Structural Design for Architecture
the least efficient type of structural element.^1
The maximum span which is possible is deter-
mined by the largest size of cross-section
which is available. In the present day the
maximum span possible with timber floor
joists is around 6 m unless larger than normal
sizes of cross-section, which are now difficult
to obtain and therefore expensive, are used. In
Greek and Roman antiquity, when larger sizes
were apparently readily available, beam spans
of up to 12 m were used.
The rafter arrangement (Fig. 6.42) is more
efficient than that of the beam because the
elements carry a combination of axial internal
force and bending. This, together with the fact
that the individual elements do not cross the
entire span, allows a greater span to be
achieved but has the disadvantage of imposing
horizontal thrust at the points of support. The
rafter system therefore carries with it the same
problem as the masonry vault, which is the
generation of horizontal forces at the tops of
supporting walls. The magnitudes of these can
be reduced if a tie is introduced (Fig. 6.43) and
if the tie is placed at the level of the supports
the lateral thrust is eliminated. The tie need
not be a single piece of timber but can be
jointed. It can also be supported from the apex
of the rafters by a second, vertical tie (Fig. 6.43).
The tied rafter with the tie at support level,
and secondary vertical tie, is in fact a simple
version of a fully triangulated truss or trussed
rafter (Figs 6.27 and 6.28), which is more
efficient than any of the various rafter systems
because the majority of the constituent sub-
elements carry axial internal forces rather than
bending.
The fully triangulated truss is a type of
element which is frequently used for long spans
in the present day. The advantages of triangula-
tion seem not to have been fully understood
until quite late in the history of Western archi-
tecture, however, and it was not until the
nineteenth century that widespread use was
made of fully triangulated, highly efficient truss
systems, which exert no horizontal loads on1 See Macdonald, Structure and Architecture, Chapter 4.Fig. 6.4 A selection of timber element types.
(a) Sawn-timber joist - the simplest type of element which
is produced in a wide range of sizes.
(b) Plyweb beam. The web of this built-up-beam is of
plywood and the flange elements of sawn timber.
Rectangular box sections are another common type of
plyweb beam.
(c) Laminated timber beam. Large cross-sections can be
built up by the laminating process which allows curved
arch and portal frame elements to be produced as well as
182 straight or tapered beams.(c)(b)(a)