<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html>   Delwin D Fandrich wrote: <blockquote TYPE=CITE><style></style>  We have just seen a bunch of numbers carried out to several decimal points of accuracy being tossed about to prove one thing or another. May I remind the gentle reader that any numbers given for such parameters as compression strength perpendicular to grain and shrinkage or expansion relative to changes in moisture content are averages. They are derived through the careful testing of a variety of wood samples within a given species but taken from a variety of locations and growing conditions. Some samples will have physical characteristics considerably higher and some considerably lower than these averages. These numbers are not, sadly, The Gospel Truth!</blockquote> Well just so we are on the same page.... I myself, am not trying to prove anything with these numbers... except perhaps to prove that I am on the learning road. I'm in Hoadleys book at a very early stage and am just pushing numbers around to see if I can the formula usages correct, and in some basic perspective. I note also that tension strength along the grain  is not even given, and is only very lightly refered too.   <blockquote TYPE=CITE>To complicate things even more, they are based on tests taking a relatively short period of time -- minutes rather than hours or days. Hoadley, in his book Understanding Wood, suggests that we must degrade these figures by approximately 40% if the time under load is going to be appreciably longer than the time taken for the specific test in question. This number also appears to be an average as I've had other wood technologists quote degrade figures of 30% to 50% for time under load periods of five to ten years. It depends, apparently, on the particular characteristic and the particular species of wood under consideration. For a characteristic such as tensile strength parallel to grain the degrade figure can be considerably lower. For a characteristic such as compression strength perpendicular to grain it will be some higher.</blockquote> Could you point me to the page and section where this is found ?  Just for edification really, as for now I have my hands more then full with getting a handle on  chapters 4,5,6, and 7. <blockquote TYPE=CITE> To use the published test figures in an attempt to show that a soundboard made to a given set of specifications can safely be taken right up to the point of short term damage without sustaining either short-term or long-term degradation overlooks the cold, hard reality of the natural variableness of wood in several ways.</blockquote> Well, thats not exactly where I am going with any of this... in case anyone had thought anything else. I am rather trying to find out just how likely and to what degree we can say the opposite.  I know, I know.... bothers some to put a question mark on this point... but hey... thats how one learns... or at least how many of us learn. The other kind of learning has always struck me more as memorizing assertions, taking for granted that they are fact. <blockquote TYPE=CITE>First, it overlooks the problem that, out of any given batch of wood, some samples are going to measure high and some are going to measure low. If we could depend on every single piece of wood that goes into a given soundboard panel testing on the high side, or at least on its being average, we'd be in great shape. But in the real world we can't count on this. Inevitably, some samples are going to test low. In fact, about half of them are going to test on the low side of average.</blockquote> Thats brings up a point I'm already wondering about. Soundboard grade material visa vi lumber grades used in these tables..... but I'll get back to that as I get more read. <blockquote TYPE=CITE>Second, it overlooks the problem that no lumber grader, regardless of training and experience, can accurately determine any of these physical characteristics without getting into some form of destructive testing which, needless to say, is not being done by any piano manufacturer. Nor has it ever been that I am aware of. At least not in production. We can't get by with random testing, you see, we would have to test every single piece of wood.</blockquote> Grin... that was fast... so.. are you saying that soundboard makers have no tools at their hands for reliably (say a 90 % confidence factor) selecting materials that will fall above the average strengths ?   <blockquote TYPE=CITE>And, finally, it overlooks the degrade factor that must be applied if the structure is going to be expected to support a load over some extended period of time. Even if we accept Hoadley's figure of 40% -- rather than the more probable 50% for a parameter such as compression strength perpendicular to grain for a soft wood like spruce (it's all that soft earlywood that does it) -- this still means that we have to work from a number closer to 350 lbs/in2 than to the 580 lbs/in2 quoted in the USDA's Wood Handbook. And all the time we have to keep in mind that about half of the lumber we use is going to test lower even than that. Just how much lower, of course, we have no way of knowing because there is no practical way of either determining or predicting the specific and exact mechanical characteristics of any given sample of a natural material like wood. Del</blockquote> Yes..but !... doesnt that kinda put any soundboard construction at like horible risk ?  And if we reduce Ron's figures by 50 %.. then it would seem likely that we would be witnessing wholesale implodic disseminating chaos going on inside of absolutly every CC board within a few hours after assembly. Or do I misunderstand your point here ? Cheers, and by the way... thanks for suggesting me to buy Hoadleys book. What an adventure. RicB -- Richard Brekne RPT, N.P.T.F. 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