Curve on Bridge Bottom

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Erwinspiano@AOL.COM=20
  To: pianotech@ptg.org=20
  Sent: May 06, 2002 5:31 PM
  Subject: Re: Curve on Bridge Bottom


  snip--snip--snip

                                            Richard, Terry
                            It doesn't take much to flatten an =
unsupported rib as it  doesn't really take on its maximum stiffness till =
after it becomes part of the ply wood componet comprising the whole =
soundboard system at which point the cumulative resistive forces of =
cross banded woods and compression takes effect.... =20


    Rather depends on the cross-section of the rib, doesn't it?=20

    Del


                Yes, but of course,  the stiffening idea of the two =
componet forces  combined was what I was trying to illuminate and the =
thicker the rib the stiffer the cumulitive effect or perhaps you =
disagree? Well ,I mean unless were talking 2 by 4s which would cut the =
boards contribution  out of the equation altogether.
  ...........Dale>>>>>>>>>>


Let's just say I agree conditionally....=20

The rib--at least in a rib-crowned board and somewhat simplistically--is =
an end-supported, center-loaded structural beam. In principle the =
load-carrying capacity of this type of beam is a function of the cube of =
its height. But, its load-carrying capacity is also a function of the =
strength characteristics of the material making up the beam and an =
assumption is made that if the height of the beam is increased it will =
be done so with a material having strength characteristics that are =
uniform throughout. Especially in its tensile and compression strength =
characteristics.=20

When loaded, this type of rib will be under tension along its bottom =
surface and under compression along its top surface. Now, a =
characteristic of woods such as spruce and pine is that they have =
reasonably high compression and tensile strength in line with their =
grain but quite poor compression and tensile strength perpendicular to =
their grain.=20

In the situation you describe the soundboard panel will be glued to the =
top of the rib and this will, indeed, increase the overall height of the =
'beam' and make the assembly some stiffer. However, when the assembly is =
loaded the soundboard panel--by virtue of its grain orientation at =
approximately 90=BA to the rib--will be placed under compression =
perpendicular-to-grain. Since its compression strength in this =
orientation will be considerably lower than that of the top part of the =
rib it will not increase the stiffness of the assembly by the expected =
relationship.=20

As well, both of these woods are prone to early compression-set when =
stressed under compression perpendicular-to-grain.=20

So, yes, gluing the soundboard panel to the top of the rib[s] will =
increase the stiffness of the assembly. But not by much and not for =
long.

Del

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