STRUCTURAL DESIGN FOR ARCHITECTURE

the interiors of the cells and the only physical
change which this causes is a reduction in
density. At a moisture content of around 30%
(the exact figure depends on the species), all
of the moisture in the cell interiors will have
been lost and only the water in the cell walls
will remain. This condition is known as the
'fibre saturation point' of the timber. Further
reduction in the moisture content causes water
to be lost from the cell walls and this gives rise
to shrinkage as well as to loss of weight. The
shrinkage is greater in the across-grain direc-
tions than in the along-grain direction and, of
the across-grain directions, it is greater
tangentially than radially (Fig. 6.16). The
shrinkage is reversible; if the equilibrium
moisture content of a timber becomes higher,
due to a change in environmental conditions,
then the timber will absorb water from the
atmosphere and a corresponding expansion
will occur. The phenomenon is known as
'moisture movement' and the extent to which
it occurs depends on the species.
The greatest amount of 'movement' takes
place during the initial drying process which
must be regulated carefully because differen-
tial shrinkage, due to variations in the rates at
which moisture is lost from different parts,
could cause damage in the form of cracks and
warping. The controlled drying out of timber
after felling is called 'seasoning'. It involves the
adoption of measures to restrict the rate at
which moisture is lost from the timber and to
restrain the latter physically so as to prevent
excessive deformation from occurring. Various
techniques for seasoning have been developed
but most timber which is used in building is
either 'air-seasoned' or 'kiln-seasoned'. In air-
seasoning the timber is stacked in well-venti-
lated sheds and allowed to dry out under
atmospheric conditions; in kiln-seasoning the
timber is dried more quickly under conditions
of close environmental control. In both cases
the stacking arrangement which is adopted is
designed both to allow air to circulate freely
between timbers, so as to promote drying, and
to restrain the timber to prevent it from
warping. A certain amount of deterioration in
the quality of the timber, due to the formation

of cracks and to warping (Fig. 6.17), is

inevitable during seasoning, however, because

differences in the rates at which moisture is

lost from different parts of each timber cannot 193

Timber structures

Fig. 6.16 The shrinkage which occurs to timber during

the initial drying out is greater in the tangential than in the

radial direction.

Fig. 6.17 Examples of seasoning defects.

Tangential movement Radial movement (about

half the tangential movement)

Twist in board which is not cut

parallel to the heart of the tree

Twist

Cupping

of plain sawn

board away from heart

Spring

In same direction as curvature of grain

Bow