Structural design for achitectureAll structure types have a potential
maximum span and the less efficient the
structure, the lower is this practical
maximum span. An indication of this is given
in Table 2.1 in which it will be observed that
the maximum span given for each type of
structure is related to its potential efficiency.
Thus, elements with simple, solid rectangular
cross-sections, such as sawn timber joists or
rectangular cross-section reinforced concrete
slabs, have relatively low maximum spans.
Simple elements with 'improvements', such
as l-section steel beams or triangulated
trusses of timber or steel, have higher
maximum spans. The highest spans are
achieved by highly efficient vaulted shells or
cable networks.
The effect of the variation in the maximum
span potential of different types of element is
that the choice of element types which is avail-
able to the structural designer diminishes as
the span increases. If the span to be achieved
is small (say 5 m) virtually all structure types
are available. At this scale the designer could
therefore choose to use any type of structure
from simple, solid beams or slabs to sophisti-
cated forms such as the arch or the vault. In
the context of contemporary architecture, it
would probably be regarded as technically
inappropriate to use a complex form for a
short span, unless some special requirement
for high efficiency existed, because much
simpler post-and-beam forms would perform
adequately. It would nevertheless be a choice
which the architect could make. As the span
increases the number of different types of
structure which would be viable decreases
until, at the very long span (say 200 m), only
the most efficient form-active types, such as
steel cable networks or thin concrete shells,
are feasible. In summary, from the point of
view of the designer, the choice of structure
type is large if the span is small and becomes
progressively more limited as the span
increases.
The most basic post-and-beam types of
structure (loadbearing-wall arrangements in
masonry or timber with simple horizontal
elements such as timber beams or reinforced
concrete slabs) are suitable for short-span
structures in the 5 m to 10 m range. The span
range can be extended by the use of more
efficient types of horizontal structure such as
triangulated trusses in either timber or steel.
The use of walls with 'improved' cross-sections
(fin wall or diaphragm wall) allows the very
basic structural system to be used for larger
enclosures with high external walls, such as
sports halls, where spans of up to 30 m have
been achieved.
The post-and-beam frame (in either
reinforced concrete or steel) is a more sophis-
ticated and therefore more flexible system than
the loadbearing wall. In the multi-storey
version the span range is slightly more exten-
sive than that for loadbearing-wall structures
(5 m to 20 m). The most basic types of element
are used at the low end of the range (solid
reinforced concrete slabs, rolled-steel sections)
and at the upper end more efficient types such
as coffered, reinforced concrete slabs and
hollow-web steel beams. Spans greater than
20 m are unusual in multi-storey buildings but
where these occur, efficient types of structural
elements must be specified (e.g. triangulated
girders used to achieve a span of around 25 m
at the Centre Pompidou in Paris).
In single-storey structures the change from
the most basic forms to more efficient struc-
ture types (e.g. space frame horizontal struc-
tures or semi-form-active structures) normally
occurs when the span is in the range 20 m to
30 m, with spans greater than 30 m usually
requiring the use of a semi-form-active struc-
ture such as a portal framework. As with short-
span structures, the type of element which is
used can vary and the tendency is always for
the level of efficiency (and therefore of
complexity) to increase as the span increases.
Thus, short-span portal frameworks (20 m)
would normally be accomplished with rolled-
steel sections such as the universal beam
while a triangulated-truss longitudinal profile
might be specified for a longer-span version
(say 50 m).
The transition from the semi-form-active to
the highly efficient fully form-active structure
occurs in the range 40 m to 60 m. 37