This is about the fourth time I have posted this message in response to
the same question. But no problem, I saved it. I'll just dump it again.
For some free software to make these calculations go to
http://www.arrl.org/qexfiles/
and download the file labled TRANVITY.ZIP. Also download the following
pdf file which gives some good explanations of the calculations.
http://www.arrl.org/qex/1123.pdf
Jerry, K4SAV
kb0fhp@comcast.net wrote:
>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 authority.....
>
>Scott
>
>-------------- Original message --------------
>From: Mike <k4gmh@arrl.net>
>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:
>
>http://www.key-to-steel.com/Articles/Art24.htm
>
>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
>B.
>
>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/2differs 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
>conditions.
>
>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
>Metallurgist
>KB0FHP
>
>
>-----Original Message-----
>From: towertalk-bounces@contesting.com
>[ mailto:towertalk-bounces@contesting.com]On Behalf Of Mike Bragassa
>Sent: Thursday, July 13, 2006 11:39 PM
>To: towertalk@contesting.com
>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 chromemoly pipe compare to a 0.25 inch schedule 80
>pipe?
>
>73, Mike, K5UO
>
>
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> 73,
> Mike, K4GMH
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