RC&S question in general Kent/Jude

[Richard Brekne]

Hi Jude

Thanks for your post. The thing is... there are several arguments out 
here being used to justify the use of RC&S boards over compression 
reliant boards.  Central to some of the most important of these is the 
issue of panel compression.  I dont think I need to get into all the 
reasonings here as they've been well discussed already. The 
self-destruct thing, predictablity thing... we've been through that 
all.  So if degree of compression is to be blamed  I think there for its 
more then fair... and hardly chasing foul of any sort to establish what 
is actually known. Coming up with a decent guestimation of how much 
compression builds up because of the panel taking on humidity is fairly 
easy... tho as Hoadly warns the actual variance of compression strength 
from panel to panel is pretty large. Still we have usable ways and means 
there and they are these that are used as the basis for the 
argumentation against compression reliant panels.

Knowing how the glued and crowned board behaves with respect to what 
compression degrees occurs when the assembly is subjected to downbearing 
is at the least a bit more complicated matter.  You are looking at at 
least a biaxial if not triaxial loading of both the ribs and the board. 
Its not like you are just pushing down on a set of ribs. The ribs are of 
interest in this question of panel compression only because the degree 
of tension and compression they find themselves in before the panel is 
loaded is needed in order to know what degree of panel compression will 
build up for any given load. Likewise the degree of panel compression 
before load needs to be known.

I'll just sketch out two rough examples to illustrate. In each it is 
assumed that all relevant data about rib dimensions, panel thickness, MC 
at glue up for both etc are known ok ?  In a CC board  ready for string 
loading one has ribs which are bent by way of the compressed panel 
tensioning the top half of the rib. Isolated, a downwards load force 
alone would of course simply relieve both the bent state of the rib and 
panel... essentially removing the compression/tension moments that have 
to do with the actual bent form of both of these.  But we know that 
thats not at all what ends up happening.  The glue joint between the 
panel and ribs causes a second load to be applied as a result of the 
downward pressure. This load goes along the surfaces of the adjoining 
panel and rib surfaces causing a net increase in panel compression and 
at the very least less relief of rib tension then if rib was not 
subjected to this increase in panel compression. Just how much resultant 
net compression there is in the panel and tension there is in the rib is 
from what I can see anyones guess.

In an RC&S board you have ribs that are machine crowned, and a panel 
that is bent over their surface. No problem figuring amount of 
compression / tension in each as the only thing going on here is the 
panel in bent form.  As it takes on humidity one still can with ease 
reasonably calculate the amount of compression in the panel.  The rib 
will experience the same kind of tensioning as in a CC board but to a 
much less degree.  But application of downbearing changes things 
entirely here. If one has as a goal to press the assembly downwards to 
say 50 % of its unloaded height then the ribs are essentially bowed  
DOWNWARDS . From their static unloaded state, the top half of the rib is 
then under compression and the bottom half is under tension... the 
reverse of a CC board.  So what happens to compression in the panel then 
?  It of course has to increase as result of the load placed along the 
surfaces resulting from the downwards pressure... which will exert a 
tensioning stress on the rib. But the top half of the rib is in a 
compressed state now... because of being bent downwards. It will readily 
comply with any tensioning force applied by the panel and would continue 
to do so until it eventually reached its unloaded state.  So how much 
net compression ends up being in this board ? 

Unless one can answer these questions and at least show the numbers that 
relate to each of the type of boards then one is essentially guessing 
and as a result stand on very weak grounds indeed when making claims 
about which kind of panel is more stable, more predictable, more durable 
etc etc all attributed to a degree of compression in the panel one in 
reality doesn't know.  If on the other hand one CAN answer these 
questions... then at the very least the whole predictability argument 
dissapears... at least to the degree with which the strength of the 
panel across the grain itself is predictable. And again... the arguments 
against compression reliant boards is at least on that point weakened.

I've been told by a few here I've been given plenty of answers and 
information and I should be greatful.  I would like to point out that 
this is really the one and only question I have raised in the recent 
discussion, and that anything close to an answer is still lacking.  
Frank at least implied he has software that may be up to the task. JD 
and yourself seem at least also interested in the question. 

Heck guys... its just a question that needs answering... and as I say... 
none of this has no bearing on the validity of the various approaches to 
building soundboards.  All have proven themselves plenty viable.  
Personally, thats where my the boarder of how thick a hair to split goes.

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,