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wrote:

On Mon, 06 Jun 2005 02:40:33 +0100, The Natural Philosopher
wrote:

Its actually fairly not good to use bent grown timber for structures.
The assymetry of the growth makes it move in quite drastic ways under
humid/dry cycles.

I suspect a lot of what Anna thinks is grown bent is, in fact, branches,
and the mediaeval chippies who used them went to some fairly extreme
lengths to accomodate the warping that they engender.

The thing about bends, knees and jowels is that they were natural
structures and the grain followed the curve, sawing was not needed
other than to square the timber (really only to accept joints) so no
fibers were cut across to cause problems with short grain. The warping
did not occur (other than causing fibres to split apart as cracks)
because, as was previously mentioned, wood only shrinks as it drops
from about 25% mc down to whatever equilibrium mc it ranges over as
the rh of the local environment changes. Then its length shrinkage is
negligible and the movement only becomes a problem if the baulk has
been cut such that the large difference in radial to tangential
shrinkage has an effect. In general the curves were used as boxed
heart, though I am told crucks would often be a halved tree. Even then
the changes would only be in cross section.

It's things like flooring boards that warp because to get the width
they were often through sawn. The real problems arrive with large
knots, as then any plane through them has longitudinal, tangential and
radial features. This is why the trees planted now will have less
value as structural timber.

Mm. I aree that you can make nice curved pieces out of curved wood, but
they liked that because the amount of effort to cut wood in those days
was immense. We don't have that problem today.

And curved wood - wood that has been grown that way - exhibits extremely
assymetric ring growth and is very prone to movement whether whole sawn
or cut into beams. In fact its better to cut it to separate the wide
ring areas from the narrow.

It's never a good idea to confuse what people did because there was
nothing better, with genuine engineering excellence.

You may want to adze down a piece of reaction wood to make an oak arch,
but frankly I won't bother.

AJH