Structural Design for Architecture
suitable for structures of moderately large
span, especially if the level of imposed load is
low. Timber is therefore used for the construc-
tion of large, single-storey enclosures and for
medium- to long-span roof structures.Trees may be classified into two types,
narrow-leaved trees and broad-leaved trees.
Narrow-leaved trees are coniferous and mostly
evergreen - an exception is larch; broad-leaved
trees are mainly deciduous - an exception to
this is holly. Many differences exist between
the physiologies and anatomical structures of
these two types of tree. The most significant of
them is that the narrow-leaved species tend to
grow much faster and to produce timbers
which are less dense and less strong than the
broad-leaved species.
Commercial timbers are subdivided into the
two broad categories of softwoods and
hardwoods, and these correspond approxi-
mately to the botanical classifications. The
softwoods are derived from the narrow-leaved
species and the hardwoods from the broad-
leaved species. The terms are misleading,
however, because in some cases they do not
correspond to the physical properties of the
timber. Balsa wood, for example, which is one
of the softest of timbers, is classified as a
hardwood because it is derived from a broad-
leaved tree. In general, however, the coniferous
species from which the softwoods are derived
are fast-growing and produce timbers with a
high proportion of spring wood. They are
therefore less dense and less rigid than the
hardwood timbers which are derived from
slow-growing, broad-leaved varieties.6.4.3 Moisture content and moisture
movement
The moisture content of a specimen of timber
is defined as the ratio of the weight of water
which it contains to its dry weight. It is always
expressed as a percentage. In the living tree
the moisture content is around 150%. After the
tree is cut down, most of the water evaporates
and the timber eventually reaches an 'equilib-
rium moisture content' whose exact value
depends on the species and on the conditions
of the environment in which it is placed; it
depends principally on temperature and
relative humidity and is usually in the region of
15 to 20%.
When timber dries out following the cutting
down of a tree, moisture is lost initially from6.4 Properties of timber
6.4.1 Introduction
In this short section only the properties of
timber which are relevant to its use as a struc-
tural material are reviewed. These are the
mechanical properties (i.e. strength and
elasticity), fire resistance and durability. Two
introductory sections, in which the internal
structure of timber and the phenomenon of
moisture movement are described briefly, are
also included. More detailed accounts of all of
these topics will be found in Desch, H. E.,
Timber, 6th ed. (revised Dinwoodie), London,
1981.6.4.2 Internal structure
The parts of the tree which are used as the
source of structural timber are the trunk and
sometimes the major branches. These, in
common with other parts of a tree, are
composed of cells; in the trunk these are long
and thin and are aligned parallel to its direc-
tion. The mechanical strength of timber is
derived mainly from the fibrous tissue which
forms the walls of the cells together with
deposits within the cells.
The most active part of a living tree trunk is
the layer which lies immediately under the
bark. This is called the cambium layer and it is
here that growth by cell division occurs. In
seasonal climates the rate of growth varies
throughout the year. It takes place mostly in
the spring and summer and is more rapid in
spring than in summer. Spring wood is softer
than summer wood, and this difference
accounts for the annual rings which are a
prominent feature of the cross-section of a tree
trunk and which are responsible for the
phenomenon of 'grain'. The denser, harder
parts of the grain are formed by summer wood
192 and the softer parts by spring wood.