laminated ribs

[Ric Brekne]

Hi Terry

Not quite me thinks, (at least as I think I understand what you are 
getting at). Certainly downward pressure on the ribs alone exert a 
stress (force) that will eventually strain the ribs. But even in RC&S 
systems pressure downwards on the combined assembly will stress the 
interface between the ribs and soundboard. And this is where it gets 
interesting. On the one hand the downward pressure on the ribs (if seen 
isolated from the assembly) will exert a compression stress on the top 
half of the rib. But on the other hand that same downward pressure will 
exert a compression stress on lower half of the soundboard, and 
compression there will in turn stress the ribs in a tensional 
direction.  So downward pressure on the entire assembly places two and 
opposing types of stress on the ribs (and the soundboard ftm)

Now one can rightly state that the ribs resistance to tension is several 
mulitples of that of the panels resistance to compression. However as 
stated in my last post, it is not greater then that the ribs upper half 
actually does strain tensionally when the panels under side compresses 
enough for the assembly to experience a strain in its crown.

Which of the opposing stresses wins out in the ribs case is less 
interesting then the fact that the result would be different if the rib 
was isolated from the assembly and experienced only a downwards strain.

Grin... and I think I've managed to keep the 
stress-strain-compression-tension- words all in their proper contexts... 
must be learning :)

Cheers
RicB



Terry Farrel writes:

stress is the force causing the deformation divided by the area to which 
the
force is applied; and strain is the ratio of the change caused by the 
stress
to the original state of the object.

For a piano rib (in a RC&S system) then, the stress would be downbearing 
and
the strain would be the amount of crown loss due to downbearing pressure.

Terry Farrell