Crown on Compression Boards

[Ron Nossaman]

> What is especially alarming is when you run some numbers on how effective
> rib support is in these areas (just starting to play with some toys).
> 
> A 500mm rib typical of a killer octave rib on a Steinway that has a
> 60'radius produces about 1.6 mm of crown.  Having just measured an original
> rib on a model O in that section at 28mm wide and 19 mm high, when loaded at
> 1.25 degrees, as those pianos typically are, about 80 lbs of load is carried
> on that rib.  The rib will deflect (according to my calculations) by 5.6 mm
> yielding a remaining crown of negative 2.4 mm.  Presumably, the balance of
> that support needed to hold the crown positive is carried by a compressed
> panel.  Seems like not great odds, at least not for very long.  
> 
> David Love

It doesn't work quite like that in a CC board. All of that 
initial 1.6mm of crown, if it actually produces that much, is 
pure panel compression. The rib supports no load as a beam 
until it passes flat and goes concave. If it's positively 
crowned, it's pulling down - negative support. To hold 
positive crown under string bearing, panel compression must 
both support bearing, AND bend the rib up to any positive 
crown it may possess. So if you're separating the beam support 
from the panel in CC board calculations, you have to throw out 
the positive crown. So an 80 pound load would give you a 5.6mm 
negative rib crown. Anything less is panel support.
Ron N