RC vs CC again

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 09, 2003 12:43 AM
Subject: Re: RC vs CC again


>
>
> Delwin D Fandrich wrote:
>
> >
> > Will the rib stretch? Sure it will. At least the top of it will. To
about
> > the same degree as Ron N's aircraft carrier will slow down as it
impacts
> > that kamikaze mosquito. Spruce is rather strong in tension, it doesn't
like
> > to stretch at all. As well, the bottom will compress a bit and there
will
> > be a neutral line somewhere towards the center. The net result is that
the
> > thing wants to straighten itself out again.
> >
>
> No arguement there.... heck the soundboard wants to straighten itself out
again
> for that matter... I dont suppose it likes being all squeezed and bent
anymore
> the the rib likes whats happening to it.
>
> I just dont see where the justifcation for writting off the ribs overall
> tension is covered in the literature. Perhaps you can point me to some
section
> or table some where ?

And I don't see the justification for claiming that some mysterious
"overall tension" is in any way supporting crown. I don't see it as any
factor at all let alone a significant factor. I doubt many others do
either. Which probably explains why you don't see many charts on the
effect. It does suggest an avenue for research, however. When can we expect
to see numbers and charts explaining the Brekne Effect?

As glued up the top of the rib is neutral relative to the soundboard panel.
If the panel and the ribs are kept at a stable moisture content there is no
stress differential between them and, obviously,  there is no crown. It is
not until the panel begins to absorb moisture and the wood fibers begin to
swell that the assembly begins to take on crown. The rib is now forced to
bend against its will and it will always be trying to return to
equilibrium -- i.e., to being the straight chunk of wood it was cut out to
be. It's whole heart and soul go into that desire until such time as its
time under load is sufficient to permanently deform the poor thing through
creep.

The exception to the above, of course, is the compression-crowned
soundboard assembly which still uses flat ribs but the assembly is glued up
at a somewhat higher moisture content using curved cauls to come up with
essentially the same amount of crown. In this case the top of the rib is
under some tension at the time of gluing. How much depends on the amount of
curve found in the caul. The end effect on the soundboard panel, though, is
essentially the same -- for a given amount of crown the level of internal
compression will be the same.

And just one more point, while I'm on the subject. Most of us are aware of
the approximate perpendicular-to-grain compression strength of the various
spruces. (The test numbers are readily available in the 'USDA Wood
Handbook.') What is generally left out of the discussion is the effect of
time under load. For a wood structure expected to survive any length of
time -- say, ten years -- these test figures have to be reduced by about
40%.

De.