RC vs CC again

[Richard Brekne]

John:

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.

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..


> >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.

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 ?



> 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.

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
?...

I think.... thats more or less where Johns reasoning above is going... yes ?.. no
?

>
>
> >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.
>

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...?

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 ?


Thanks for whatever help you can offer in helping me clear these questions I am
struggling with up.

Cheers
RicB

--
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