evaluating sdbd. crown & bridge downbearings in a new piano

[Richard Brekne]

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Well...lets see here...Still scratching my head a bit.. but no doubt a bit brighter then I was
a few days ago. After initially having problems with this idea of a convex side of a curved
board being in compression, and after Del was so kind to clear this up with a language use
appropriate to one with my understanding of these things, I am left wondering if Frank would be
equally kind to attempt to do the same after reading the example that follows. I have never
tried replaceing soundboards (with good reason.. grin) but am moving in that direction in my
buisness and will sooner or later be confronted with my first attempt. So I need as much
clarity as is possible into these basics, or further "education" will be sidetracked, hindered,
fogged, or what have you of such negatives.

My example will be very simplistic, as I need a couple very simplistics basics cleared up. I
will use just one humidity figure, no cauls, presses or any such things. Just two pieces of
wood, a panel and a rib, and an oven, and a given room humidity of 50% relative.  This ought to
be real easy for you to explain then. The example will also be a bit unrealistic in terms of
length numbers involved, but that doesnt matter as it is a very basic principal that I am
after.

Lets say we have a panel exactly 1 meter across grain, and a rib that is 998 cm long in a room
with 50% relative humidity. The panel is put into the oven and dried to the point that it is
also 998 mm long. Now if we take the panel out and simply let it re-adjust to the room humidity
it will "grow" back to 1 meter in length. The top and the bottom of the panel will both shrink
the same, and grow back the same amounts... ok so far ??  But if we quickly attach the ribs
while the panel is at 998 and then allow it to re-grow to what ever length the room humidity
and this constraint from the rib allows for, its length will be less the 1 meter. (This
following what I think I got from Dels description.) Ok.. assuming this is a correct picture so
far, stretching a measuring line across the top of this  ribbed panel after its re-adjusted to
room humidity shows that it measures less then 1 meter.

If, (Frank) you accept that this is correct so far,  I would appreciate it very much to know
the reasoning behind why this top half of the panel is not (if it is not) to be considered in
compression. As initially stated, please follow Dels kind "laymans language" approach to
demonstrating the reasoning.

Richard Brekne
I.C.P.T.G. N.P.T.F.
Bergen, Norway

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