Fix what's not broke?

[Ron Nossaman]

> There is a 'natural' balance in all structures and within that balance there 
>is a range of capabilities. Should a 'balanced' structure never be pushed 
>beyond its inherent capabilities there is no reason for that structure to 
>fail. For instance if a soundboard is produced with 10% emc (illustration 
>purposes only) and rib crowned with ribs also at 10% emc then there is a 
>natural range wherein this assembly will operate perfectly well and provide 
>long service without any structural deformity/damage.  However should this 
>structure be pushed beyond its capability, in either direction, then damage 
>of some type will begin to occur.

Not quite. True, there is a natural balance at work here, but the balance
isn't always as equitable as it could be. In the rib crowned board you use
as illustration, the panel is still under compression and the ribs are
still being bent. While the forces involved may be in equilibrium, that
doesn't mean the component parts are suffering equal abuse. Compression set
will still occur in the panel, and creep deformation will still occur in
the ribs, so some cumulative damage will occur even if the fiber stress
proportional limits of the materials are not exceeded. In that old Steinway
board that started this discussion, as in many more boards from other
manufacturers through the years, the ribs weren't crowned at all and the
panel is not only doing all the work, but having to bend the ribs as well.
That means that the "natural balance" of forces built into a compression
crowned board leave the panel compression much closer to (or farther past)
the fiber stress proportional limit than those same forces in a rib crowned
board exhibiting that same natural balance.


>Dry it out too much and it will split/separate/flatten...Add too much 
>moisture and it will develop pressure ridges, tear away from the ribs, crush 
>wood cells, etc.

Both the tendency of the board to flatten when it's dry, and the
compressive panel damage from high humidity cycles are less in a rib
crowned board than in a compression crowned.


 
> Now if we take that balanced structure (soundboard) and restrain it in some 
>manner........... we alter the balance of that structure.  Let's say we glue 
>it in the rim of a grand. By doing so we alter, or limit, the 'usable' range 
>of any balance within that board...........and for the sake of discussion 
>let's say that we now have 30% of the 'balanced' range to work 
>with...............as long as what we do does not demand more than that 
>remaining 30% of capability then the board will not suffer any deformity or 
>damage.  But now we put in bridges and a download from the strings
>which produce forces of their own.........still if we don't exceed that 
>remaining 30% capability of the board then there will be no structural damage 
>and for all practical purposes you will have a balanced structure in-toto.

Ok - first, the string bearing load will be more of a factor here than the
gluing of the assembly to a rim. Panel compression levels escalate much
more quickly than string tensions and bearing loads with a humidity
increase. Check the leverage moments, and rough in the math. In a CC
assembly, the panel takes the full abuse because it not only has to hold up
the strings, but bend the ribs more too. The RC board has the ribs helping
the panel hole up the load, so the compression level in the panel won't be
as high. Again, damage in the form of compression set will still occur in
both panels, but will be statistically more severe in the CC panel.

Second, the RC assembly will statistically lose less crown with time and
humidity fluctuations than the CC assembly because the ribs are carrying
some of the bearing load.


>Different board fabrication techniques will provide different parameters of 
>"remaining" capability for us to work with. Probably a rib crowned board will 
>give us the largest percentage of 'residual' capability to fool around with 
>and a compression crowned board will give us the least.

Absolutely right.


> So the real answer to your original question is.......... Yes. :-)
>My thoughts.
>Jim Bryant (FL)

Good thoughts. Now as to why that 115 year old CC S&S board still has crown
- the statistical probability of any single person winning the lottery or
being bitten by a Gaboon viper is so small as to be unthinkable.
Nevertheless, every year, a number of people are bitten, as others win
lotteries. Whatever combination of grain densities and directions, rib
warpage, bearing load and distribution, and climate history resulted in
this impressive current condition, I believe I'd check the action for lost
lottery tickets. Ever cautious of vipers.


Ron N