<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html>   Farrell wrote: <blockquote TYPE=CITE><style></style> Richard wrote:"This assertion that ribs don't support the bearing load also is bothersome. Why wouldn't the ribs in a CC board support both crown and bearing... just because the panels compression forces the ribs to bend?"</blockquote> Grin.. I eat vectors for breakfast. And actually thats one of the reasons why I asked. Could you share with us your vector analysis of the forces involved here.?     <blockquote TYPE=CITE>When flat ribs are glued to a very dry flat panel, no force vectors exist in the system. As the panel takes on moisture and the ribs bend, the ribs will always be trying to pull the panel flat. Period. If that assembly has crown, the ribs will always try to go straight by pulling down on the panel. The compression forces in the panel are the only thing that has provided the force necessary to bend the rib into a crown. Whether or not you ever load that soundboard on top, the ribs will always be fighting the panel compression to go flat.</blockquote> Yes... the ribs will always be fighting the panel compression. But that fact in itself doesnt mean the ribs can react independently to some other force..... or what ?   <blockquote TYPE=CITE>When you do apply a load to the top of the soundboard, the compression on the panel will increase, but the ribs will do nothing other than still try to go flat.</blockquote> Yes... this is where we need that vector analysis...:) I am not sure, however, that downbearing simply increases compression over the entire panel. As downbearing tends to flaten the panel,  the ribs would cooperate by your reasoning the compression in the panel at the interface between ribs and panel would decrease... necessarilly. But this isnt what happens... the board is pressed down, and compression increases on the top half of the panel, and perhaps  a little if at all at the rib panel interface. This was the bit Frank Weston got all hung up on a couple years back. He was seeing things in terms of an absolute number line where positve and negatives are directions and not a matter of being plus or minus zero. Ergo he reasoned that the upper part of the panel was in tension while the lower part was in compression, and when downbearing was applied then and only then was the whole board under compression. There was a certain logic to his reasoning... but it only works in terms of relative compression levels. In anycase... looking at the crowned board before bearing is applied.. the top and bottom are at different levels of compression, and flattening the board will further compress the top half. But the bottom half would want to expand... which of course the ribs wont allow... so it would seem to me that the ribs bending pressure would increase instead of decreaseing.   <blockquote TYPE=CITE>  As the crown lessens under increasing load, the ribs will pull downward a bit less, but only because they are being bent upwards less by the panel.</blockquote> I'm not sure I buy this yet.. see above. <blockquote TYPE=CITE>I  think any thoughts of the ribs providing support to a CC soundboard are related to thinking of the rim acting as a buttress (I'm not sure of the terminology here - I'm referring to church architecture from the middle-ages where they used arches that were supported by a solid foundation). Perhaps some still subscribe to this train of thought. Is that perhaps where you are coming from?</blockquote> Sort of... but only in as much as the ribs sort of force an arched structure to the cellular structure of the wood.   <blockquote TYPE=CITE>   Stiffness? That's still quite amorphous to me! Terry Farrell</blockquote> Wifes complaining... so I gots to get to bed.. :) Cheers RicB   -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway <A HREF="mailto:rbrekne@broadpark.no">mailto:rbrekne@broadpark.no</A> <A HREF="http://home.broadpark.no/~rbrekne/ricmain.html">http://home.broadpark.no/~rbrekne/ricmain.html</A> <A HREF="http://www.hf.uib.no/grieg/personer/cv_RB.html">http://www.hf.uib.no/grieg/personer/cv_RB.html</A>  </html>