Experimental Study on the Damaged Pillars of the Noto Cathedral
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Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 Experimental study on the damaged pillars of the Noto Cathedral L. ~inda',A. saki', R. De ~enedictis*& S. ~rin~ali~ I - DIS Dept. of Structural Engineering, Politecnico di Milano, Italy 2 Designer Abstract The paper describes the on site and laboratory investigation carried out on the remaining pillars of the Noto Cathedral, in order to verify their state of conservation in view of the rebuild of the church, and how the designers had to take the decision of demolishing them. 1 Introduction In December 1990 an earthquake hit the Eastern part of Sicily damaging old and contemporary buildings in different towns. Noto, known as the "Baroque city" was among them and several of its most beautihl buildings were seriously hurt. Also the Church of St. Nicolo, the Cathedral had damages to the vault, the lateral domes and to the pillars, apparently no more than other buildings. Provisional structures and scaffoldings were set up to sustain the damaged parts waiting for the repair and strengthening intervention. The partial sudden collapse occurred on March 13, 1996 fortunately without any casualty, left the Noto community astonished by the loss of one of its most famous buildings. The church had been built in different phases suffering several casualties from 1764 over a previous smaller church opened in 1703 to the public and demolished in 1769170 as the new Cathedral was growing. The Cathedral was opened in 1776. In 1780 the dome collapsed and the church was reopened in l8 18. In 1848 the dome collapsed again under an earthquake and then it was rebuilt and the church reopened again in 1862 but the dome was not completely finished until 1872. In 1950 the Cathedral was restored with new renderings and paintings and the timber roof substituted with a concrete structure; the work continued until 1959. Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 90 Strrcctrrral Srudir~,Rrpam atrd Marnrorancr of Hrriragr Aldzirrcruw V111 The losses caused by the collapse were the following: 4 pillars of the right part of the central nave and one of the 4 pillars sustaining the main dome and the transept, the complete roof and vault of the Figure 1 : Plan of the remains and of the tested elements central nave. three quarter of the drum and dome with the lantern, the roof and vault of the right part of the transept and part of the small domes of the right nave (Fig. 1). The extensive experimental and numerical investigation carried out after the removal of the ruins by a team of experts together with the designers [l], [2] clearly showed that the collapse started from one of the pillars, due to the damaged situation they were bearing before the earthquake. Taking into account the clear weakness of the collapsed pillars, the designers asked for a further careful investigation on the remaining pillars of the central nave which also where damaged by the collapse. The first idea was to repair and preserve these pillars during the reconstruction of the Cathedral. The paper will describe the on site and laboratory investigation carried out and how the designers had to take the decision of demolishing them. Other cases of similar damages occurred in Sicily will also be described showing the importance of investigation in order to prevent future failures. 2 Investigation on the remaining parts of the collapsed pillars: Layout of the section and material characteristics After the study of the collapse mechanism, carried out also on site during the removal of the ruins [3], the attention of the consultants was focused on the careful study of the peculiar features of the collapsed pillars. The removal by layers of the components of the collapsed pillars allowed to understand the poor technique of construction used for them. Layers of large round river stones with thick mortar joints, where the mortar appeared very weak and dusty, were found in the core of the structure, surrounded by an external leaf made with regular blocks of more compact limestone at the base of the pillars (Fig. 2). Since only the base had remained after the collapse and the symmetric pillars were still covered by plaster, the hypothesis was made at first that this limestone had been used for the external part of the whole pillars. This material, compact but not very strong, came from sedimentary carbonatic depositions which can be found in the area and are still used as quarries for the building industry [4]. Inside the rubble filling also pieces of a material full of voids were found which was called travertine; this material is of the same nature of the limestone, but deposited in the presence of turbulent Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 waters and it is rich in voids of various shape and dimensions which previously contained vegetarian and organic parts later on dissolved. The height of the blocks varies from 25 to 30 cm and the thickness, small compared to the pillar dimensions, is ranging from 25 (stretcher) to 40 cm (header). No really effective connection was realised between the external leaf and the core. The stones of the pillar strips supporting the arches, vault and domes Figure 2: A collapsed pillar have no connection either to the internal masonry or to the other parts of the external leaf (Figs. 3 and 4). The inner part of the pillars represents the 55% of the entire section, while in the pillars sustaining the dome it is the 58%. This part is a rubble masonry made with irregular stones but it could be seen from the ruins that approximately up to the half of the total height was made with large round river pebbles. The courses of these Figure 3: Horizontal pillar section stones are rather irregular without any transversal connection or small stones to fill the -- voids and with thick mortar joints. Nevertheless every two courses of the external leaf (about 50 cm) a course made with small stones and mortar was inserted in order to obtain a certain horizontality (Figs. 2 and 4). Scaffolding holes were left everywhere, some crossing the whole section. The mortar appeared to be very weak made with lime and a high fraction of very small calcareous 1 aggregates. Also the bond between the mortar and the 'r stones was very weak; in fact it was possible to remove stones and pebbles from the interior of the i pillars without any difficulty and with the stones being completely clean. This poor technique of construction and the use of the weak limestone (actually called "Noto stone") typical in the Noto region, was probably the cause of the damages to the pillars of the Cathedral, even if a clear crack pattern was reported to have appeared only after the 1990 earthquake. The walls were built -L.- similarly; nevertheless, the internal part was made Figure 4:Reconstruction with smaller sharp stones alternated with a slightly of a pillar section. Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 92 Strrcctrrral Srudir~,Rrpam atrd Marnrorancr of Hrriragr Aldzirrcruw V111 stronger mortar, in some way a better masonry. Some stones were sampled from pillars and walls and mortar samples were taken from horizontal, vertical joints and from the interior of the masonry (Fig. 2). The samples were sent to the DIS Laboratory in Milan and tested in order to find the material characteristics [l], [5]. The investigation (Fig.-. 5) has shown Figure 5: Detail of the foundation that the foundations df pillars and walls were sufficiently well constructed; rubble walls but with enough load carrying capacity for the weight of the above structures. The soil was a sort of natural compact silt and clay thick layer from where also the aggregates of the mortars were taken. Up to this point of the investigation even if the weakness of the material used seemed to be the cause of the high damage suffered from the earthquake, but it was not still clear why the pillars reached the collapse. 3 Survey of the remaining pillars The left pillars, still covered with a thick plaster, seemed to have suffered minor damages; only small and diffused vertical Figure 6:Large crack in a pillar cracks were present on the plaster. and example of a crack filled with mortar. Nevertheless the doubt that the damage could be deeper inside and perhaps even present before the 1990 earthquake, suggested to cany out on these pillars to a more accurate survey. As the plaster made at the end of the works done in the fifties was partially removed, a series of vertical large cracks were found, some of which filled with the gypsum mortar used for the plaster (Fig. 6). This finding gave the authors the first evidence that the damage was already present in the fifties. The pre-existing crack pattern was clearly a damage from compressive stresses, a long range damage dating probably even long time before the fifties. The lesson after the collapse of the Civic Tower in Pavia and the subsequent research taught the authors that the damage would probably have progressed even without the earthquake, which only accelerated the collapse. After the recognition of the damages, the removal of the plaster from all the pillars was planned in order to survey the crack pattern. Fig. 7 shows a reconstruction of the crack pattern of the pillar before removing the plaster from all .the pillars.