Civil Engineering Reference
In-Depth Information
Timber damaged by air pollution is considered to be normal quality, as long as
it is not mouldy in any way.
Felling
Both deciduous and coniferous trees intended for construction purposes should
be felled in winter when the quantity of sap is at its lowest and the state of
swelling, acidity, etc. are at their most favourable. Timber felled during spring is
more readily susceptible to mould. Trees to be used in damp earth or in water
should, however, be felled during the sap-period. Another advantage of the win-
ter felling of ordinary construction timber is that the sawn timber dries out more
slowly and is therefore less likely to split. Some felling traditions were related to
the phases of the moon. Coniferous trees were felled at full moon, because the
resins were well drawn out of the roots and into the trunk.
It has been assumed that the large amount of mould damage to newer Swedish
timber buildings, especially in windows and the outside panelling, relates to the
fact that the timber was felled during the summer - a usual occurence in Sweden
during the 1960s (Thörnquist, 1990).
Storage
Although newly-felled timber should be treated as soon as possible, it is usually
some time before this can be done. The timber should be stored in water, where
there is hardly any oxygen. This reduces the risk of mould and insect damage. If
it is stored in salt water, there is a risk of attack by marine borer.
Timber stored in water during the summer often becomes porous through the
action of anaerobic bacteria which eat the contents of the cells and pore mem-
branes. This can dramatically increase its resistance to rotting later, because the
timber can easily cope with damp.
Splitting
The trunk is transported to the site where it is to be milled. Splitting should take
place while it is still very damp. For log construction and certain other forms of
building the log is used as it is, occasionally with its sides trimmed slightly flat
with an axe. Pine has a longer life span if it is split in this way along two sides,
because the hardness and amount of resin increases towards the centre of the
trunk. Spruce should not be chopped along its sides, because the outside wood
is stronger and heavier than the wood in the middle of the tree.
The oldest way of splitting a trunk is by cleaving through the core of the tree.
The halves can be used as logs almost as they are, or they can be trimmed to a
rectangular cross section. They can be further cleft along the radii, giving trian-
gular profiled planks.
With the invention of the vertically-adjustable saw during the sixteenth cen-
tury, splitting timber by saw became the dominant technique. The method was
