As with all  calculations, it is best to verify the calculations, and if 
necessary contact  the appropriate engineering or other cognizant  


-------------- Original message  -------------- 
From: Mike <> 
Thanks Scott for the  clear and complete explanation of the difference.  Nice 
to get a  knowledgeable answer about mast material.  

At 12:44 AM 7/14/2006,  you wrote:

There are really two questions there combined into one  question.  Schedule
80 is a pipe wall thickness designation....It does  not specify alloy - there
are approximately 30+ alloy designations for  pipe:

But making it  simple, it it is assumed that the material is standard black
steel pipe to  ASTM 53, typical strength has a 52,000 psi yield.  That would
be about  70,000 psi ultimate strength, and a hardness of about 79  Rockwell

Normalized 4130 pipe, has a hardness of about Rockwell C  30, with an
ultimate tensile strength of about 130,000, and a yield  strength
approximately of 110,000 psi.

Just based on strength, the  4130 is better.  But our failure criterion is
bending, or plastic  deformation.  Now we have to compare the wall
thicknesses and the  strength, so see which works better.  This is where the
metallurgist,  being the jack-of-all-trades, instantly transforms into a
stress  engineer.

The maximum stress in a beam is given by s=Mc/I, where s is the  stress, M is
the moment, c is the distance of the outer fiber from the  neutral axis, and
I is the moment of inertia.

I for a thin walled tube  (good enough in this illustration) is: I =pi*t8r^3
Assuming a 2" diameter,  the I for the two cases are:
4130 (2" dia. x 0.18" wall): 0.565
A53 (2"  dia x 0.25" wall): 0.785

c is the fiber distance from the neutral axis,  and is c = r + t/2 differs for
each one because of the wall  thickness:
4130 (2" dia. x 0.18" wall): 1.09
A53 (2" dia x 0.25" wall):  1.13

M is unknown, and is a function of the length of the mast, and  the
windloading.  For the purposes of this, the masts are the same  length, and
the windloading is the same.

Failure is yielding of the  material, or when the stress equals the yield
strength (less any safety  factor - we will ignore that for the time being).

For the 4130 pipe,  yield is 110,000 psi = s = Mc/I = 1.93M
For the A53 pipe, Yield is 50,000 psi  = s = Mc/I = 1.44M

The max moment at failure can be calculated by  rearranging:
for 4130 M = 110,000/1.93 = 56,994 ft-lbs
for A53 M =  50,000/1.44 = 34,722 ft-lbs

In other words, the 4130 can take about 1.64  times the moment than the A53
pipe, even with a thicker wall.  Since the  lengths of the mast are assumed
to be the same, then the 4130 can take 1.64  times the load that A53 can
before bending can occur, for this specific  example, and specific material

The next question is, is  this strength necessary for the application?  It
depends on the windload  of the antennas, and the length of the mast.  From
that calculation, and  a suitable safety factor, the maximum antenna loading
for a specific mast can  be determined.

I hope that this clarifies some things - finnaly I have  been able to answer
someone's question and contribute, instead of just asking  questions.....  :)

Scott MacKenzie,  PhD

-----Original Message-----
[]On  Behalf Of Mike Bragassa
Sent: Thursday, July 13, 2006 11:39 PM
Subject: [TowerTalk] Chrome-moly VS:  steel
Importance: High

For you metallurgists in the group:
Re:  20 ft mast pipe

How does a 0.18 inch chrome-moly pipe compare to a 0.25  inch schedule 80

73, Mike,  K5UO


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