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Steve,

The point that stands out for me in this discussion re: the neutral axis is that the two laminations are bent BEFORE they are bonded together. So I'd think DavidO's comment is right. If you look at the sandwich, you have tension-compression-adhesive-tension-compression. This may explain some of why these laminated sides are stiffer (of course, as I write this, I'm thinking that the whole reason of doing the heat bending is to minimize/eliminate the tension/compression at the outer fibers, so maybe this argument falls apart). So in the end, maybe the fact that the two laminations are different, with variable grain directions and stiffnesses is the main reason for the increase in stiffness. I've gotta believe that is real, given the number of knowledgeable people that have testified to that fact.

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

[QUOTE=SteveS] [QUOTE=davidO] Todd,
I am not an engineer, but I am a carpenter (I think that's a little like being
a TV doctor ). A little wood engineering analysis would go something
like this:

Typical rim construction would put the inside wood in compression and
the outside in tension due to bending it.

In Kens experiments with weights on top of the arches he put the outside
(top of arch) in compression and the inside (bottom of arch) in tension.

What I a getting at here is that each surface experiences a different type
of opposing force either tension or compression............
[/QUOTE]
David,
That doesn't really explain it because the outer fibers are in tension/
compression just the same whether or not they are solid or plywood.
What matters most is the distance those fibers are from the central axis.
For some reason, plywood of the same thickness is stronger than solid
wood.
The difference is the adhesive, so it must have an affect that accounts for
this. I'm thinking that it fills in a lot of the small voids and pores in wood
and that makes it stronger.
I don't know for sure, but that's my story, and I'm sticking to it!     [/
QUOTE]

No Steve, I think you almost got it but laminated wood is only stronger if
there is a curve laminated into the layup.

Take one piece of wood and saw it into three slices and glue them back
togeher flat. Now take an identical piece of wood and thickness it so it is
identical to the 3 sliced wood but minus the kerf width. These would be
similar in strength (modulus of elestacity). I did not do this experiment,
but from what I have read, it is true.

Keep in mind that changing orientation of the laminate layers changes
things. Interestingly, if the middle layer is 90 degrees to the face layers,
the bending strength along the grain is similar to a similar layup with the
middle layer running parellel. This is because the spacing of the two
"skins" largely determines the strength in the long axis.

So your three ply sides will be the best of both worlds. Strength from
cracking along the grain while retaining the strength from the both how
far the skins are placed apart and that bent lamination strength that we
can't define.