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Figure 1. Plan and longitudinal vertical section of the Basilica before the 1832 earthquake (di Boveglio & d'Isola Maggiore 1834, pl. 4).

phases of its construction history are reported here and neglecting the information about the convent.

After the death of Saint Francis (1226) only small buildings, a refectory, some dormitories, a chapel, a choir, an infirmary, were built in the area.

In 1568 Pope Pius V decided to construct an adequate Basilica, because of the importance of the site and because of the mass of pilgrims already attracted. The Friars asked the Perugia (Central Italy) architect Galeazzo Alessi to provide a model for the design of the new building which he did during the same year. The following year materials were supplied to the site and the prominent architect Jacopo Barozzi da Vig-nola visited the place, probably as design supervisor on behalf of the Pope. On March 25th the church was officially founded. Alessi delivered a plan of the building on March 1570, and died in 1572 without having the chance to come back again to the building site.

After five years the foundations were completed and it took twenty years to finish the masonry work of the three naves and related chapels. In 1593 their wooden roof was built (p. 111). During the first decade of the 17th century three pillars of the dome were built. It was in 1622 that the construction of the church restarted slowly in the choir. In 1637-40 the fourth pillar was erected, and in between 1644 to 1646 three main arches under the dome and the choir vault were completed.

In 1645-48 the right transept was built, but the left one was raised in 1650-52. Even though the construction of the dome started in 1662, just a year later was stopped by a harsh controversy (both monetary and technical) between the master builder and the Friars. The works restarted between 1668 and 1674, when they were stopped due to an out-of-plumb of the façade. The damage was ascribed to the too limited thickness of its upper section, and the void created by the stairs in the lower one. Between 1674 and 1675 the façade was strengthened and partially rebuilt. Only after that the works on the dome started again and were completed on September 25, 1677. Then, the bell tower was built in 1679-85, thus completing the church with the exception of the façade (Bartelli 1989).

2.2 Earthquake damages

The first information about earthquake damages in the church date to the April 17, 1747 earthquake (VII MCS felt intensity, Guidoboni et al 2007): a survey reported cracks in the choir, damages at the naves pillars, out-of-plumb of the façade (Boschi et al 1998, 81 and 86).

On July 27, 1751 another earthquake (VI MCS, Guidoboni et al 2007) hit the Basilica again in the choir (Boschi et al 1998, 62). It is possible that is was after these earthquakes that the lantern was looped by means of iron ties at three different levels (Menegotto 1993 and 2003).

Generic slight damages are reported for the July 28, 1799 earthquake (Boschi et al 1998, 122), felt with a VI MCS intensity (Guidoboni et al 2007).

A very severe seismic sequence struck the Basilica between 1831 and 1832 (di Boveglio & d'Isola Maggiore 1834, ch. 14; Perilli 1842, p. 6-7; Vignoli 1989,147-153; Boschi et al 1998,150-151). On October 27 the main door and the small cupola (on top of the main dome) were damaged. On November 6 such damages were worsened. The January 13 and 27,1832 and the March 13 shakes cracked the vaults and the pillars on the naves. Finally, the March 15 event caused the collapse of the vaults of the main and the left naves, as well as of the pillars between the two (Figure 2). The right nave, although not crumbled, was damaged badly, and probably survived because two of its pillars have been previously looped with iron and timber. On the contrary, the dome was not severely stricken.

The first emergency interventions, under the direction of engineer Antonio Mollari, were the confinement of the survived pillars, through iron and timber, and the construction of wooden pyramid for the protection of the "Porziuncola".

On August 11, engineer architect Giuseppe Brizi was asked to advice for permanent interventions. Among other secondary indications, he suggested to rebuild the roof on trusses not resting on the vault,

Figure 2. The left and central naves of the Basilica after the March 15, 1832 shock (di Boveglio & d'Isola Maggiore 1834, pl. 7).

although this would have reduced the drum of the dome.

On August 22 engineer architect Luigi Poletti (1792-1869) is first mentioned. Poletti was a preeminent practitioner at that time (Dezzi Bardeschi et al 1992) and during his professional lifetime was frequently involved in interventions after catastrophic events (Reale etal 2004).

Poletti recommended to shore the choir's vault, the right nave's arches, and the three arches between the naves and the transept.

Between 1832 and 1835 the master builder appointed to execute his instructions added iron rings to the dome, repaired two pillars in the right nave and tear down the upper section of the façade. The interventions on the two pillars were not faultless. According to Perilli (1842, 5) the original masonry of the pillars was a rubble core one, with a thin external brick masonry skin, and the master builder did simply repair them. The description of the construction technique is very interesting, since it may help to explain the severe damages observed in the building.

After many discussions in 1836 Poletti was confirmed in his role of designer and Mollari was appointed as supervisor of the execution. Between March and September of the same year the right nave was torn down and rebuilt (with solid brick masonry, Perilli 1842, 8) up to the arches, while the vault was completed between April and June 1837. BetweenJuly and October 1836 the demolition of the top of the façade was completed.

The central nave vault was rebuilt in 1838. The construction of the roof took place in 1839. Finally, the façade was partially rebuilt between March and August, 1840.

On February 12, 1854 another earthquake hit the area (VII felt intensity in Assisi, Guidoboni et al 2007).

The Basilica suffered cracks in the choir apse, in the transept, in the cupola, in the lateral naves, and in some chapels. The dome and the main nave were only slightly damaged (Boschi et al 1998, 164). Architect Giovan Battista Tiberi suggested extensive use of iron ties (Boschi et al 1998, 165), especially in the lateral naves and in the chapels. His proposals have been partially carried out, avoiding the visible ties, and locating them in the attic (refer to pictures in Lunghi & Lunghi 1989, 194-198; and description in Boschi et al 1998, 166).

Shear and spalling cracks occurred in the lantern above the dome by the April 19, 1984 earthquake (Boschi et al 1998, 214), whose felt intensity was VII MCS (Guidoboni et al 2007). Such damages induced careful provisional and permanent interventions (Menegotto 1993 and 2003), which have lived through the following seismic event.

The 1997 Umbria-Marche seismic sequence has also damaged the Basilica: the façade (rebuilt for architectural reasons during the 20th century) rotated slightly, cracks were detected on the lateral naves vaults and at the junction between naves and dome (refer to: Capalbini et al 1999, where also a complete geometric survey of the basilica is available).

Although from an architectural point ofview, Poletti restoration was substantially faithful to the 16th century design (Perilli 1842, 7; Vignoli 1989,152), from a structural point of view he introduced important innovations which will be discussed in the next section. It is important to stress that the portions reconstructed under his supervision survived all the subsequent events with only minor damages (Boschi et al 1998, 151). As will be shown, the strategy pursued by Poletti was not to recur to special earthquake-resistant solutions. Instead he tried to reduce the vulnerability of the edifice first of all by means of rule-of-art construction techniques, such as solid brick masonry instead of rubble core masonry. Secondly he saved weight on vault. Finally, he reduced the thrust exerted by horizontal structures, both vault and roof.

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