[TowerTalk] Ratio of mast inside tower vs outside tower?

[Stan Griffiths]

>You may remember me with the 'painting a mast' question.  The general
>consensus seems to be painting is for cosmetic reasons not structural.
>I am leaning towards using the cold galvanizing spray.
>
>Now a new question.
>
>I needed 19 feet of 2'od 4130 mast for my design.  However, a 24' length
>was delivered.  To cut or not to cut, that is the question.
>
>Are their any guidelines as to what the minimum percentage of the mast
>that should be in the tower?  I'm concerned that if I use the whole 24',
>with 4.5 feet inside and 19.5 feet outside,  that I will over stress the
>tower and rotor.  The tower is the 9' Create Roof tower.  Rotor is Yaesu
>G-800SDX.
>
>If I stick to my original plan I will have 4.5 feet of mast inside the
>tower with 14.5 outside the tower.
>
>Comments?
>
>7 3
>
>Bob - N2SCJ

Hi Bob,

I am trying to imagine what this system will look like on your roof.  I
assume you need the tall mast because you plan to stack at least three HF
Yagis, but I could be wrong.  Otherwise, why wouldn't you use a taller tower
and a shorter mast?  I also assume you will guy this tower.  Will your roof
really support this load?  The load will be much much greater than the
weight of the system under windy conditions, assuming you guy it.  Guy wire
tension is has a downward component that has to be supported by the tower in
addition to the weight.  The guy wire tension (and the downward component
supported by the tower base) will increase a LOT with the wind.  So I
seriously wonder if this system will stay up even if you do cut the mast
down to 19 feet.

On the other hand, if your roof is a steel reinforced concrete slab, it will
probably hold the thing up OK.

I think you are correct in being concerned about whether a Create roof tower
will withstand the forces applied with 19.5 feet of mast out the top.  Where
can you get any engineering data on the Create?

Stan  w7ni@teleport.com


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