[450] K1FSY Mast Strength Analysis

[John Foege]

December*


On Fri, Nov 15, 2013 at 5:04 AM, John Foege <john.foege at gmail.com> wrote:

> It's very rare, afaik, that we, here inland, get wind gusts over 50mph -
> however, in Decemeber it has be known to get up to 55+ mph gusts once in a
> blue moon according to historical wind data I've been looking through for
> Waterbury!!
>
>
> On Fri, Nov 15, 2013 at 4:51 AM, John Foege <john.foege at gmail.com> wrote:
>
>> After eyeballing your pic "towering above" from earlier - I guesstimated
>> that you've got 16' of 1-1/4 trade size EMT above the thrust bearing, so
>> making all the same assumptions from the earlier analysis, here are your
>> magic numbers:
>>
>> Exceed yield strength, resulting in irreversible bending @ 53 mph
>> (bending stress: 35,767 psi)
>> Exceed absolute tensile strength, resulting in snapping @ 67 mph (bending
>> stress: 57,161 psi)
>>
>> Moral of the story? If we are going to get over 50 mph gusts, point your
>> antenna into the wind - and you should be fine. It will be a real show to
>> watch from the outside though :-)
>>
>>
>>
>> On Fri, Nov 15, 2013 at 4:34 AM, John Foege <john.foege at gmail.com> wrote:
>>
>>> Results below also attached as *.TXT file for better
>>> formatting/readability.
>>>
>>> Analysis of Antenna Mast Strength
>>> ----------------------------------
>>>
>>> Produced for: K1FSY
>>> Performed by: W1QEX
>>> Date: 11/15/2013
>>>
>>> All calculations assume:
>>> * Single M2-9SSB antenna with 1.2 ft^2 cross sectional wind loading area
>>> is at top of mast
>>> * 1-1/4 Trade Size EMT (1.51 in. OD / 0.065 in. nominal wall thickness)
>>> * 36,000 PSI elastic yield strength / 58,000 absolute tensile strength
>>> (standard EMT conduit)
>>> * Antenna is broadsided into the wind for maximum wind contact surface
>>> area
>>> * All wind speeds are rounded to the nearest whole mph value
>>> * Point of failure is at thrust bearing / top of tower and mast interface
>>>
>>> Case #1
>>> -------
>>> * Mast protrudes 20' above thrust bearing
>>>
>>> Results:
>>>
>>> Bending stress of 35,908 psi occurs @ 45 mph wind speed. Wind speeds in
>>> excess of
>>> 45 mph will exceed the elastic yield strength of EMT conduit, resulting
>>> in an irreversible
>>> bending of the mast.
>>>
>>> Bending stress of 57,612 psi occurs @ 57 mph wind speed. Wind speed in
>>> excess of 57 mph
>>> will exceed the absolute tensile strength of EMT conduit, resulting in
>>> breaking of the mast.
>>>
>>> Case #2
>>> -------
>>> * Mast protrudes 17' above thrust bearing
>>>
>>> Results:
>>>
>>> Bending stress of 36,194 psi occurs @ 51 mph wind speed. Wind speeds in
>>> excess of
>>> 51 mph will exceed the elastic yield strength of EMT conduit, resulting
>>> in an irreversible
>>> bending of the mast.
>>>
>>> Bending stress of 58,792 psi occurs @ 65 mph wind speed. Wind speed in
>>> excess of 65 mph
>>> will exceed the absolute tensile strength of EMT conduit, resulting in
>>> breaking of the mast.
>>>
>>> Case #3
>>> -------
>>> * Mast protrudes 15' above thrust bearing
>>>
>>> Results:
>>>
>>> Bending stress of 36,369 psi occurs @ 56 mph wind speed. Wind speeds in
>>> excess of
>>> 56 mph will exceed the elastic yield strength of EMT conduit, resulting
>>> in an irreversible
>>> bending of the mast.
>>>
>>> Bending stress of 58,462 psi occurs @ 71 mph wind speed. Wind speed in
>>> excess of 71 mph
>>> will exceed the absolute tensile strength of EMT conduit, resulting in
>>> breaking of the mast.
>>>
>>> Case #4
>>> -------
>>> * Mast protrudes 10' above thrust bearing
>>>
>>> Results:
>>>
>>> Bending stress of 36,125 psi occurs @ 74 mph wind speed. Wind speeds in
>>> excess of
>>> 74 mph will exceed the elastic yield strength of EMT conduit, resulting
>>> in an irreversible
>>> bending of the mast.
>>>
>>> Bending stress of 58,4291 psi occurs @ 94 mph wind speed. Wind speed in
>>> excess of 94 mph
>>> will exceed the absolute tensile strength of EMT conduit, resulting in
>>> breaking of the mast.
>>>
>>>  73s de W1QEX
>>> John
>>>
>>>
>>
>
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