<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <META content="MSHTML 6.00.2800.1226" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY> <DIV><FONT face=Arial>Richard wrote:</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>"This assertion that ribs don't support the = bearing load also is bothersome. Why<BR>wouldn't the ribs in a CC board support both = crown and bearing... just because the<BR>panels compression forces the ribs to = bend?"</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>I've been following this thread with great = interest. Me and anyone else that knows anything about anything knows that I am no = expert on soundboard theory - I've barely scratched the surface. But, I did pretty = dern tootin' good in college physics, and hey, I was a <EM>scientist</EM> in = another life! So here goes......</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>A simple vector analysis of forces in this = system would make things quite clear. If you are familiar with simple vector = analyses, think of it that way.</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>I think it is pretty clear to all how it is that = the laminated- or machine-radiused rib will support soundboard crown. = As you add downbearing to the RC&S board, the ribs will provide = most all the resistance to bending, with the panel supplying a little bit (as the = ribs loose crown and the panel compresses a little bit) as the = assembly squishes down under load.</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>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 <EM>always</EM> be trying to pull the panel flat. = Period. If that assembly has crown, the ribs will <EM>always</EM> 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. 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. 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.</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>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?</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Stiffness? That's still quite amorphous to me!</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>----- 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: Sunday, October 05, 2003 9:35 = AM</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><FONT = face=Arial size=2>> John:<BR>> <BR>> Sat on all this for a couple days, = and the thought I had that equally dimensioned<BR>> and crowned CC vs RC = boards would result in the CC board being stiffer... and find<BR>> myself bothered = by something that doesnt quiet add.<BR>> <BR>> First, there is this = bit about just how valid it is to say that the ribs in a CC<BR>> board do or do = not support both crown and bearing. I think I see where you and Ron<BR>> = diverge in your thinking here.... perhaps as I walk through this you can = correct<BR>> as neccessary for me. Thanks..<BR>> <BR>> <BR>> > >If you = cut a rib from a Panel crowned soundboard (parallel to the<BR>> > = >rib but half way between the adjacent ribs) you have a laminate made<BR>> = > >from a rib and a strip of cross grain spruce. The two pieces of = wood<BR>> > >form their crown just the same as the above laminated rib. Once = formed<BR>> > >this structure is going to behave just the same = as if the crown were<BR>> > >formed from a crowned rib.<BR>> = <BR>> What you are saying here is that the structual strength of the = soundboard<BR>> assembly  is independent of the crowning method ?  That = whatever stresses are<BR>> involved in forming a CC board are independant from = the stresses that will be<BR>> applied to the board when bearing is = applied ?<BR>> <BR>> <BR>> <BR>> > This doesn't equate in the = real world with actual wood. Wood is<BR>> > anisotropic, and compresses = considerably more across the grain than it does<BR>> > along the grain. It = takes considerably more dimensional change in a cross<BR>> > grain = spruce panel to generate the cross grain compression levels it takes<BR>> > to = both bend the rib (which, itself is not made of multiple layers = formed<BR>> > and glued, but is one solid piece that resists bending far more than = a<BR>> > stack of laminations of the same overall depth), and support the = bearing load.<BR>> <BR>> This assertion that ribs dont support the bearing = load also is bothersom. Why<BR>> wouldnt the ribs in a CC board support = both crown and bearing... just because the<BR>> panels compression forces the = ribs to bend ?  I mean.. ok... so the ribs resist<BR>> bending and will = want to bend back... or rather they will resist the board<BR>> expansion = while taking on humidity with whatever degree of force it takes to bend<BR>> them = in the first place.... but once that bending is done... and the assembly = is<BR>> in equilibrium... why wouldnt the ribs directly help support the assembly against<BR>> down bearing ? Its not like pushing down on the board = will be seen by the ribs as<BR>> a drop in humidity or anything... the = compression of the panel at its interface<BR>> with the ribs isnt going to change = with an increase in downbearing. The ribs, bent<BR>> as they are... will just = see a downwards force wanting to push them flat, and<BR>> unless there is = some reduction in the panels compression  keeping them bent...<BR>> = they arent going to want to bend flat.... so why isnt this support against bearing<BR>> ?...<BR>> <BR>> I think.... thats more or less = where Johns reasoning above is going... yes ?.. no<BR>> ?<BR>> <BR>> = ><BR>> ><BR>> > >I admit that it took force to form the crown in a = panel crowned<BR>> > >Soundboard but once formed it will have just = the same stiffness as a Rib<BR>> > >crowned soundboard. The method of = crowning has no effect on its stiffness.<BR>> ><BR>> > We aren't = talking about stiffness, but about the panel supporting both the<BR>> > = string bearing load, and whatever crown the rib is forced into.<BR>> = ><BR>> <BR>> I dont see where anyone has pointed any information that = establishes this<BR>> stiffness amount bit either way.  But its right at the = center of my present<BR>> headache :)  It the ribs are contributing the = same amount of support in both the<BR>> CC and RC otherwise identical assemblies... then why wouldnt the increased<BR>> compression on the = CC board increase its stiffness over the RC board. And  If its<BR>> the = other way around... that  the stiffness of these same two otherwise idenditcal<BR>> panels is the same... then how can the ribs = contribute the same amount  to the<BR>> overall stiffness in both = boards...?<BR>> <BR>> one other thing... isnt "stiffness" and "how the panel supports = string bearing<BR>> load" sort of very much interelated, for not to say more = or less the same thing ?<BR>> <BR>> <BR>> Thanks for whatever help you = can offer in helping me clear these questions I am<BR>> struggling with up.<BR>> <BR>> Cheers<BR>> RicB<BR>> <BR>> --<BR>> = Richard Brekne<BR>> RPT, N.P.T.F.<BR>> UiB, Bergen, Norway<BR>> = </FONT><A href="mailto:rbrekne@broadpark.no"><FONT face=Arial size=2>mailto:rbrekne@broadpark.no</FONT></A><BR><FONT face=Arial = size=2>> </FONT><A href="http://home.broadpark.no/~rbrekne/ricmain.html"><FONT = face=Arial size=2>http://home.broadpark.no/~rbrekne/ricmain.html</FONT></A><BR><FO= NT face=Arial size=2>> </FONT><A href="http://www.hf.uib.no/grieg/personer/cv_RB.html"><FONT = face=Arial size=2>http://www.hf.uib.no/grieg/personer/cv_RB.html</FONT></A><BR><FO= NT face=Arial size=2>> <BR>> <BR>> _______________________________________________<BR>> pianotech list = info: </FONT><A href="https://www.moypiano.com/resources/#archives"><FONT = face=Arial size=2>https://www.moypiano.com/resources/#archives</FONT></A><BR><FON= T face=Arial size=2>> </FONT></BODY></HTML>