Introduction

The present scientific theories aimed at analysing the mechanical behaviour of buildings started to be developed in the second half of the 17th century. Only then were the mechanics of materials taking their first steps with Mariotte and Robert Hooke's research (Timoshenko 1953; Timoshenko 1956). This followed the road which Galileo had opened indicating experimental observation as the basis of scientific knowledge. In his last work Galileo himself had presented the first observations on the "new science relating to the mechanics" of structures (Galilei, 1638).

The first applications of the new scientific methods to structural problems started to be enunciated between the end of the 17th century and the early 18th century. It was then, in 1741, that Benedict XIV commissioned three mathematicians Roger Joseph Boscovich, Fran├žois Jacquier and Thomas Le Seur from the "Repubblica Romana dei Dotti" (Boscovich & Al. 1742) to carry out a historical assessment. Serious concerns had arisen over the static conditions of Saint Peter's dome, where significant cracks had appeared. Interesting studies had already been developed on the subject, accompanied by learned and heated debates. This had led to the compilation of authentic treatises, which also aimed to order the knowledge on the subject. Historical documents also report other experts' opinions on the state of the dome, including that of the well-known mathematician from Venice Giovanni Poleni (Poleni, 1748).

The three Mathematicians' study stood out for its important innovation. It contained an assessment based entirely, perhaps for the first time, on a scientific criterion aimed at interpreting the mechanical behaviour of an architectural building. Its historical importance lies in the fact that, unlike the previous practices, which were based on empirical rules, generally of a geometric nature (see for example Poleni's studies on the statics of arches), theoretical conceptions, this time of a scientific nature, were used and applied to the study of a structural problem. Although not entirely correctly, the PVW (Principle of Virtual Work) was adopted in the assessment, and used as an instrument for measuring the metal rings to be applied to the drum of the dome (Capecchi, 1999; Capecchi, 2002). In an attempt to determine an important date, a number of experts (von Halasz 1969) regard this

Figure 1. Saint Peter's dome (Curcio, 2003).

assessment as the historical moment when the change took place from engineering based on artisan traditions, of an empirical nature, to engineering based on the application of the new scientific theories; theories, which were just starting to become established.

The three Mathematicians' assessment was presented towards the end of 1742 and printed in 1743. The study method thus introduced could truly represent the historic beginning of modern civil engineering. Unlike the previous practices, which used rules dictated by intuition and experience, a scientific process was applied to assess a building's characteristics of resistance and state of stress. This consequently started a process, which does not yet appear complete (Di Pasquale 1996).

2 "SAINT PETER'S" DOME

The building of "Saint Peter's" Dome was started on 15 July 1588 under Sisto V however, it was interrupted on 13 May 1590, just before the Pope's death. Following Michelangelo's project, the building work had reached the placing of the drum. It was completed by Giacomo Della Porta at the beginning of the 17th century (Ack-erman, 1968). The first cracks were discovered back in 1603, under Clement VIII, just after the building was finished, on completion of the mosaics on the vaults. Subsequent damage was then recorded after 1631, as

Figure 2. Section of Saint Peter's dome (Curcio, 2003).

we can see in Gianlorenzo Bernini's biography written by Filippo Baldinucci. It was suspected that the statics of the dome had been compromised by the insertion of the spiral staircase by Bernini, set within the pillars, under the pontificate of Urban VIII.

The controversy relating to Bernini's work soon quietened down. Baldinucci himself observed that a number of cracks inside the dome on the cornice and on the drum had been found before Bernini. Some had attributed the damage to phenomena of settling of the great dome and to the different working techniques used for its construction. However, the three Mathematicians claimed in their study that the damage described by Baldinucci was not the damage found in 1742. The numerous criticisms raised against Bernini referred to evident conditions of instability which were present already in 1742. Subsequently, the instability had increased and was developing.

"Il Tempio Vaticano" by Carlo Fontana, published in 1694 (Curcio, 2003) made it possible to carry out an in-depth analysis of the phases of construction of Saint Peter's Basilica and its dome, until the complex took on its present arrangement. The work covers the events of the construction of the building from the beginning, when the emperor Constantine wanted the basilica built near the tomb of the apostle Peter, until the end of the 17th century.

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