Timber trusses in Italy: the progressive prevailing of open-joint over closed-joint trusses

5th

International Congress on Construction History

TIMBER TRUSSES IN ITALY: THE PROGRESSIVE PREVAILING OF

OPEN-JOINT OVER CLOSED-JOINT TRUSSES

Emanuele Zamperini1

Keywords

Italian timber trusses, Carpentry joints, Iron reinforcements for trusses, Structural analysis

and the development of structural forms

Abstract

The time and the place in which trusses were invented is uncertain; however, it is certain that

they were already in use during Roman Empire. The oldest extant material evidences date back

to the 6th

century AD and witness the simultaneous presence in Italy of two constructive types

that coexisted for centuries. The two types of truss are distinguished from each other based on

the different relationship between posts and tie-beams: in closed-joint trusses, posts are

connected to tie-beams with carpentry joints (e.g. tenon-mortise); in open-joint trusses, the posts

are physically detached from the tie-beam but possibly linked to it with a metallic strap.

The paper will outline the idea that, under the effects of vertical static loads, there is no

significant difference in structural behaviour between the closed- and the open-joint trusses.

Indeed, under ordinary loads, in Italian-style trusses the deflection of the tie-beam is greater than

the lowering of the bottom of the post, and carpenters were aware of this since the post/tie-beam

joint is always traction resistant; therefore, the tie-beam is usually hanging from the post.

Subsequently, the joined analysis of many clues (structural behaviour, evolution of carpentry

joints, evolution and spread of iron reinforcements) will be used to support the thesis that the

progressive prevalence of the second type over the first (which essentially disappeared since the

mid-18th

century) is due to the progressive understanding of the behaviour of damaged trusses

and to the gradual diffusion of metallic elements for joints reinforcement.

When in a closed joint truss a material decay affects the rafter/tie-beam joint, the tie beam

starts to be charged by a concentrated load, which generates a flexional action even though no

sensible displacement is visible; decay progression can eventually lead to tie-beam breakage,

especially if it is weakened by a mortise.

When the same decay affects an open-joint truss, as soon as the decay starts, if there is no

iron reinforcement, the bottom of the post suddenly hits the tie-beam, and there is the risk that it

could collapse. On the contrary, if the rafter/tie-beam joint has iron reinforcing elements, it

acquires a certain ductility and, in case of decay, the bottom of the rafter slowly and

progressively slips, causing a lowering of the post towards the tie-beam, which denounces the

beginning of a decay and warns against the risk of collapse.

Hence, progressive affirmation of open-joint trusses can be seen as the achievement of a

higher safety in event of material decay of the rafter/tie-beam joint.

1 DICAr, University of Pavia, via Ferrata 3, 27100 – Pavia (Italy), [email protected]

Timber trusses in Italy: the progressive prevailing of open-joint over closed-joint trusses

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INTRODUCTION

No ancient Greek or coeval timber roof has come down to modern times. Many hypotheses

has been made about the types of structures used to support roofs of Greek temples: on the basis

of the archaeological remains, most authors think that timber roof structures of Greek temples

were formed by principal horizontal beams (lintels) supported by the longitudinal walls of the

naos (cella) and bearing vertical props to support the ridge and eventually other purlins

(Apollonj et al. 1937, Tampone 1996). In his De architectura, while describing the roof of the

Etruscan temple, Vitruvius depicted a similar system.

On the basis of indirect evidences that can be found in stone remains, some authors suppose

that timber trusses were already in use in the Greek temples at Selinus (Sicily, Magna Graecia) in

the 6th

century BC (Hodge 1960, Klein 1998).

Iconographic evidence of the 6th century BC, consisting of clay models of houses produced

by the Italic populations of the present Basilicata, represents triangular roof structures (Ruggieri

2012), but it is not possible to state whether their static behaviour was that of the truss or not.

Furthermore, some Etruscan tombs of the 6th

century BC (e.g. the so-called Tomba del dado in

Tuscania) and the Temple of Hercules at Cori (2nd

century BC) present a full-sized stone

reproduction of a different type of timber roofs: the horizontal beam seems to support a couple of

rafters – which define the slope of the double-pitched roof – through five equidistant props.

