Buttressed Arch. Question for Ron N.

[Ric Brekne]

Back to the origional point of this thread... the article itself.  There 
was a few things I found really interesting about the article.  The 
experiment itself showed a few things I'd like to have a bit of 
clarifications on.

When Ron put the newly ribbed (and still dry)  panel in the restraining 
caul (if I may call it that) and allowed it to take on humidity, the 
thing didnt crown.  This is as Ron stated to be expected given his basic 
postulate about the assembly not being buttressed to begin with.  On the 
other hand the part of the panel/rib assembly being restrained is panel 
endgrain. 

As I have understood earlier discussions along this arch thread,  
compression failure of end grain far exceeding its ability to support 
the arch even if there was a real buttress was one of the main points 
argued against the arch idea. But the experiement would seem to show 
that the end grain did not fail at all and acted as a kind of reverse 
buttress if you will, preventing the panel from crowning when restrained.

Then too is the point about the centerline of the rib/pannel assembly 
being below the buttress.  This raises a nut that needs addressing.  If 
the underside of panel itself when crowned actually does behave like the 
upper half of the rib in this respect... then it is expanded... 
stretched as it were along with the bending ribs upper half.  But if 
that is the case... then this same underhalf of the soundboard has 
experienced a degree of tensioning because of that stretching. It would 
seem that tho the underside of the panel in general is under very much 
compression, this same compression is then lessened to some degree by 
the ribs stretching in the opposite direction. 

How one is to figure the net degree of compression in the panel then 
strikes me as difficult at best... but certainly seems worth knowing.

Cheers
RicB