[TowerTalk] The last aluminum mast comment

[UpTheTower@aol.com]

In a message dated 98-10-08 18:11:05 EDT, you write:

> > Question 2.  I have a 2 inch 6061-T6 1/4 wall 24 foot piece of tubing
>  that
>  > I acquired to use as a mast in the above tower.  Want to mount my TH7DXX,
>  my
>  > KLM 40M monopole, and a 12 ele 2M beam. My intention was to place a plate
>  > with a bearing where the original rotator goes, and put a new rotator
>  plate
>  > further down the tower, to take any stress off the rotator.  My concern
is
>  > how much mast I could (should) leave out with the antennas noted and if I
>  > could sneak in a  small 12/17M beam. >>
>  
>  Let's examine the question and put some numbers on it.
>  
>  
>  Here are numbers I got from Hygain and Force12 catalogs (sorry, no KLM
>  catalog) which should be about right for your situation (Hygain has lowered
>  their effective areas (available on their web site, see archives for
>  discussion of this point) so these are conservative for the Hygain
>  antennas):
>  
>  Antenna                             area (sq ft)    load (#) (@ 80mph using
>  25#/sq ft)
>  TH7                                         9.4                  235
>  Disc1 (40m 1 el)                    2.7                  68
>  214FM (2m 14el)                   1.7                  43
>  WARC 2/2 (12/17m yagi)     3.1                  78
>  mast                                        .1/ft                 2.5/ft
>  
>  Calculated mast bending moment resisting strength: 1628 ft# (see Leeson's
>  "Physical Design of Yagi Antennas" for the technique).
>  
>  One possible solution:
>  
>  Antenna      Hgt above tower(ft)    bending moment (ft#)
>  TH7               .5                                   118
>  WARC 2/2     5                                   390
>  Disc1          10                                    270
>  214FM        12                                    516
>  mast             12                                  180
>  
>  Total                                                  1474
>  
>  This load should be safely supported by your mast.  Other solutions exist
>  and depend upon what you consider to be adequate stacking distances.

      I think that you forgot to multiply by the wind pressure. At 70.7 MPH,
it's 
20 psf, 80 MPH is about 25 psf. Your analysis doesn't allow for the load
distribution up the mast. (Sorry, I'm not an engineer.)

     Anyway, I'll assume a windspeed zone of 70 MPH since it's the lowest
rating they give (but I think the original poster was in an 80 MPH zone). With
8 feet of mast in the tower and 16 sticking out, I'll spread the above
antennas equally spaced up the mast five feet apart. The MARC Program gives a
bending moment of 25,992 in-lbs and recommends a mast with a yield strength of
50,000 psi and 0.250" wall. 

      This load will NOT be safely supported by the proposed mast. Your 35K
psi yield aluminum mast is toast with that configuration. 

Cheers,  Steve   K7LXC
Tower Tech
Champion Radio Products

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