soundboardinstal again

[Ron Nossaman]

>Ron,
>
>Me think (may be wrongly) that on the opposite, the fact that a so
>small dimension is involved show us the importance of a better joint
>(last news from the day !)

It doesn't work that way.


>The inserting under tension of the soundboard seem the only way to
>have some tension in the panel, in the rib direction (across grain)
>the modulus of elasticity is said to be 3 Kg cm2 vs. 100 Kg cm2 in
>direction of grain.
>So if we wish to have some tension in the panel that looks like a
>convenient method, more than compression crowning.

Again, it doesn't work that way. It's not tension you're putting in the 
panel, it's compression. Modulus of elasticity and compression resistance 
are not the same thing, and MOE is meaningless in this context. Soundboard 
crown is still not an end supported arch, so buttressing the edges will 
have no significant effect on the crown. In the example rib I used, of 36" 
(915mm) and 60' (22M) radius, the outward thrust of the "arch" will be 40 
times the load it carries. This means that at 580psi fiber stress 
proportional limit of cross grain compression of Sitka spruce, that rib can 
only hold up to 14.5 pounds load before the wood is crushed. It gets much 
worse with shorter ribs. Once again, the rim has nearly nothing whatsoever 
to do with the maintenance of crown, nor the retention of compression in 
the panel. The ribs do that all by themselves.

>Stephen have forced back some boards with shims on the straight side
>with good results he say, when we see the dimensions involved, that
>looks like a possibility.

Yes, I have also heard (or heard of) all sorts of folks saying all sorts of 
things. The physical fact is that this doesn't work. The physical 
structural limits of the materials just don't make it possible.


>But I seem to understand that in the actual conception in the USA the
>soundboard assembly is viewed as an auto supported device is not it ?

Yes, but it's not a view. It's a fact, as is easily demonstrated that a 
soundboard assembly has crown, and will support a full load without being 
anywhere near a rim.

Ron N