Civil Engineering Reference
In-Depth Information
The best quality conifers grow in
lean soil. Heartwood timber shrinks
less than other timber and is more
durable, making it well-suited to the
construction of doors, windows or
external details. The demands of qual-
ity are lower for the production of cel-
lulose, internal panelling etc.
In order to be economical with the
use of heartwood timber, it used to be
worked while the tree was still stand-
ing. This process, called self-impreg-
nation, is known in most cultures from
the British Isles to Japan. The most
common method is to chop the top of
the tree and remove a few strips of
bark from the bottom to the top. Three
or four of the highest branches are left
to 'lift' the resin. After two to seven
years the whole trunk is filled with
resin. There is little growth during
these years, but it produces a high tim-
ber quality. The method is especially
effective on pine, which contains three
times as much resin as spruce. Economically speaking, it is quite possible that the
reduced growth is balanced by the higher strength and the reduced amount of
impregnation needed, both of which are valuable assets.
Before timber for felling was categorized, people tried to find suitable features
in timber for use as diagonal ties and bracing in post and lintel construction or
framework construction. Crooked trees and round growths on the roots of trees
proved particularly interesting. The tree could be worked with while it was still
growing to achieve certain effects. English framework is, in many cases, based on
bent timber. A 'bulge' occurs when a coniferous tree that was bent straightens
itself up, the bulge occurs on the underside of the bend. Timber at this point is
close knit and strong and has been used for exposed items such as thresholds.
There is no great value in hand picking timber with today's production tech-
niques. Even the quality of timber is given little attention apart from the desire
for straight trunks with few knots, and concern focuses upon volume. However,
there are still possibilities for small, more specialized industries in this field.
In Sweden, research is now being undertaken to evaluate the possibilities of
differentiating qualities of timber in modern forestry, in order to return to a situ-
ation where the best quality timber is used in the most exposed situations.
Figure 10.2: A traditional method of cultivating special
qualities in timber.