In his treatise (probably composed about 15 BC), Vitruvius also wrote about a Roman type of

timber roof to be used «si maiora spatia sunt» (i.e. for larger spaces) and described the roof of the

basilica he designed in Fanum; since the Renaissance, these roof structures described by

Vitruvius have been almost always interpreted as purlin roofs with trusses and many drawings

have been made of it; nevertheless, the obscurity of the Vitruvian text precludes the certainty of

correctness in the interpretation.

In this context of knowledge, a certain date for the invention of truss cannot be defined.

It is known, thanks to the representations made by Palladio and others, that the roof of the

pronaos of the Pantheon in Rome (2nd

century AD) had reticular structure made of bronze or

bronze-plated wood (Valeriani 2005), and similar structures were present in the Basilica Ulpia

and in the Baths of Caracalla (Apollonj et al. 1937). The oldest known timber trusses are those

no longer existing of the ancient basilicas of St. Peter in Vatican (early 4th

century AD) and St.

Paul Outside the Walls (late 4th

century AD), known through historical depictions. However, the

high complexity of these trusses makes it difficult to imagine that they were based on a recently

formed building tradition.

According to many experts, the oldest, still existing trusses are those of the church in the

fortified monastery of St. Catherine on Mount Sinai, dating back to the mid-6th

century AD.

FALSE TRUSSES, CLOSED-JOINT AND OPEN-JOINT TRUSSES

The examples reported in the previous paragraph show the simultaneous use of various types

of triangular roof structures.

The first type is the one that seems to appear petrified in the abovementioned Etruscan tombs

and in the Latin Temple of Hercules at Cori, the so-called “false truss”. The false truss has a

structural behaviour similar to that of the “post and lintel” roof, which is supposed to be used in

Greek temples: the horizontal timber element plays the role of bent beam under the load of one

or more vertical posts, which in turn support the rafters, also subject only to bending (fig. 1).

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Figure 1: Types of triangular structures: false truss (above), closed joint truss (below on the left), open joint truss

(below on the right) (from Laner 2000, redrawn by the author with some modifications).

The second and third types of triangular structure differ between each other in the

relationship between posts and tie-beams: they are called closed-joint and open-joint (Laner

2000), depending on whether the posts are connected with a carpentry joint to the tie-beams or

not (fig. 1). In the closed-joint trusses, posts could be connected to tie-beams with tenon-mortise

joints or with dowels, as in the basilicas of St. Peter and St. Paul in Rome. In open-joint trusses,

the posts are physically detached from the tie-beam – as in the trusses of St. Catherine on Mount

Sinai – but possibly linked to it with a metallic strap. To these types of triangular structure, it is

possible to add the simple truss made only of tie-beam and rafters typical of Sicilian architecture

of the 12th

-18th

centuries (Copani 2005), but diffused also elsewhere for minor span trusses.

A review of the technical literature concerning examples of timber trusses in the Italian

territory allows to build an albeit partial Italian “geography” of trusses typologies from Middle

Ages to the present day; in particular, it was possible to find many examples in some conference

proceedings on the restoration and conservation of timber structures (Tampone 1983, Id. 1987,

Id. 1989, Id. 1990, Id. 2005, Biscontin, Driussi 2009). As already stated by other authors (e.g.

Valeriani 2005) and as testified by the works of Renaissance treatise writers (Zamperini 2013c),

for a long period of time closed joint and open-joint trusses coexisted in the works of Italian

carpenters of different regions. However, since the 16th

century, areas of predominance of

closed-joint trusses progressively decreased and it almost completely disappeared towards the

end of the 18th

century; it returned in use only at the turn of 20th

century in trusses that were

inspired by central and northern European tradition (Laner 2000, Zamperini 2013c) and belonged

to a completely different technical culture, distant from the empiricism of previous centuries

(Zamperini 2013a). For a better understanding of the reasons for the gradual prevalence of the

open-joint trusses, it is necessary to analyse the construction details of the two types of structure,

to comprehend their static behaviour and to take into account the general technological context

of the period of progressive recession of closed-joint trusses.

