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<DIV><FONT size=2></FONT><FONT face=Arial>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:</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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. </FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.)</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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. </FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Let's try this thought on for =
size. Upon taking
the dried flat soundboard out of the hotbox, the panel hydrates and =
forms
crown.</FONT> <FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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?</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Is that kinda where your questions were going
Richard?</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Can anyone out there shed light on =
this?</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Terry Farrell</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>----- Original Message ----- </FONT>
<DIV><FONT face=Arial size=2>From: "Richard Brekne" <</FONT><A
href="mailto:Richard.Brekne@grieg.uib.no"><FONT face=Arial
size=2>Richard.Brekne@grieg.uib.no</FONT></A><FONT face=Arial
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>To: "Pianotech" <</FONT><A
href="mailto:pianotech@ptg.org"><FONT face=Arial
size=2>pianotech@ptg.org</FONT></A><FONT face=Arial =
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>Sent: Wednesday, October 08, 2003 5:59
PM</FONT></DIV>
<DIV><FONT face=Arial size=2>Subject: Re: RC vs CC =
again</FONT></DIV></DIV>
<DIV><FONT face=Arial><BR><FONT size=2></FONT></FONT></DIV>
<DIV><FONT face=Arial size=2>> Delwin D Fandrich
wrote:<BR><BR>SNIP</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>> > For the time your =
sandwich was
being<BR>> > held in a crown by the stress-interface between the
longitudinal rib and<BR>> > the cross-grain top piece do you =
suppose it
somehow -- miraculously -- lost<BR>> > its desire to get itself
straightened out? I can't conceive how. That bent<BR>> > rib is a =
spring,
what are its lines of force?<BR>> ><BR>> <BR>> Hmm... Like I =
say..
the only thing I dont understand is what happened to that<BR>> =
tensioning
(pure stretching) force on the rib.? I mean.. the rib forces the<BR>> =
panel
to compress, and the flip side of that coin is that the panel is at
least<BR>> going to attempt to force the rib to lengthen. The degree =
it fails
in doing so<BR>> results in the bending of the rib and the formation =
of
crown.. yes ?? But what<BR>> about the degree it succeeds (if any) ? =
If the
rib is at all tensioned in the<BR>> sense that it is lengthened in =
addition
to the tensioning that occurs in the<BR>> top half from bending... =
then I
dont see how the rib could not be supporting<BR>> crown, because if =
thats the
case there is more going on then just the ribs<BR>> desire to =
straighten
out.... It also has a desire to not tension any more,<BR>> which =
downbearing
on the panel will tend to do.<BR></FONT></DIV>
<DIV><FONT face=Arial size=2>SNIP</FONT></DIV></BODY></HTML>