RC vs CC again

[Farrell]

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Okay Richard, a light bulb went off in my head while reading this post =
and I think I finally understand what you are asking. Let me state my =
understanding of your question in my words and let me know if I am on =
the right track. And if I am right, I also see that it could raise the =
questions you ask. Here goes:

CC soundboard. Panel dried way down, glued to ribs, then exposed to =
normal room environmental conditions. Panel hydrates and tries to =
expand, but because it is glued to relatively unstretchable ribs, it =
causes the ribs to bend and we get our crowned soundboard.=20

Let's examine the stresses at this crowned but unloaded point. There is =
compression in the panel. There is tension in the top area of the rib =
(panel is trying to expand the rib top, but has only very limited =
success). There is compression in the lower area of the rib. And as Ron =
N. stated, there will be a line/plane going through the rib where these =
is no stress. If somehow you were able to instantly de-bond one of the =
ribs on this soundboard from the panel, it would pop right back to its =
original straight configuration, i.e. all the ribs in this soundboard =
are constantly trying to go flat.

Now comes the tricky part - and as I am writing this it is becoming =
clear to me that I am not clear on this either. (Below is my third =
attempt carrying on with this thought.)

Now we add downbearing. What happens to stresses? Compression in panel =
increases. Now I understand you to suggest that tension in the upper =
part of the ribs will increase - for the same reason the tension =
increased in the top portion of the rib as the panel took up moisture =
after being taken out of the hotbox.=20

But does it? This is where about three different theories pop in and out =
of my head and I'm really not sure what makes sense to me. To tell you =
the truth, I'm not sure where tensions/compressions increase/decrease in =
the rib upon loading. I suspect tension decreases in the upper portion =
and compression decreases in the lower portion.

Let's try this thought on for size. Upon taking the dried flat =
soundboard out of the hotbox, the panel hydrates and forms crown. The =
ribs bend. The ribs are tensioned a bit on the top side and compressed a =
bit on the bottom side. Is the tension on the top side simply from the =
rib bending - whether a panel is glued to it or you are just holding on =
a table and bending it with your hand (applying pressure to the middle =
of it)? I suspect, and I don't know for sure, that almost all the =
tension in the upper portions of the soundboard ribs are from the rib =
simply being bent (convex side is being stretched along a larger arc =
than the concave side) and very little tension in the rib is actually =
related to the rib-panel interface.

If what I stated above holds any water, now when we load the soundboard =
(as in adding downbearing), the panel compresses, panel compression - =
via the panel-rib interface - does not significantly affect stresses in =
the top of the rib, tension in the upper part of the rib decreases and =
compression in the lower part of the rib decreases. As more load is =
applied to the board, those trends will continue until the board is =
flat, at which point the ribs will be essentially unstressed and back to =
where they were trying to go ever since getting out of the hotbox.

But now we have all this gobs of compression in the board (it has tried =
to expand with moisture intake - compressing the panel - plus we have =
squished this crowned structure down into a pancake - thus increasing =
panel compression even more). Surely all this panel compression has to =
affect the rib in some way. I don't know. Seems like a reasonable =
question. Now that the panel is flat, is the rib unstressed? If the =
panel has all this compression in it, there has to be something out =
there in this universe countering the internal compression in the panel. =
Is it the rib somehow? Could the rib be stressed by the compressed panel =
in such a way that the net effect is neutral (doesn't seem likely if the =
panel is glued to one side of the rib). If the rib is flat, happy, =
relaxed and unstressed, is the counter force somehow the downbearing?

Is that kinda where your questions were going Richard?

Can anyone out there shed light on this?

I think I was correct a post or two ago saying that vector analyses of =
the stresses and strains here can adequately describe what is going on. =
But it is also now clear to me that I don't have much of a clue as to =
how to conduct such an analyses - it appears much more complex than I =
had originally thought.

Terry Farrell


----- Original Message -----=20
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: Wednesday, October 08, 2003 5:59 PM
Subject: Re: RC vs CC again


> Delwin D Fandrich wrote:

SNIP

> > For the time your sandwich was being
> > held in a crown by the stress-interface between the longitudinal rib =
and
> > the cross-grain top piece do you suppose it somehow -- miraculously =
-- lost
> > its desire to get itself straightened out? I can't conceive how. =
That bent
> > rib is a spring, what are its lines of force?
> >
>=20
> Hmm... Like I say.. the only thing I dont understand is what happened =
to that
> tensioning (pure stretching) force on the rib.? I mean.. the rib =
forces the
> panel to compress, and the flip side of that coin is that the panel is =
at least
> going to attempt to force the rib to lengthen. The degree it fails in =
doing so
> results in the bending of the rib and the formation of crown.. yes ?? =
But what
> about the degree it succeeds (if any) ? If the rib is at all tensioned =
in the
> sense that it is lengthened in addition to the tensioning that occurs =
in the
> top half from bending... then I dont see how the rib could not be =
supporting
> crown, because if thats the case there is more going on then just the =
ribs
> desire to straighten out.... It also has a desire to not tension any =
more,
> which downbearing on the panel will tend to do.

SNIP
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