evaluating sdbd. crown & bridge downbearings in a new piano

[JIMRPT@AOL.COM]

In a message dated 9/21/1999 1:39:41 PM, klavier@annap.infi.net writes:

<<"A so called "compression" crowned soundboard in an unloaded state will 
have the fibers in the top half of the board subjected to tension and those 
in the bottom subjected to compression">>

Frank;
 What is an "unloaded state" in a compression crowned board?.....do you mean 
a board with no pressure, i.e., downbearing placed on it?  A compression 
board by definition is "loaded" with or without downbearing...isn't it?

<<"(it's the rule, you got to have both).">> 

 CoExpansion/contraction = .00585" per 1% moisture increase or decrease per 
lineal inch of spruce board across the grain. (CoEx pertains to boards from 
4% to 10% moisture as measured vis a vis EMC and is an average CoEx as 
closeness of growth rings affects this CoEx, sometimes dramatically)

 Using this CoEx on a board 46" across, as measured perpendicular to the 
grain, as for instance in a S&S model 'A', and increasing the board from a 4% 
to a 8% EMC will effectively add  .2691 of a inch to the boards 
width..............question... where does this extra width go?  To crown? 
Since the bottom of the board is captured by the ribs isn't it reasonable to 
assume that the crown will increase?  Why does the crown increase?  Because 
of uncaptured pressure in the top surface of the board?  If there is pressure 
in the top surface of the board how can there be "tension"....if the 
definition of tension is "stretching"? 
Perhaps the statement of "(it's the rule, you got to have both)." is not 
quite accurate, vis a vis the board, for if it were wouldn't the induced 
tension of the top of the board tend to pull the board flat, negating some of 
the effects of the compression?

  If a board is 'rib crowned' I would have no argument with saying the top 
surface of the board is under tension and the bottom under compression but I 
don't think the argument works with "compression boards"...does it? 


<<"As downbearing load is applied, the tension in the top half of the board 
will be decreased, as will the compression in the bottom half.">>

 Does this work for "rib" formed....... and........... "compression" formed 
boards?

On the "rib" formed board I can go along with lessening of tension....... but 
since a compression formed board is by definition under compression doesn't 
downbearing increase the compession on said board?
My thoughts, but I went to FSU, not Ga Tech, and can't spell enginear :-)
Jim Bryant (FL)