RC vs CC again

[Ron Nossaman]

>I see where you are going with this but here is a little problem for you 
>to solve.

But I wasn't going anywhere with it. That was it.


>When string bearing is applied to a soundboard a portion of the crown is 
>lost supporting the load. This is usually about 2.5 to 3.5 mm. It would be 
>very simple to figure out how much load the panel supports. Take a 
>measurement of the cross grain distance along a rib before you load the 
>board. Then apply the down bearing pressure and remeasure the panel. The 
>difference is how much the panel is compressed. You then need to figure 
>this as a proportion of the total length. If you do this you will see that 
>the panel is supporting very little of the bearing even on a CC board. If 
>you look at how changes in moisture content compress the panel you will 
>see that this is a lot more of a stress on the panel that down bearing.

I hardly know where to start, John, but I didn't particularly want to go 
back to the very beginning again. In a CC board: --- First, I don't have 
equipment capable of measuring the difference across the curve of a crown 
without load, and depressed. It's only going to be a few thousandths of an 
inch. Second, this difference measurement can't tell you anything about the 
compression level in the panel under load unless you know the compression 
level in the unloaded panel. The unloaded panel is already under 
considerable compression because it has already expanded enough to bend a 
flat rib into a crown. Third, seasonal humidity changes are indeed 
extremely abusive to the panel, but only because it is constrained under 
already damaging compression levels by both the downbearing load and the 
resistance of the rib to bending. An unconstrained panel will happily 
expand and contract with humidity changes indefinitely without significant 
damage. I don't know exactly what the limits are, but without constraint, 
there is no severe compression set and subsequent permanent deformation.


>You can use a similar method to find how much the ribs are bearing; just 
>look at how much they bend under the load. I am sure you will find that 
>the ribs are holding much more than the panel. You will be perplex to find 
>that both together do not equal the total load of bearing. So what else is 
>supporting the board?
>
>I think you will find that how the crown is created will not make a lot of 
>difference to how much support the panel gives to the down bearing.
>
>John Hartman RPT

In a CC board, panel compression is not only supporting all of the bearing 
load, but bending the flat ribs into a crown as well. The ribs in a CC 
board are supporting none of the bearing load. They are, in fact, adding to 
the load on the panel. How much down load do you have to apply to the top 
of a flat rib to get it to bend up in a crown? If you try it, you will find 
that the answer is "none", you have to apply lift from beneath to bend the 
rib up into a crown, just like you have to apply lift from beneath to 
support string bearing. Ribs don't support bearing at all in a CC board 
until the crown is concave.

Ron N