Civil Engineering Reference
In-Depth Information
The wedge is more sensitive with wood than the axe, and the axe is more sen-
sitive than the saw. By using a wedge, the cells are kept whole when the wood is
split; the saw cuts straight through the cell walls. This is critical to the timber's
absorption of water, which governs the risk of attack by mould or insects. In
spruce, which when whole has an impermeable membrane between the pores,
this is particularly important. A carefully-divided spruce can be as durable as
pine heartwood.
Timber from deciduous trees often has high inner tensions. To avoid this devel-
oping into twisting in the sawn timber, it is important to keep to smaller dimen-
sions, preferably not above 50 mm.
Drying
Some researchers say that the drying routines for freshly sawn timber are much
more important for its durability than the time of felling. Spring- and summer-
felled timber should be dried as soon as possible (Raknes, 1987).
Timber shrinks 15 times more in its breadth than in its length when being
dried, so when a newly-felled log dries it forms radial splits. By putting a wedge
into one of these splits, further splitting can be controlled. In the same way, sawn
timber has a tendency to bend outwards on the outer side when wet, and out-
wards on the inner side when it is dried. This is why the way in which a log has
been sawn determines the degree of movement in a sawn plank.
In order to use newly sawn timber, 70-90 per cent of the original moisture in
the trunk must be dried out, depending upon the end use. The sawn timber is
stacked horizontally with plenty of air movement around it, and is dried under
pressure. The stack can be placed outdoors or in special drying rooms. The out-
door method is more reliable for drying winter-felled trees during spring, as arti-
ficial drying produces some problems. Certain types of mould tolerate the tem-
peratures used in this technique, and develop quickly on the surface of the wood
during drying, emitting spores which can cause allergies. It has also been noted
that the easily soluble sugars which usually evaporate during the slow drying
process are still around in artificially dried timber, and become the perfect breed-
ing ground for mould. It is also possible that the natural resins in the timber do
not harden properly. This could be, for example, the reason why there often are
considerable emissions of natural formaldehyde in buildings made purely of
timber. Formaldehyde is an unwelcome substance to have in an indoor climate,
and can cause irritation in the ear, nose and throat, allergies, etc. Another reason
for drying timber outside is the lower energy consumption, which for an ordi-
nary load of timber rises by 300 per cent when dried artificially.
Drying outside is best carried out in the spring. The number of months
required for drying can be roughly estimated by multiplying the thickness of the
timber in centimetres with 3.2 for spruce and 4.5 for pine. Normal planks take
about three months, deciduous trees take longer.
