RC&S question in general Kent

[Jude Reveley/Absolute Piano]

> I'd restate my question... but it seems like really no one knows how to 
> figure how much compression is imparted to a panel for a given 
> downbearing with known starting values for rib strength and orientation 
> and panel compression for a constant RH.    And if THAT be the case... 
> then how on earth can we be certain of just how much compression is in 
> an RC&S panel when loaded ? Those ribs are stiff suckers... downbearing 
> forces compression in the panel as it strains against the ribs.... its a 
> fair question.
> 
> 
> Cheers
> RicB
> 

Ric,

Symantecs aside, it seems like a fair and interesting question to ask to me.

I've built my rib data spreadsheet to calculate the sag by entering the pressure of the string bearing force, rib dimensions, modulus of elasticity of rib material, bending and resisting moments & moment of inertia. Many of these factors come from static tables in old industrial standard texts for the given material (ie sitka spruce, sugar pine etc.) and are based upon some sampling once upon a time. I know it's been brought up that the modulus of elasticity, for example, will vary from rib to rib even with the same species and dimensions (this by the way is definitely one of the advantages of the laminated ribs in that the elasticity coefficient is averaged out). 

Here are some potential drawbacks:

  1.. Even with all this data, some of the static values and constants are averages or aproximations. 
  2.. Everyone I know, including myself, uses the formula for center loaded beams, which isn't exactly the case in the piano, where there may be two to three different loads on a given rib and even when there is only one load it is not necessarily in the center. This does make a good case for the symetrical design though, if for no other reason than to make the math easier.
  3.. A judgement still has to be made as to how stiff you want your assembly to be. 
Nevertheless, it's still a pretty good tool, a point of departure shall we say. I don't know how much compression this translates into but I would sure like to know. And I would like to experience how this affects the sound. All in due time I hope, unless this is my Moby Dick.

Ron N. makes some very good points about staying out of the "swamp of details" and staying aware of the diameter of the hair we are trying to split (definitely goes in the Ron N. Greatest Hits of List Quotes), but there's no harm in asking questions.

Cheers,

Jude Reveley, RPT
Absolute Piano Restoration, LLC
Lowell, Massachusetts
(978) 323-4545
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