Structural Design for Architecture
are the practice of stress grading and the
evolution of greatly improved jointing techniques.
The stress grading system, which is
described briefly in Section 6.5, overcomes one
of the fundamental problems of timber which
is the variability of the material. It allows the
strength of timber elements to be specified
within fairly precise limits. This in turn allows
factors of safety which are used in the design
calculations to be low. The material is there-
fore used efficiently.
As was seen in Section 6.2.3, traditional
carpentry joints involve the cutting away of a
considerable amount of material - sometimes
as much as half the cross-section of a struc-
tural element. The joint was therefore the
weakest part of the element and the resulting
efficiency was very low. In modern timber
joints the quantity of material which is
removed is minimised. In addition, fastening
techniques, based on improved glues and
mechanical components, have been developed
which minimise the concentration of stress
which occurs at joints (see Section 6.6.6).
One of the main consequences of the avail-
ability of these improved jointing techniques
has been the development of a wide variety of
built-up elements such as laminated timber
beams, portal frames and arches, ply-web
beams and trussed rafters of various configur-
ations. The availability of efficient types of
joint has also allowed timber to compete effec-
tively with steel for triangulated plane and
space frameworks for roof structures.6.3 The material, its properties and
characteristics
6.3.1 Introduction
The technical advantages and disadvantages of
timber in relation to other structural materials
are reviewed in this section.6.3.2 Advantages
Strength
Timber possesses tensile, compressive and
flexural strength and is therefore suitable for
all types of structural element.Lightness
Timber is a lightweight material with a high
ratio of strength to weight. It therefore
produces lightweight structures with com-
ponents which can be easily transported and
handled on site.Tractability
Timber can be easily cut and shaped with
simple tools and the erection of timber struc-
tures is therefore straightforward. Other
components can be easily attached to timber
with simple fasteners, such as nails or screws,
and this simplifies the detailing of timber
buildings.Performance in fire
Timber is a combustible material but the rate
at which it is consumed in a fire is relatively
low and it does not lose its structural proper-
ties when it is subjected to high temperatures.
It therefore continues to function in a fire until
the cross-section of elements become reduced
to the point at which excessive stress occurs
(other materials, particularly metals, lose their
strength at relatively low temperatures). The
performance of timber structures in fire is
therefore good.Durability
The constituents of timber are relatively stable
chemically and the material does not suffer
chemical degradation in environmental condi-
tions (such as high humidity levels) which
might prove detrimental to metals. It is,
however, susceptible to insect infestation and
fungal attack (see Section 6.4.4.4).
Appearance
Timber is a material which has a pleasing
appearance which matures rather than deteri-
orates with age. It can therefore serve in the
combined role of a structural material and a
finishing material.6.3.3 Disadvantages
Lack of strength
Although the strength-to-weight ratio of timber
is high, its actual strength is low compared to
190 other structural materials such as steel and