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 Lange3 (Fig. 6.6) shows a truss-and-purlin

arrangement.

2 See Mark ( e d ) , 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

Fig. 6.7 St Peter's Basilica,

Rome, 4th century CE. A fully triangulated timber structure, which exerts no lateral force on the walls, spans the central part of the building. The use of this type of timber structure has allowed the creation of a large interior space without the need for the elaborately buttressed supporting walls which would have been required had a masonry vault been used (after Mark).

Fig. 6.8 Left. St Paul's Outside the Walls, Rome, 4th

century CE. This perspective drawing illustrates well the advantage of using a triangulated timber roof structure rather than a vault. The thinness of the walls is well shown (after Piranesi).

Fig. 6.9 Below. St Paul's Outside the Walls, Rome, 4th

century CE. Cross-section (after Rondelet).

6.9). Both of these buildings are of the fourth century CE. They each consisted of a high central nave flanked by lower aisles. The naves, which were each approximately 24 m wide, were roofed in timber by truss-and-purlin systems and the flanking aisles by lean-to structures, also of timber. The use of tied trusses over the naves eliminated side thrust at wallhead level and enabled the builders to adopt very slender walls. The buildings were of light and delicate appearance, as is well illus- trated by the seventeenth-century drawing of St Paul's Outside the Walls by Piranesi (Fig. 6.8).

The importance of the timber-roofed basilica was that it was a building with a relatively large interior which did not have the massive buttressing walls which were required to support a masonry vault (the only other way of achieving a large span at the time). It was an early example of a building type which would make a very significant contribution to the development of Western architecture - the large building with large interior spaces and thin walls. It was the development of the technology of the timber truss, from the Roman period onward, which made this possi- ble.

Examples of this type of building can be found in all subsequent periods of western architecture. The palaces and large houses of the Italian Renaissance, the country houses and public buildings of the classical period in Britain, Northern Europe and America (Fig. 6.10) and churches from all subsequent periods have structural armatures of this type. In most of these buildings the timber struc- tures which spanned the large interior spaces were entirely hidden from view and made no obvious contribution to the architecture. The forms of the buildings would have been impos- sible to achieve, however, without the technol- ogy of the timber truss.

In the nineteenth century timber gave way to iron and then to steel as the principal material from which large trusses for long-span roofs were constructed. The tradition of using large timber trusses did not die out, however. Twentieth-century examples have frequently

Fig. 6.10 Banqueting House, London, 1619-22. Inigo

Jones, architect. The roof trusses which span the width of the Banqueting House in London (17m approx.) are typical of the structural arrangements which were used to create large interior spaces in the buildings of the European Renaissance.

been exposed in the interior of buildings and used as part of the architectural language.

6.2.3 Timber loadbearing-wall structures