soundboard stresses

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: October 14, 2003 12:47 PM
  Subject: Re: soundboard stresses


   =20
  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.=20

First, I mentioned no names. Don't take everything so personally.

Second, your point is?



   =20

    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.

  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.=20

Not specifically. Look for something about time under load or TUL.



    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.

  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 ?=20

Strength and the ability to withstand compression perpendicular to grain =
are two different things. And no, there is no such tool that I am aware =
of. Which is not to say one couldn't be made if one was interested =
enough in doing so.

 =20


  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 ?=20

I don't think you really misunderstand my point but you do exaggerate. =
It simply means that suitable care must be taken to insure the long term =
stability and integrity of the thing. Assuming, of course, that such =
long term stability and integrity are an issue of concern and that =
pushing the limits of structural integrity are not a design feature.

Del

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