STRUCTURAL DESIGN FOR ARCHITECTURE

supporting walls. Most earlier truss systems
were hybrid arrangements in which some trian-
gulation was used but which also relied on
rigid joints or continuity of elements through
joints for satisfactory structural performance
(see Fig. 6.29). Both of these expedients induce
bending into the sub-elements of the truss and
reduce their efficiency. The consequence was
that larger cross-sections had to be used than
would have been necessary had full triangula-
tion been adopted.

6.2.2 The architectural significance of the
use of timber as the horizontal elements in
large-scale loadbearing-wall masonry
structures
Timber beams were used in the roofs of the
temples of Greek antiquity - probably in the
form of simple beam systems (Fig. 6.5). It seems
unlikely that their presence affected the external
appearance of the buildings, which is the chief
legacy of Greek builders to the vocabulary of
Western architecture. The use of timber would
have allowed larger interior spaces to be created
than would have been possible had the build-
ings been constructed entirely in stone but, as
the Greeks seem not to have used any form of
built-up-beam or truss, the maximum span
(around 12 m) was determined by the largest
size of timber which was available. There is
some evidence that the Greeks were aware of
the principle of trussing but that, as in the case
of the arch, they chose not to use it.^2
The Romans, who were more adventurous
technically than the Greeks, did adopt the
principle of trussing. A basic triangulated
arrangement was described by Vitruvius and he
also provided a description of a timber-roofed
basilica with a span of 60 Roman feet (approx.
20 m). A further example from the first century
BCE is that of the basilica at Pompeii. The
conjectured reconstruction of this building by
Lange^3 (Fig. 6.6) shows a truss-and-purlin
arrangement.

2 See Mark (ed), Architectural Technology up to the Scientific
Revolution, Cambridge, MA, 1993, Chapter 5.
3 Lange, Basilica at Pompeii, Leipzig, 1885.

Fig. 6.6 Cross-section of Basilica at Pompeii, 1st century

BCE. A form of semi-trussed timber structure is used here

to span the wide central space (after Lange). 183

Fig. 6.5 Cross-section of the Parthenon, Athens, 5th

century BCE. In this conjectured reconstruction of the

Parthenon the roof structure consists of timber beam

elements (shown shaded). The spans are kept short by the

subdivision of the cross-section by walls and rows of

columns. Note the very deep beam which is required to

cross the central part of the interior, between the rows of

columns which flank the statue, and which carries the

posts which support the ridge beam (after Coulton).

Examples of later Roman timber-roofed

basilicas are that of the original St Peter's in

Rome (Fig. 6.7) and the church of St Paul's

Outside the Walls, also in Rome (Figs 6.8 and

Timber structures