It's hard to imagine that someone would go through al the trouble to make
their own tower sections when you can buy used Rohn 25 for $25-40 a section.
Then you know what you have.
Julio, W4HY
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Jim & Velma Woods" <woods@grantspass.com>; <towertalk@contesting.com>
Sent: Monday, November 22, 2004 7:40 PM
Subject: Re: [TowerTalk] Octahedron Towers
> At 03:53 PM 11/22/2004 -0800, Jim & Velma Woods wrote:
> >I am new to the tower-talk group but have interest in building an unusual
> >tower for a small HF beam or quad. It is based on a design from the 80's
> >where 1-inch EMT conduit is used to build sections of the tower in a
> >series of octahedrons. Height would be about 44 ft. Plans are sketchy
> >and engineering analysis appears to be almost non-existent. The towers
> >were used to support wind generators. Most wind generators have quite a
> >bit of wind loading compared with ham antennas.
> >
> >Any initial comments?
> >
> >73,
> >
> >Jim Woods, W7PUP
> >woods@grantspass.com
> >Grants Pass, OR
>
> I've fooled with using EMT as a building material quite a bit over the
> years. (see, for instance,
http://www.luxfamily.com/jimlux/robot/emtsphere.htm)
>
> In general, EMT is terrible as a structural material. It cracks on the
> welded seam in particular.
>
> However, one could build something out of EMT and then proof test it to
> several times the expected loads. That is, bolt the base of the tower to
> something sturdy so that it's hanging out like a canteliever beam a few
> feet off the ground. Start hanging weights on the structure to replicate
> the design wind loads. If you get to 2 or 3 times the load and the
> structure survives without permanent deformation you're in pretty good
> shape. It will deflect under load, but you want to make sure it comes back
> to where it was before you loaded it. Use a tape measure! Don't estimate
> the deflection by eye.
>
> Make sure you do this in several orientations.
>
> Some other things to think about:
> Fatigue failure. Fortunately, you'll probably have the structure
> overdesigned enough (because of the crummy material properties for EMT)
> that you'll never have enough load on it so that fatigue is a
> problem. However, if you haven't analyzed it, you don't know.
>
> Buckling. In a structure like this where the materials are sort of iffy,
> you can't really be sure how the stresses are distributed. You might be
> perilously close to buckling some member. Proof testing helps, but,
still,
> if you didn't happen to load it just the right way.
>
> I'd be interested in seeing more about this approach, for a variety of
> reasons.
>
> Maybe someone should get bold and build a tensigrity tower.
>
> Jim Lux
> W6RMK
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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