Constructive details of closed joints

In close joint trusses, the links between posts and tie-beams are almost always obtained by

making a trapezoidal tenon at the bottom of the post and an analogous but wider mortise in the

tie-beam (i.e. a half-dovetail tenon and mortise joint) and by fixing each other with a wooden

wedge tucked inside the mortise (fig. 2-3). This kind of joint was quite complex to be realised: to

make the mortise, the carpenter needed to make a series of holes close together with a hand drill,

to remove the wood between them and to regularise the edges with a chisel (Zamperini 2013c).

Furthermore, the mortise weakened the strength of the tie-beam.


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Figure 2: Types of closed joints between post and tie-beam (from the left): half-dovetail tenon and mortise fixed

with a wooden wedge (Saldini 1886-88); double post clamping the tie-beam (Laner 2013); double timber

reinforcement clamping the bottom of the post and the tie-beam (Laner 2013).

Figure 3: Closed joint between post and tie-beam made with a half-dovetail tenon and mortise fixed with a wooden

wedge in a truss of the monastery of San Francesco in Bobbio (Emilia) (photo by Umberto Aimé)

Examples of this kind of joint can be found in various parts of northern and central Italy:

Tuscany (Giorgi 1990, Tampone 1996, Chiellini 1997), Emilia (fig. 3, Zamperini 2013c),

Lombardy (Pianazza 2008, Zamperini 2014), Veneto (Mannucci 2005, Menichelli et al. 2009). In

Tuscan and Venetian trusses, these joints are usually reinforced with metal straps nailed to the

posts and enclosing the tie-beam from below.

Particularly interesting are Tuscan examples: some closed-joint trusses are equipped with one

or two joists positioned under the central part of the tie-beam to reinforce it and are fastened to it

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by metal brackets; some others have no tenon-mortise joint, but only the strap (Giorgi 1990,

Tampone 1996); examples of open-joint trusses appeared later in which these reinforcement

joists under the tie-beam were present (Tampone 1996, Bigazzi, Vivarelli 2005, Carlomagno,

Ferrara 2005).

An evolutionary process that was similar in the outcomes, but different in the development,

can be found in Venetian territory. In this area, there are examples in which the trusses are

reinforced by adding a second beam under the tie-beam and in which closed joints are obtained

by making each post (or only its bottom part) with two timber elements that are tightened as a

clamp around the tie-beam and fastened with a horizontal stirrup (fig. 2; Laner 2000, Id. 2013).

Static interpretation of open-joint and closed-joint trusses

A simple static analysis of ordinary span timber trusses under vertical loads can provide

important information about the static behaviour of trusses. The analysis has been carried out

assuming a load due to snow equal to 1.6 kN/m2 in addition to the self-weight and to the weight

of the roof. The results of this analysis show that, in absence of links between the post and the

tie-beam, the vertical deflection of the middle point of the tie-beam is greater than the vertical

displacement of the bottom of the post; obviously, this difference will be higher in absence of

snow loads or in presence of an additional load on the tie-beam (e.g. floors, false vaults, ceilings,

chandeliers). Furthermore, the analysis shows that the difference increases with the truss span.

This behaviour was certainly well known to carpenters through empirical knowledge gained

from direct experience. Indeed, both closed joints and open joints with metal straps were

designed to be tensile-resistant joints; therefore, tie-beams are usually hanging from posts.

HYPOTHESES ABOUT THE CAUSES OF PREVAILING OF OPEN-JOINT TRUSSES

Assumed that the static behaviour of closed joint and open-joint trusses seems to be the same,

it might appear quite difficult to speculate on the motivations of the co-existence of the two types

for a long period and then of the progressive prevalence of the latter on the former. However, we

can suppose some structural, technologic and executive reasons that in a first period allowed the

existence of the two types, and later determined this change.

As we already said, the structural behaviour of the two types is quite similar, but this occurs

only when the truss is in full structural efficiency, i.e. in the absence of forms of decay.

