Sorry to cause confusion.
Loading forces applied to the mast create stresses in the mast. Temporary
deformation (bending) of the mast occurs while the stresses are present. If
the stresses present exceed the yield strength of the mast material, then
permanent deformation (failure) of the mast occurs. If the stresses exceed the
ultimate strength of the mast material, then material fracture will occur
(which usually results in rapid catastrophic failure - "breakage"). We
therefore select the mast and position the loads so that the stress resulting
from maximum loading does not exceed the yield strength of the material. In
fact, prudent design requires a margin of safety wherein material yield
strength exceeds the peak anticipated stress by some factor of safety.
Increasing the tube wall thickness of the mast reduces the peak stress
developed for a given loading. In the case of a mast installation, the bending
stress in the tube usually comprises the vast majority of peak stress. The
percentage of bending stress reduction that can be realized by increasing the
tube wall thickness was given in the previous post for a 2" mast without
changing loading or length of the mast.
Increasing the mast diameter, or changing the loading and support points on the
mast also change the bending stress developed - so placement of antenna loads
and how & where the mast is supported also determine the safe "payload" limit.
Similarly, increasing the wall thickness or using a material with a higher
yield strength also increase the safe "payload" limit.
Hope helps and also clears up the confusion.
Matt
KM5VI
-----Original Message-----
From: Chris Pinholster [mailto:k4win@mac.com]
Sent: Wednesday, February 26, 2014 4:07 AM
To: Matt
Subject: Re: [TowerTalk] FW: Mast Wall Thickness
Hi Matt,
Thanks for your reply. However I am confused on the stress part.
If 2" OD ⅛" wall shows a strength of 42000 psi and ¼ " wall is 61% stronger,
wouldn't the math be 1.61x42k ?
When i plug the resulting number into that MARC mast calculator I get stats i
think i can live with.
Understanding also that this is for aluminum and also on top of a UST MA-40.
Where am i going wrong?
Thanks, Chris
Sent from my iPad
> On Feb 26, 2014, at 2:09 AM, Matt <maflukey@gmail.com> wrote:
>
> For a 2" OD tube mast with 1/8" wall thickness, increasing wall
> thickness will decrease stress for any given bending load as follows:
>
> 1/4" wall: 61% of 1/8" wall stress
> 3/8" wall: 49% of 1/8" wall stress
> 1/2" wall: 44% of 1/8" wall stress
> 2" solid rod: 41% of 1/8" watt stress
>
> Another way to express this is the increase in strength = 1 / the
> percentage stress reduction.
>
> Hope this helps.
>
> Matt
> KM5VI
>
>
>
>
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
> Jim Lux
> Sent: Tuesday, February 25, 2014 7:47 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Mast Wall Thickness
>
>> On 2/24/14 9:39 AM, Chris Pinholster wrote:
>> I have been researching tower mast material. Both aluminum and steel.
>>
>> If aluminum 60601 tubing with 2"OD and .125 wall fails at 35000 ps,
>> what happens when you increase wall thickness?
>
> Not much it might seem, as it happens. The outer part of the wall
> carries the peak stress and fails first (think about holding a bunch
> of spaghetti in your hand and bending it.. which strands break first?)
>
> This kind of thing goes as the 4th power of the radius.
>
> To the first order, you could estimate it as
>
> 1^4 - 0.875^4 =0.414 -> 1/8" wall
>
> 1^4 - 0.75^4 =0.684 -> 1/4" wall
>
> about 60% more strength for about twice as much metal.
>
>>
>> If you increase the thickness of the mast wall, wouldn't that
>> increase make a difference in the bend or failure rating? The charts
>> at the metal company I visited seemed to indicate that would be true.
>> (I was looking at aluminum 2"OD and 1.5"ID)
>>
>> Also I ran across a chart that showed that using a 14 ft mast, with 4
>> ft inside the tower and 10 ft above was stronger than an 11 ft mast
>> with only 1ft inside the tower and 10 ft above.
>>
>> Any opinion or science from this learned group?
>>
>>
>>
>> CHRIS PINHOLSTER k4win@mac.com
>>
>>
>>
>> _______________________________________________
>>
>>
>>
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