[TowerTalk] ALUMINUM MASTS

[T A RUSSELL]

On Wed, 07 Oct 1998 22:07:27 -0400 Paul McInnish - K4BET
<pmcinnish@worldnet.att.net> writes:
>
>Yep... I agree!  I use a 24' piece of STRUCTURAL 6061-T6 
>(1/4" wall thickness, 2.5" OD) aluminum tubing for a mast 
>and it has stood up to some god awful stress... 

	A  LARGE part of your success results from your
	use on a larger diameter (2.5 inch vs. 2.0 inch)
	mast.  The maximum allowable bending moment
	is a 4th power function of inside and outside diameters.
	This means that your 2.5 inch dia. mast is roughly
	TWICE as strong as a 2.0 inch dia. mast.   de  N4KG

 Been in some tornado winds and mine stood vertical (some 
>damage to antennas... none to mast) and many, many friends 
>in area lost HIGH strength steel masts with LESS wind loading 
>and NOT as tall with recorded tornado winds less than what was 
>recorded near my QTH!

	This sounds somewhat suspicious.  Are you SURE
	they had a high strength material?  Or did they try to
	get by with SOFT  WATER  PIPE?   What DIAMETER
	are you talking about?  What WALL  THICKNESS?
	All of these factors must be accounted for.  de N4KG
		

>Pound for pound, etc., this structural grade (expensive $200 +) 
>'stick' is many, many times stronger than steel!  Also, to approach 
>the capabilities of  this material, steel would be many times heavier 
>and will bend to crimp/crease easier... NO flex! Without flex, something
gotta give!
>
	Aluminum may be stronger on a per pound basis BUT,
	the bottom line is WHAT  IS  THE  MAXIMUM  ALLOWABLE
	BENDING  MOMENT for the given DIAMETER and WALL
	thickness used?

	A 2 inch diameter 1/4 inch wall ALUMINUM mast does NOT
	come close to the bending moment achievable with a 2 inch
	diameter 1/4 inch wall STEEL  mast.

>I cannot imagine the weight of a 24' piece of steel in the air, on top 
>of my tower, with all the wind loading,  that would weigh as much as it 
>does. Scary!
>
	What is so hard to imagine about a 100 to 150 pound mast
	in a tower that is rated for THOUSANDS of pounds vertical load?
	NOT scary at all. 

	The scary thought is the uninformed guy who tries to put 
	a 2L40 meter beam 10 ft above a big tribander on
	a 2 inch diameter 1/4 inch wall ALUMINUM  MAST.
	THAT   is  SCARY !   (Better have a plan in place for removing
	the bent aluminum  mast and antenna from your tower before
	you attempt such a foolish installation.)    

	de   Tom  N4KG



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