RC vs CC again

[Richard Brekne]

Delwin D Fandrich wrote:

>
> 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?

I didnt say there was such a claim... its just what seems to make sense to
me... assuming there is an <<overall tension>> in the rib, Nor do I find it
mysterious at all to question about whether or not tension in the rib exists
beyond that which simple bending can account for. As for the Brekne Effect...
grin.. raeallly Delwin...even if I did manage some noteworthy discovery....
(which is doubtfull at best)  I am not that vain.

Im just an egg here. :)

>
> 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.
>

Well, that puts us back at the starting point I guess, and I'm none the wiser
for it. I want to find out what happens to all the tension placed on the rib,
thats all. If you say it all goes into a simple bending of the rib then ok..
but I would like to know where thats covered in the literature.

I do have an experiement in mind tho... that should tell me just how much (if
at all)I can expect the rib to be elongated. I'll take a few pictures and post
them when I get it together.


> 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.
>

See... what I dont get is why the ribs are simply being treated as if they were
just being bent, yet the soundboard is treated as being bent AND stressed
horizontally. The only real difference is the direction of the grain. Otherwise
the horizontal stress is applied equally to both.. its just that the panel
gives more... (horizontally) and so every thing bends. But that doesnt mean the
horizontal stress along the grain applied to the rib just dissapears or
something....

If its enough to elongate the ribs at all.. then it would seem that the ribs
would resist this as best THEY can...and any increase in stress levels would be
increasingly resisted. If one can declare the compresion in the soundboard is
what supports crown, then why is any tension in the ribs not doing the same
thing ? Its just two sides of the same coin... or what ???



>
> 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.
>
> ___

Cheers
RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html