[200q20v] Re: 200q20v digest, Vol 1 #73 - 14 msgs

[Bernie Benz]

Obviously, you know from experience on this and the 20V lists, that I do
enjoy good list dialog, and the laws of physics are always in need of
defending.

> From: George Sidman <sidman at montereynet.net>
> Date: Wed, 07 Mar 2001 16:30:50 -0800
> To: Bernie Benz <b.m.benz at prodigy.net>
> Cc: 200q20V mailing list <200q20v at audifans.com>
> Subject: Re: [200q20v] Re: 200q20v digest, Vol 1 #73 - 14 msgs
> 
> Bernie:    Don't you just love a good list dialog??? Your
> points are well taken, and indeed, you are right; EMT is
> plated steel (wot was I thinking?). I just was trying to
> avoid cutting up the sheet metal, and it seemed to me that a
> small spacer of hefty diameter would reduce or avoid cutting
> and introducing rust points. As you say the lateral forces
> are in the neighborhood of 200 lbs., so it seems that a
> small increase in the length of the lever arm would be of
> little consequence. (wL squared over 12 I believe.) By
> welding one end and finding a way to leave the other end
> adjustable but constrained against both forces, a much more
> rigid solution would have to be the result.
My point was that the worst case latteral force is trivial reletive to the
column strength of a straight piece of EMT.  The bending stress on the strut
rod end is a much more complex matter under this circumstance. First, it is
preloaded in tension by the nut. Bending forces from the strut brace are
taken on the compression side by the tower bushing sleeve and your extension
sleeve washer in series, but this force is added directly to the preload on
the tension side.  Thus, the geometery factor plus the stress concentration
factor could multiply the strut force induced stress at the root of the
shaft end by 20 to 50X.  You could probably mount the ball end an inch
higher on the strut rod with no problem, but why fight when it is much
better design with it mounted as low as possible.  Cutting sheet metal is
trivial and, as I remember, you don't have room to move the brace up anyway,
as you will discover when you actually lay straight edge across between
strut rods and detail measure.
> 
> I believe that your solution is excellent, but I have always
> considered EMT good for not much more than wiring and
> hanging garage door openers.
You could pay $50 for the exact same tubing without the EMT label on it.

Also, IMO you are asking for trouble, ineffectiveness, in planning to
operate this or any other type strut brace in a bidirectional stress mode,
ie. tension and compression.  Impossible to keep the same degree of
stiffness, all the joints and fastners move as the sign changes.  That's the
beauty of simplicity.  The compression only strut to strut brace is about as
stiff as is possible to achieve.  Don't screw it up!  KISS.
> Also, I could not possibly
> approach such a delightful project without gilding it
> somewhat. I am sure you would agree that all designs can
> bear minor improvement here and there. Also, you can
> probably tell that my former profession as an Architect with
> a specialty in structural engineering is getting
> rusty......... And, yes, I do prefer silicon.
> -- 
> George Sidman, President
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