# [TowerTalk] Aluminum mast

Bruce Ferratt uglyatc at msn.com
Wed Mar 15 22:18:20 EST 2006

```Wind load is 40/12 or 3.3 square feet.
This was calculated by Jim Lux 'Doc'.
Thank you.
Bruce
K4BOF

Check this out:

-mast is 1 1/4" OD.   It is solid, round.

So, you can apply a bit of simple mechanics..
bending stress is =Moment/section modulus

Section modulus for a solid rod is pi/32 * D^3 or 0.19 inch^3 for your rod.

Yield for 6061 aluminum is probably around 40,000 psi, so rearranging a bit:

40,000 lb/in^2* 0.19in^3 = 7700 in lb.

You've got 8 ft (96 inches) sticking out, so 7700/96 = 80 pounds.

One generally likes to leave a reasonable safety factor to allow for
imperfections in the material, etc.  Maybe, a factor of 2 would be
reasonable here?  So, your at 40 pounds load at the end of the mast.

I believe you said 70 mi/hr.  For air drag, load (lb/ft^2) = V(mph)^2/391..
12 pounds/square foot.

So, you get a total windload area of 40/12 or about 3.3 square
feet.  (don't forget to allow for the half square foot or so for the
mast).  Doesn't look like you're going to be putting up a full size 80m
yagi on this<grin>.

----- Original Message -----
From: RICHARD BOYD<mailto:ke3q at msn.com>
To: Bruce Ferratt<mailto:uglyatc at msn.com>
Sent: Wednesday, March 15, 2006 7:43 PM
Subject: Re: [TowerTalk] Aluminum mast

Even with it just being 1-1/4", I'm surprised it was only \$66!

When I was a kid (13-16), I used a lot of 1-1/4" steel mast for both masts and booms and, I highly recommend going to at least 2" for anything you do.

I agree that the weight of the tribander and 2M and 440 antennas shouldn't be a concern, but what is is the wind load.

I'd feel a lot better about your setup if you'd go to 2" OD steel mast.  I think you'd be okay then.

73 - Rich, KE3Q
----- Original Message -----
From: Bruce Ferratt<mailto:uglyatc at msn.com>
To: ke3q at msn.com<mailto:ke3q at msn.com>
Cc: towertalk at contesting.com<mailto:towertalk at contesting.com>
Sent: Wednesday, March 15, 2006 4:33 PM
Subject: Re: [TowerTalk] Aluminum mast

Rich-
The mast is 1 1/4" OD.  Solid, round.  Side writing on the states it is ASTM
6061-T6511 grade aluminum.  It was \$66.  Weighs about the same as the seet
6' mast that came with the tower.  I just don't like mounting a rotor right
in the open section of Rohn25g,... it's gotta be lower inside the tower.
The beam is a Mosely TA-33M Tri-bander.  Longest element is 26ft---13ft each
side of boom.  Boom is 14ft long.
The beam and stacked vertical 2m/440 will probably weigh under <40lbs at the
most!
73
Bruce
K4BOF

>From: "RICHARD BOYD" <ke3q at msn.com<mailto:ke3q at msn.com>>
>To: "Bruce Ferratt" <uglyatc at msn.com<mailto:uglyatc at msn.com>>
>Subject: Re: [TowerTalk] Aluminum mast
>Date: Wed, 15 Mar 2006 09:06:13 -0500
>
>Sheesh!  That's quite an item you have there!  You might consider the scrap
>value of aluminum and sell it to a scrap metal dealer and use the money to
>buy a steel mast.  The piece of stock you have there could be worth many
>hundreds of dollars, in the value of the aluminum.
>
>Aluminum is inherently soft compared to steel.  A solid aluminum mast may
>be more prone to bending than a hollow, thick wall, one.  I have some thick
>wall aluminum tubing that previous hams probably acquired to use as masts,
>and they are very heavy.  I plan to use them only for short (like two feet)
>splice pieces for booms for large HF monobanders.  It's very expensive
>stuff, and I believe steel masts are better and stronger and more
>economical too.
>
>I buy "heavy duty fence post" steel tubing stock from Long Fence.  I have
>bought a 24 foot piece of 3" steel mast for as little as \$50 (they are
>often confused and don't know what to charge for it so the prices vary).
>3" is much stronger than 2" and nicer to climb if you end up climbing the
>mast above the tower, as I sometimes have to.
>
>73 - Rich, KE3Q (Bowie, Maryland)
>   ----- Original Message -----
>   From: Bruce Ferratt<mailto:uglyatc at msn.com<mailto:uglyatc at msn.com>>
>   To: towertalk at contesting.com<mailto:towertalk at contesting.com<mailto:towertalk at contesting.com<mailto:towertalk at contesting.com>>
>   Sent: Tuesday, March 14, 2006 10:56 PM
>   Subject: [TowerTalk] Aluminum mast
>
>
>   Hello and thanks for letting me post a question.
>   How strong is a solid aluminum mast compared with a walled/hollow mast?
>   I secured a solid, round 12' aluminum mast 6061-T6511 grade.  What is
>the likelihood of bending when loaded with a TM-33A and VHF 2m/440 vertical
>at a rated 70mph wind load in Northern Virginia?  8ft of mast will be
>exposed above the point of the tower peak (Rohn 25G), with about 4 ft
>inside the tower to rotor plate.
>   Thanks
>   Bruce
>   K4BOF
>   _______________________________________________
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