List: Apologies for barging in in the middle of a thread--I have followed a few iterations. I have to concur with Ron--I believe that the board and ribs are analagous to a stressed-skin panel--you might as well say that the glue joint between the ribs and the board holds up the downbearing forces as say that the ribs do it alone. I know I am not telling anyone something new when I say that wood, and especially spruce, swells much less in an axial sense than radially or tangentially. The system, the stressed-skin panel, curves because the bottom of the sound board is restricted in its expansion by being fixed to the axial grain of the rib. The top surface of the rib must be stretched in order for the board to expand, and the bottom surface is therefore compressed, but it must be the system as a whole that supports the load. Cracks in the soundboard just turn the system into several parallel stressed-skin panels, each of which will still remain curved. There would have to be an enormous number of cracks to reduce the overall curvature of the soundboard/rib system barring failure of the board/rib glue joint, I think. There are stressed skin panels made out of 3/4" plywood glued to curved ribs holding up 60 lb/sq ft snow loads on hockey and curling rink roofs all across Canada and the northern US. It is quite a useful feat of engineering. The WWII Mosquito fighter-bomber , I believe, is another example of plywood stressed-skin panels in action. Tim Keenan Noteworthy Piano Service Terrace, BC
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