[450] K1FSY Mast Strength Analysis

[John Foege]

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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