Rib Support

[Farrell]

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Oops! This note was not finished - it was supposed to go to my outbox =
for completion later. I deny having anything whatsoever to do with this =
post!

Terry Farrell
  ----- Original Message -----=20
  From: Farrell=20
  To: Pianotech=20
  Sent: Sunday, October 26, 2003 9:31 AM
  Subject: Re: Rib Support


  Hello Richard. I know some have express some disenchantment with your =
persistant theoretical explorations of soundboard crown, but I am rather =
enjoying it. It's a good mental excercise. And I also understand that =
even though you may well never build a soundboard, it is still a great =
thing to try and understand how they might work. There are not very many =
authoritative references available to find answers to these questions.

  So with that in mind, let's proceed to do the best job we can leading =
each other into an advanced state of total confusion............=20

  Comments interspersed below:

  Terry Farrell
    ----- Original Message -----=20
    From: Richard Brekne=20
    To: Newtonburg=20
    Sent: Sunday, October 26, 2003 5:47 AM
    Subject: Rib Support


    Hi List=20
    I wanted to try and better explain why I am having difficulty in =
accepting that the ribs in a CC board do not support crown against down =
bearing. Please follow the following reasoning and diagrams and you will =
see where I am stumped.=20

    First is a panel that is simply bent across the grain and held into =
place with a cable appropriately attached on the ends of the panel. =
Obviously, when down bearing is applied, the outward pressure on the =
cable is increased. This entire situation is going to be the same if one =
first dries out the soundboard, then securing that same cable tightly, =
let the panel take on enough MC to form the same crown.=20

    =20
     =20

    Replacing the cable with a piece of wood doesn't really change a =
whole lot conceptually.  The rib will  be under the same amount of =
tension as the cable was, though the amount of physical strain may =
differ.  Pressing down on the panel is definitely going to be resisted =
by both the panel and the rib. If the assembly is only resting on =
supports at the edge, down bearing might even result in some downwards =
bending of the rib.. but I will have to try this to see. Still, the =
panels edge will <<bend>> in a downwards direction, and there might be =
some reaction in similar direction by the ribs. In any case, the =
assembly as a whole will resist any down bearing for the same basic =
reasons as above.=20

    =20
     =20
     =20

    Now if the rib is secured to the panel such that it bends a bit in =
the same direction as the panel, (i.e. glue say about 10 inches in on =
each side)  then the only thing that has really changed is that the rib =
bends upwards a bit... as in the below drawing. Yet the same basic =
condition exists. Downwards pressure in the panel increases the outwards =
pressure, yet this is resisted just in the same way as in the previous =
two examples, so in response both panel and rib will feel an increased =
stress and will strain against that stress until one of the pieces fail, =
which of course will be the panel.=20

    Keep in mind here that stress and stain, although related, are two =
very different things. Stress is a force that is applied to an object - =
or an object's ability to resist deformation. Stain is the deformation. =
With the soundboard, I suppose stress is delivered to both panel and =
rib, but strain is negligible for the rib, and orders of magnitude =
larger for the panel. I've searched for numbers that describe this =
relationship, but have found none.=20

    But until that point is reached, the entire system will resist down =
bearing vigorously. No doubt the rib will straighten at some point, and =
the relative tension/compression on the top and bottom sides will =
equalize, but it will have to experience an increase in overall tension =
for this to happen, which it will resist. The panel will also straight =
out at somepoint... but nobody believes for one second that it wont be =
under stress when its flat.. So why should the rib be any different ? =
Opposite side of the same coin. Both will physically strain against the =
stress, the panel a good deal more then the ribs, but the stress levels =
are the same on both.=20

    =20

    The only thing that remains is to simply glue the entire rib to the =
soundboard, but I cant see that that changes anything. If the above =
three steps hold, then it seems in error to say that the only thing that =
supports crown in a CC board is compression in the panel. The entire =
system, panel and ribs alike will strain against down bearing.=20

    Ok... so this seems to make perfect sense to me, and though ribs are =
not acting as beams in their support, they can neither be said to be =
purely fighting crown. Quite the opposite. The fact that the rib is bent =
in the process, and of course resists this bending can be said about the =
soundboard as well. But I dont see that simple bending of either =
describes their condition adequately.=20
     =20
     =20

    Cheers=20
    RicB=20

    --=20
    Richard Brekne=20
    RPT, N.P.T.F.=20
    UiB, Bergen, Norway=20
    mailto:rbrekne@broadpark.no=20
    http://home.broadpark.no/~rbrekne/ricmain.html=20
    http://www.hf.uib.no/grieg/personer/cv_RB.html=20
     =20

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