PSI - Issue 44

Romina Sisti et al. / Procedia Structural Integrity 44 (2023) 1116–1123 R. Sist et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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3.1. The seismic damage after 1997 earthquake and the relevant renovation works Before 1997 the building was subjected to some structural modifications that altered its original characteristics: the frequent change in position of the openings on the walls and the widespread cracks pattern, progressively caused by the repeated earthquakes, weakened the masonry walls that therefore resulted far from being compact despite their considerable thickness. In addition, few horizontal structures were weighed down with screeds and marble flooring without strengthening the supporting masonry walls. Despite the low intensity of the seismic events of 1997, the Palazzo Comunale suffered extensive damage, mainly due to the poor quality of the masonry and the lack of wall-to-wall connections: the activation of the simple overturning of the main façade was the most significant damage. By contrast, the contiguous theater was not damaged. To repair the earthquake damages and improve the seismic behavior of Palazzo Comunale, strengthening measures were designed and realized starting from 1999 (Fig. 4). The project aimed to achieve a seismic capacity equal to 65% of the design actions set by the regulations in force at the time. Traditional strengthening techniques were adopted: the mechanical characteristics of all existing walls were improved by means of grout injections; the wall irregularities were reduced walling up the wall niches and hollows; the shear capability of some masonry panels were increased inserting diagonal tie rods; the compression stress of some masonry panels were increased by inserting prestressed vertical Dywidag Threadbar; the original wooden floors and roofs were either replaced, when too poor, or stiffened, when too flexible; and finally the wall-to-wall connections were improved inserting several steel tie rods. No interventions involved the theater. Fig. 4. Strengthening interventions carried out during 1999 renovation on the first storey of Palazzo Comunale. Red lines identify the introduction of steel tie rods; green color represents the insertion of diagonal or vertical Dywidag Threadbar; blue color represents the construction of new block masonry and fuchsia shows the niches or hollow that were walled up. 3.2. The damage after 2016 seismic sequence The 2016 seismic events allowed to evaluate the effectiveness of the strengthening solutions adopted in 1999. In situ surveys were carried out to detect the damage caused by recent earthquakes. Cracks on the interface between the main facade and the vaults at the ground ( Errore. L'origine riferimento non è stata trovata. a) and first floor ( Errore. L'origine riferimento non è stata trovata. b) were detected. However, the lack of damage at the top floor excludes that the main facade, or a central portion of it, was subjected to a vertical overturning ( Errore. L'origine riferimento non è stata trovata. c), while the vertical arch mechanism ( Errore. L'origine riferimento non è stata trovata. d) may have been activated. Wide-spread millimetric cracks affected the load-bearing walls, whereas only few cracks involved the entire thickness of the walls (Fig. 6a). In these cases, there are often discontinuities in the masonry layout, like the one shown in Fig. 6b, that may have facilitated the crack development. Severe damages occurred in the non-structural components and decorations as shown in Fig. 6c (e.g. commemorative plaques, stuccos, frieze). Inappropriate construction details caused local damages, as in the case of the connection of the wooden truss and the masonry wall underneath it shown in Fig. 7a. The theater experienced the most severe damage: the external layer of the masonry gable crumbled, due to the poor quality of the masonry, and caused the partial collapse of a portion of the underlying roof structure (Fig. 7b). In addition, the earthquake caused a disarrangement of the theater’s roof covering, and , unfortunately, the needed countermeasures were realized only after three years, so the rainwater penetrations led to the collapse of the camorcanna vauts of the gallery (Fig. 7c).