Soundboard stiffness variances

[Richard Brekne]

All of this is based on some assumptions about variability in wood 
strengths that aren't well enough enough spelled out or perhaps 
understood in some cases here. To begin with variability in wood 
strengths applies in all directions and for all types of stresses. Ribs 
seen as beams are no less subject to variation in load carrying 
strengths as a panels is to compression strengths. The relevant 
cooeficients are approximately 15 for along the grain tension, 25 for 
along the grain compression, and 28 for perpendicular to the grain 
compression. On top of this approximately 85 percent of the variability 
in compression perpendicular to the grain can be accounted for by 
variation in specific density of the individual pieces (Institute for 
Strength of Materials, Vienna University of Technology, Karlsplatz 13, 
A-1040 Vienna, Austria. Study on Norwegian Spruce) When normalized for 
this variation in specific gravity... our unpredictability factor 
decreases quite a bit... well within usable parameters.

Another point that needs straighting out is just what the heck we are 
talking about when Hoadley and other such sources are quoted regarding 
compression strength perpendicular to the grain. There are two relevant 
planes. Tangential and radial. And they are often as not unequal to each 
other in their strength characteristics... depends on the type of wood 
really. Compression strength values given in tables are most generally 
averages of the two. In the bargain I'd like to mention that our friend 
Thumpy a year or so back claimed that ribs should be alligned so that 
their radial face was the one glued to the bottom of the soundboard 
exactly because of a supposed increase in strength compared to loading 
the rib against the Tangential surface. Hoadley supports this suggestion 
if anything.

    " Published values for strength properties commonly list a singel
    value for perpendicular to grain compression strength that is the
    average of both radial and tanential properties. In some species,
    there my be insignificant differences between the two, but in others
    the anotomical structure causes noteworthy radial and tangential
    differences. For example in ring-pourus hardwoods such as ash or
    catalpa and in uneven graned softewoods such as southern yellow pine
    or Douglas fir, a piece of wood stressed in the radial direction
    will be no stronger then the weakest layer of early wood....."

    "Ash and other similar woods suppor greater loads when loaded
    tangentially (i.e. against the radial face) ((as Thumpy suggested))
    because the layers of stronger latewood share the stress equally"

There are several ways of testing for individual specific gravity and 
strengths that are within the grasp of your average shop and yield 
results with an acceptable degree of resolution. I don't see that the 
line of reasoning that leans on the variability of wood strengths 
properties says anything else then that these need to be accounted 
for... which as far as I can see is a doable.

As far as the claim that the degree of unpredictability being 
proportional to the degree with which the assembly is reliant on 
compression.... where is the study that supports this ? Proportional has 
very a specific meaning when used thus. Strikes me right off that given 
the fact that the rib is just as likely to show wide variability in its 
strength properties... particularly bending strengths... such a claim is 
taken out of thin air. JMMV

Cheers
RicB



     > The problem is that each panel’s compression characteristics will be
     > different or, you could say, have its own distinct personality.

        The problem is also that you don't have control of the
        different requirements for stiffness in different areas of the
        scale, as you do with rib supported assemblies.

     > If one
     > insisted on having compression in the panel then the best way is
     > probably a hybrid system where you underbuild the precrowned rib
    support
     > somewhat counting on a certain amount, but less, compression to
    achieve
     > the rest of the requisite stiffness. You still have the
     > unpredictability of the panel’s unique ability to withstand
    compression,
     > but reducing the amount might create a somewhat more stable system.
     >
     > David Love

        Yes, with the unpredictability being proportional to the
        degree to which panel compression supports crown.
        Ron N