RC vs CC again

[Richard Brekne]

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Terry... et al

I wanted to get back to you on this with a better explaintion of what
bothers me about this position you take below.

Consider the following.

When ever a board is bent the, convex side will be stretched relative to
the concave side. This is the same regardless of the over all
compression level found in the board.  Pressing downwards on this curved
board will tend to have the opposite effect... ie stretch the concave
side and compress the convex side.

Ribs glued to the concave side will fight any tendency of the panel to
stretch on its concave side. Thats what causes them to bend in the
process of crown formation in  the first place. Downbearing applied to
the convex side of the panel will tend to compress the convex side, and
stretch the concave side... but the ribs resist stretching of the
concave side.

So how does that equate to

     "As the crown lessens under increasing load, the ribs will
     pull downward a bit less, but only because they are being bent
     upwards less by the panel."

?

Seems to me the ribs will be even more stressed by down bearing...erg
they support crown against applied downbearing.

Yes ???

RicB

Farrell wrote:

> Richard wrote: "This assertion that ribs don't support the bearing
> load also is bothersome. Why
> wouldn't the ribs in a CC board support both crown and bearing... just
> because the
> panels compression forces the ribs to bend?"  When flat ribs are glued
> to a very dry flat panel, no force vectors exist in the system. As the
> panel takes on moisture and the ribs bend, the ribs will always be
> trying to pull the panel flat. Period. If that assembly has crown, the
> ribs will always try to go straight by pulling down on the panel. The
> compression forces in the panel are the only thing that has provided
> the force necessary to bend the rib into a crown. Whether or not you
> ever load that soundboard on top, the ribs will always be fighting the
> panel compression to go flat. When you do apply a load to the top of
> the soundboard, the compression on the panel will increase, but the
> ribs will do nothing other than still try to go flat. As the crown
> lessens under increasing load, the ribs will pull downward a bit less,
> but only because they are being bent upwards less by the
> panel.   Terry Farrell

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


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