Introduction

Preservation of historical structures often includes evaluation of timber roof trusses. Satisfactory performance of existing wood truss system in terms of both resistance to applied service loads and long-term response is dependent on wood species, truss proportions and timber connections (Tampone, 2007). Generally, stress concentrations are particularly critical at joints where a component is connected to others, such as those caused by notches (load to angle grain) or other sudden changes in cross section.

This is confirmed by survey and inspections of timber buildings damaged after extreme natural events, which often point out to inadequate connections as the primary cause of damage (Derinaldis & Tampone, 2007). Common connections adopted for modern constructions can be designed to fit hinge requirements or even to ensure full stiffness and strength, so that continuity can be assumed at joints. This circumstance does not apply to existing and historical wood artefacts are concerned.

In such cases, connection response can play a relevant role both at ultimate and serviceability limit states of historical timber structures (Branco et al. 2006; Seo et al. 1999) and modern European building codes (prEN 1995).

This means that the level of analysis has to be enhanced and semi-rigid behaviour of connections properly accounted for. Obviously, this kind of approach is by far more complex and requires a thorough investigation of the specimens and advanced numerical modelling, being of moderate interest for practical purposes. The objective of the present paper is to investigate the rotational behaviour under service loading conditions of typical joints of historical timber trusses.

Numerical analyses of the joints have been performed using a standard software package used by practical engineers (http://www.hsh.info). Orthotropic elastic behaviour of wood was accounted, and contact elements friction-based between the timber elements were considered.

Reference results of the numerical simulations are herein discussed and compared with experimental and numerical data provided by Parisi et al. (1997) and Parisi & Piazza (1995, 1998, 2000).

The model showed the capacity to predict the elastic stiffness of the joints if contact elements are properly simulated. Calibrated model is used for the numerical investigation of the elastic behaviour of the connections of the roof system assemblage of the Royal Palace in Caserta (Italy). It is an interesting case study involving a timber roof truss of the 18th-Century. Some

Figure 1a. View of the Royal Palace in Caserta, Italy.
Figure 2. Partial view of the Royal Palace roof truss.

Figure 1b. Plan view of the Royal Palace.

critical issues related to modelling of this kind of timber truss are also discussed.

1.1 The timber roof truss of the Royal Palace in Caserta

The Royal Palace in Caserta is one of the most famous historical establishment of the Italian Baroque (Caroselli 1967, Chierici 1984).

It represent a masterpiece of the creative genius of the Italian architect Luigi Vanvitelli (1700-1773) and it was probably the most representative monumental building erected in Europe in the 18th-Century (Figure 1a). Due to its magnificence with its surrounding natural landscape, the site has been included in the World Heritage List by the world Heritage Committee in 1997 (http://whc.unesco.org/en/list/549).

Works started in 1752 and were completed in 1847. The building has a rectangular plan and four close courtyards which are also rectangular, as schematically reported in Figure 1b. It is 36 metres high and has five storeys in addition to the underground level. The original design drawings of the building can be found in Vanvitelli, (1756). For the construction of the roof system, Vanvitelli has been mainly inspired to the timber roof truss of the Basilica of San Paolo Fuori le Mura erected in Rome in the 4th Century a.C. and rebuild in 1824 after a serious and wide damage due to fire.

The timber roof of the Royal Palace is made of trusses of Chestnut species, span of about 23 meters are found. They are typically 3 meters spaced from d.

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