Indeed, we must understand what happens in a closed joint truss in the probable event that a

rafter/tie-beam joint progressively loses resistance due to excessive load (being typically the

weakest point of the structure) or due to decay phenomena of timber. In this case, the tie-beam

gradually turns to support the post, whereas initially it was supposed to be suspended to it. This

happens at least until the tie-beam breaks right in the middle, where it is most stressed and also

weakened by the possible presence of a mortise.

On the contrary, an open-joint truss has less degrees of hyperstaticity and robustness, so a

small deficit of resistance of a rafter/tie-beam joint can suddenly lead to large displacements and

possibly to the collapse of the structure.

This type of consideration was not foreign to the empirical culture of medieval carpenters.

Palladio himself made similar remarks in his treatise. He wrote that it is appropriate that the

intermediate walls of buildings rise up to the level of trusses to support them also in intermediate

points, thus the roof could not collapse even if the end of a beam rot (Palladio 1570).

The hypothesis proposed in this paper is that carpenters started from the observation of this

kind of problems to evolve empirically their construction techniques. Indeed, in Tuscany


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reinforcement joists were added under the tie-beam of closed-joint trusses, just in the point

where it was more likely to break; later carpenters avoided to weaken tie-beams with mortises,

entrusting the support of the entire tie-beams to iron straps. In open-joint trusses, carpenters

acted on rafter/tie-beam joint and they strengthened them and made them more ductile with nails

and metal stirrups, thus preventing possible sudden collapses.

From a technological point of view, all of these steps could only be achieved thanks to the

decrease of the cost and the increased availability of iron elements, factors that occurred since

the 14th

century, thanks to the progress in iron industry (Zamperini 2013c).

These arguments seem to justify the abandonment of the mortise and tenon joints, but they do

not explain why carpenters did not keep making posts in contact with tie-beams (as they appear

in many Renaissance treatises; Tampone 1996, Valeriani 2005).

The paltry savings of timber due to the shorter length of post cannot compensate for a huge

executive advantage that closed-joint trusses had over open-joint ones. Closed-joint trusses could

be easily built “standing”, without the need for significant support scaffoldings – necessary to

put together an open-joint truss whose parts where lifted singularly – and for complex tools or

machineries to lift the entire truss – needed in the case the whole truss was assembled at ground

level. During the installation of a closed-joint truss, the post, resting on the tie-beam, could

directly support the rafters. On the contrary, open-joint trusses either need to be assembled on a

horizontal plane (be it on the roof or on the ground) and then rotated to be placed vertically or

require complex provisional scaffolding to keep the post lifted until it is held up by the heads of

the rafters.

The real advantage of open-joint trusses – once rafter/tie-beam joints are reinforced with iron

stirrups that provide ductility – seems to be that they openly denounce the possible onset of

collapse: the progressive decay of a rafter/tie-beam joint causes a gradual lowering of the post

towards the tie-beam; in a visible truss, this displacement can be easily seen and perceived as a

warning bell, drawing attention to verify the state of decay of the structure (Zamperini 2013b).

CONCLUSIONS

The research presented in this paper is based on the joint use of multiple sources of various

kinds: iconographic, written (ancient, modern and contemporary) and material. The study of

these sources is addressed in an articulate way, trying to integrate structural, technological and

executive features to reach a hypothetical reconstruction of the manufacturers’ mind-set and of

the reasons that motivate them to act in a certain way.

Further evidence supporting the thesis here presented may come from the analysis of possible

archival or material sources related to collapses and repairs of timber structures, but also from

the realisation of an organic geo-referenced database of construction types and details and its

association with the study of the manufacturers’ movements.

Ultimately, we can say that the analysis of presented building events disclosed a consistent

trend of carpenters to gradually “correct” their own way of working with the aim of increasing

the strength and duration of their own works. Therefore, the progressive affirmation of open-

joint trusses can be seen as the gradual achievement of increased safety in event of material

decay of the rafter/tie-beam joint.

E. Zamperini

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Timber trusses in Italy: the progressive prevailing of open-joint over closed-joint trusses

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