Hi Sarah I'm going to let the RC & S gang answer to your questions about how that system ends up, but I would assume that drying the panel even to 6.5 % would introduce a certain degree of compression into the assembly.... especially in places like... Florida... Iowa... Kansas... in the summertime :) As for the structual strengths maths and assumptions being tossed around. Seems a bit curious to me that most of this stuff is based on information known about beam strengths, applied to a panel that is loaded in a completely different fashion. Heck.. its not even loaded remotely like a floor when it comes down to it. Consider the panel itself. Bridge runs pretty much parallel to the grain, and that grain is supposed to be quartersawn with the radial side up. Pressing down on this bridge... the bridge itself will want to constrain the footprint surfaceof the panel from compressing (in the face of downbearing) in the direction along its length... along the grain... but perpendicular to the grain things are different. Strength data for bending stiffness in this direction is lacking because of course you are asking for bending stiffness in the case we only measure shear strength. I mean... take a flatsawn board and give it a karate chop in the wrong direction (perpendicular to grain) and you will get my point here. So here we are... discussing bending stiffness for the panel either ignoring bending along the grain, or applying strength properties that apply to beams in the perpendicular direction with the load applied tangentially ! In short.. if one is to constertate just what the panels stiffeness properties relative to things like degree of cross grain compression, it seems a bit ridougulous to not measure the E in that direction. After all.. we are dealing with something more akin plank loaded widthwise... as in a plate, then we are a beam. Once thats all figured out.. then you can figure that into the assembly as a whole with the ribs attached. No doubt tho... this all is out to lunch :) Cheers RicB hi Ric, >/ To the direct claim that stiffness (in any direction) does not increase />/ with compression. This is simply false. It does. Wood is usually sorted />/ into grades that are related to density, straightness of grain, grain />/ consistancy... etc. Looking at the tables in just about any such manual />/ one sees quickly that E, and for that matter just about all strength />/ properties, increase as density increases. Since a compressed panel is />/ essentialy a more dense panel then it would be without the compression, it />/ goes without saying that it becomes stiffer. / Thus my caveat about the SB components *not* being ideal springs. I just didn't know *how* non-ideal! It would appear the material is highly nonlinear, which could account for some of its unique acoustic properties. Strange stuph, wood... ;-) It seems to me that a completely *uncompressed* RC/S panel would have to be made thusly... (1) Determine the total desired downbearing of the strings. (2) Determine the initial curvature of the ribs needed to support said downbearing with the desired final crown (which would be flatter than the initial, unloaded curvature). (3) Stabilize the panel wood at the target ambient RH (e.g. 42% RH). (4) Glue up the ribs under load, with the curvature of the setting/drying/curing assembly being the same as that of the final assembly under load. (Do this preferably with epoxy.) When released, the panel would spring back somewhat, putting tension, not compression, on the panel. But when the panel is mounted into the rim and placed under load, the panel tension would be relieved, and the net tension/compression on the panel would be zero. Is this how it is done??? Peace, Sarah
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