structure are, however, more likely to influence
the choice of structural material than the
strength required.
Skeleton-frame structures can be
constructed in steel, reinforced concrete or
timber. The use of timber is rare for multi-
storey buildings, however, and the timber
skeleton frame is most usually associated with
single-storey enclosures in which the timber
structure is exposed to view. Often the struc-
tural elements are of spectacular appearance
(large triangulated trusses) and the structure
therefore contributes significantly to the
appearance of the building. The relationship
between structure and architecture must
normally be of the 'structure accepted' or 'true
structural high-tech' type rather than the
'structure symbolised' type, when timber is
used for skeleton frames. This is because there
is normally insufficient reserve of strength to
allow the structural performance to be com-
promised for purely visual effect.
Reinforced concrete is rarely used for single-
storey buildings because the high self-weight
of the structural elements renders them
unsuitable for situations in which the imposed
loads are small. For the same reason,
reinforced concrete is rarely used for roof
structures in multi-storey buildings except for
flat roofs, in the form of terraces or roof
gardens, to which access is permitted and
which are therefore required to carry greater
than normal roof loadings. Steel is suitable for
both multi-storey and single-storey buildings.
The normal choices available to the designer
of a skeleton-frame structure are therefore
between steel and timber for single-storey
frames and between steel and reinforced
concrete for multi-storey frames. The relative
advantages and disadvantages of the materi-
als, from a structural point of view, are
outlined in Chapters 3, 4 and 6.
The normal reasons for the selection of steel
are its high strength, its appearance (especially
in the context of the 'structure symbolised'
approach) or the speed of construction which
it allows. The first of these is obviously impor-
tant if long spans are involved or if there is a
need to produce a structure which is of low
volume and which has slender elements.
Appearance is frequently also the principal
reason for the selection of timber for a skele-
ton framework although the lightness of the
elements can also be a factor. Timber may also
be selected due to its durability, for example
where hostile internal environments, such as
occur in swimming pool buildings, would
cause corrosion of a steel structure.
Reinforced concrete can be selected for a
number of reasons. Often, it will provide the
lowest cost structure due to both the low
inherent cost of the material and the elimina-
tion of the need for additional finishing materi-
als or fire proofing. The good durability of
concrete is one of its considerable advantages.
Concrete may also be selected due to the
opportunities which it provides in relation to
form. The mouldability of the material and the
ease with which it allows structural continuity
to be achieved favour its use in situations in
which complex structural geometries are to be
produced, especially if these involve curvilinear
forms.
All materials impose constraints as well as
provide opportunities. Thus, unless the 'struc-
ture ignored' method is being practised, the
limitations of the material must normally be
recognised once it has been chosen. The
nature of these constraints is something which
is taken into account during the selection of
the material to ensure that it is compatible
with the architectural intentions.2.5 Determination of the form of the
structure
The final form and general arrangement of a
structure is normally an adaptation of one of
the basic forms described in Chapters 3, 4, 5
and 6. This is now discussed in relation to
reinforced concrete structures to illustrate how
the process of adaptation is carried out to
produce buildings in which structural and
architectural requirements are reconciled. The
methodology described is applicable to all
structural materials. 41Structural design for achitecture