reinforced concrete. This restricts the size of
span which can be achieved and the number of
storeys which can be constructed in an all-
timber structure.
Jointing difficulty
Although the material is tractable it is difficult
to make joints in timber which have a good
structural performance. Joints which are made
with mechanical fasteners, such as nails,
screws and bolts, suffer from the problems of
stress concentration. They also tend to work
loose due to the phenomenon of moisture
movement (see Section 6.4.3). Although very
significant advances have been made in recent
decades in the development of new types of
mechanical fastener, these problems have not
been entirely overcome.
The development, in the twentieth century,
of reliable glues has made possible the virtual
elimination of both stress concentration and
the development of looseness at joints. To
achieve a satisfactory connection with glue,
however, it is necessary that the contact
surfaces should be very carefully prepared and
also that the glue should be properly cured.^4
Both of these are difficult to achieve on site
and this requires that gluing be carried out
under factory conditions only. The size of
component which can be assembled with
glued joints is therefore restricted by consider-
ations of transport.
Component size
The sizes of individual timber planks or boards
are obviously determined by the size of avail-
able trees. A consequence of the depletion of
the world's resources of timber is that lengths
greater than around 6 m and cross-sectional
dimensions in excess of 300 mm by 100 mm
are difficult to obtain in most of the species
which are used for structural purposes. This,
together with the jointing problems described
above and the low relative strength, restricts
the overall size of the structures which can be
constructed in timber in the present day.
4 See Gordon, The New Science of Strong Materials,
Harmondsworth, 1968.
Susceptibility to rot and decay
Timber components are susceptible to various
kinds of infestation, notably by fungi and
insects. The likelihood of fungal attack is
minimised if the timber is kept dry and this
can be achieved through suitable detailing of
the structure. Insect attack is inhibited by
application of insecticides but the effective-
ness of these is limited.Variability
Timber, being a natural material, exhibits
considerable variability in its properties.
Different species produce timbers with quite
different properties and even within a single
species the variability in properties such as
strength, elasticity, durability and appearance
can be high. The problem has been overcome
by the adoption of a grading system for
commercial timber in which individual planks
and boards are inspected and placed into
categories according to their particular charac-
teristics. Timber which is used in construction
is specified by grade and this ensures that its
properties can be relied upon to be within
known limits.6.3.4 Conclusion
To sum up, timber is a material which offers
the designers of buildings a combination of
properties which allow the creation of light-
weight structures which are simple to
construct. Its relatively low strength, the small
sizes of the basic components and the difficul-
ties associated with achieving good structural
joints tend to limit the size of structure which
is possible, however, and the majority of
timber structures are small in scale with short
spans and a small number of storeys.
Currently, the most common application of
timber in architecture is in domestic building
where it is used as a primary structural mater-
ial, either to form the entire structure for a
building, as in timber wallframe construction
(also called timber-frame construction) (Fig.
6.14), or to make the horizontal elements in
loadbearing-masonry structures (Fig. 6.2).
The high ratio of strength to weight which is
found in timber also makes it particularly 191Timber structures