soundboard stresses

[Ron Nossaman]

>O.K. Ron. So you are saying that a cross grain spruce reacts to stress in 
>a non linear way. It resists compression more and more as it is 
>compressed? As I see it, with my limited understanding, it should react to 
>compression forces in a linear and predictable fashion within the elastic 
>range. Check out a stress strain graph to see what I mean.

Yes, John, I see what you mean. So let's go with that. Looking at the 
stress/strain chart in Hoadley's "Understanding Wood", I also note that the 
elastic limit is less than 1%, which is also stated in the text. Using the 
supplied formula with the appropriate expansion/shrinkage average for Sitka 
spruce, we get

Dimensional change = Lin*0.043*((MC%2-MC%1)/0.28)

with MC%1 being the lower MC and MC%2 the higher.

We see that a panel that is 36" wide at 4.5%MC will expand by 0.359" when 
taken to 11%MC. It's proportional, so any width will produce the same 
percentage size change (1% with these MC ranges). That will happen at 70° 
@  60% relative humidity. Constraining this panel to a flat rib will bring 
it under compression beyond the fiber stress proportional limit, indicated 
in this same reference at 580psi radially perpendicular to the long grain, 
and induce some degree of immediate and permanent compression set. I submit 
that it is highly unlikely for a piano built this way to remain under 11%MC 
forever to avoid immediate panel damage, never mind long term compression 
set which occurs at a rate proportional to internal compression. I also 
submit that it is highly likely that the piano will reach an MC 
significantly above 11% before it even gets out of the factory, so 
permanent panel compression damage almost certainly occurs before the piano 
is even finished.

Note that this is irregardless of the ribbing material used, bearing, or 
panel thickness, and is merely the result of assembling a compression 
crowned panel with flat ribs.

This looks to me to entirely contradict your claim that building CC boards 
with flat ribs does not put the panel at damagingly high compression 
levels. Immediate compression damage seems to, instead, be a realistically 
unavoidable consequence.

Incidentally, I had always heard 4%MC quoted as the preferred dry down 
level for this type of construction. When did it become 4.5%, and why?

Ron N