This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment 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?" 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 scientist in = another life! So here goes...... 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. 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. 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. 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. 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? Stiffness? That's still quite amorphous to me! Terry Farrell ----- Original Message -----=20 From: "Richard Brekne" <Richard.Brekne@grieg.uib.no> To: "Pianotech" <pianotech@ptg.org> Sent: Sunday, October 05, 2003 9:35 AM Subject: Re: RC vs CC again > John: >=20 > Sat on all this for a couple days, and the thought I had that equally = dimensioned > and crowned CC vs RC boards would result in the CC board being = stiffer... and find > myself bothered by something that doesnt quiet add. >=20 > First, there is this bit about just how valid it is to say that the = ribs in a CC > board do or do not support both crown and bearing. I think I see where = you and Ron > diverge in your thinking here.... perhaps as I walk through this you = can correct > as neccessary for me. Thanks.. >=20 >=20 > > >If you cut a rib from a Panel crowned soundboard (parallel to the > > >rib but half way between the adjacent ribs) you have a laminate = made > > >from a rib and a strip of cross grain spruce. The two pieces of = wood > > >form their crown just the same as the above laminated rib. Once = formed > > >this structure is going to behave just the same as if the crown = were > > >formed from a crowned rib. >=20 > What you are saying here is that the structual strength of the = soundboard > assembly is independent of the crowning method ? That whatever = stresses are > involved in forming a CC board are independant from the stresses that = will be > applied to the board when bearing is applied ? >=20 >=20 >=20 > > This doesn't equate in the real world with actual wood. Wood is > > anisotropic, and compresses considerably more across the grain than = it does > > along the grain. It takes considerably more dimensional change in a = cross > > grain spruce panel to generate the cross grain compression levels it = takes > > to both bend the rib (which, itself is not made of multiple layers = formed > > and glued, but is one solid piece that resists bending far more than = a > > stack of laminations of the same overall depth), and support the = bearing load. >=20 > This assertion that ribs dont support the bearing load also is = bothersom. Why > wouldnt the ribs in a CC board support both crown and bearing... just = because the > panels compression forces the ribs to bend ? I mean.. ok... so the = ribs resist > bending and will want to bend back... or rather they will resist the = board > expansion while taking on humidity with whatever degree of force it = takes to bend > them in the first place.... but once that bending is done... and the = assembly is > in equilibrium... why wouldnt the ribs directly help support the = assembly against > down bearing ? Its not like pushing down on the board will be seen by = the ribs as > a drop in humidity or anything... the compression of the panel at its = interface > with the ribs isnt going to change with an increase in downbearing. = The ribs, bent > as they are... will just see a downwards force wanting to push them = flat, and > unless there is some reduction in the panels compression keeping them = bent... > they arent going to want to bend flat.... so why isnt this support = against bearing > ?... >=20 > I think.... thats more or less where Johns reasoning above is going... = yes ?.. no > ? >=20 > > > > > > >I admit that it took force to form the crown in a panel crowned > > >Soundboard but once formed it will have just the same stiffness as = a Rib > > >crowned soundboard. The method of crowning has no effect on its = stiffness. > > > > We aren't talking about stiffness, but about the panel supporting = both the > > string bearing load, and whatever crown the rib is forced into. > > >=20 > I dont see where anyone has pointed any information that establishes = this > stiffness amount bit either way. But its right at the center of my = present > headache :) It the ribs are contributing the same amount of support = in both the > CC and RC otherwise identical assemblies... then why wouldnt the = increased > compression on the CC board increase its stiffness over the RC board. = And If its > the other way around... that the stiffness of these same two = otherwise idenditcal > panels is the same... then how can the ribs contribute the same amount = to the > overall stiffness in both boards...? >=20 > one other thing... isnt "stiffness" and "how the panel supports string = bearing > load" sort of very much interelated, for not to say more or less the = same thing ? >=20 >=20 > Thanks for whatever help you can offer in helping me clear these = questions I am > struggling with up. >=20 > Cheers > RicB >=20 > -- > 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 >=20 >=20 > _______________________________________________ > pianotech list info: https://www.moypiano.com/resources/#archives > ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... 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