2.1. Historical and Architectural Description
The ancient building presently known as Palazzo Gualtieri (also referred to as Palazzo Ximenes-Bonomo), formerly Palazzo Alfieri (then Alfieri-Ossorio), is located in the inner core of the historical centre of L’Aquila, dating back to the Middle Ages. The main façade of the building forms the southern front of Piazza Santa Giusta, an L-shaped public square named after the Romanesque Church of Santa Giusta, free-standing in the north-eastern corner. Palazzo Gualtieri is embedded in a rich context of stately historical architectures, including also Palazzo Centi (western front) and Palazzo Dragonetti (northern front). The main part of the building, denoted as “principal body” in the following, is free-standing on the front, right and back sides, while the left side, facing the so-called Via dei Francesi is partially connected with the coeval adjacent building, characterized by strongly similar structural features and denoted as “secondary body” in Figure 1
, where the main part is contoured in blue and the secondary body in red. The structural connection bridging the two buildings, hosting two small rooms at the first and second level, is sustained by a masonry arch, which covers the passageway known as Arco dei Francesi.
The building, presently holding different private apartments for residential use, is traced by local historical tradition to the fifteenth century [32
]. However, the architectural elements that, in the absence of more reliable documents, allow an approximate dating, that is, window frames and doorways, tend to delay the construction date to the late sixteenth century. The internal courtyard can be dated back to the mid-seventeenth century with sufficient confidence, while minor constructional aspects recall late-Renaissance standards. The two monumental lateral fronts, in particular, are strongly featured by a valuable severe regularity, not tormented by the openings of convenience which have altered the original formal order of the principal façade over the years. In any case, the building—in its original layout—is certainly more ancient than it presently appears due to the architectural additions from the sixteenth and seventeenth centuries. The major historical and architectural value can be undoubtedly attributed to the courtyard. Even if not well-maintained, its original, elegant configuration is still correctly recognizable, and some of the original resistant elements can be clearly identified. Indeed, a regular sequence of thick columns, sustaining round arches, is revealed by decorative lesenes, despite the arcades originally surrounding the court are presently infilled.
2.2. Past Strengthening Interventions of Structural Retrofitting
In the early 1990s, the building was interested by an important strengthening intervention of structural retrofitting. However, the ownership fragmentation, together with the recurrent time delay and progressive downsizing of the original plan of structural interventions, caused by continuous reductions of the available financial resources, did not allow an integrated, organic design oriented to the general improvement of the global structural behaviour.
Nonetheless, numerous local interventions allowed the technical solution of the most evident and critical issues, mainly related to, first, the low quality of the rubble masonry, made of an irregular stonework and poor mortar, and, second, deficient connections among orthogonal walls. To this end, large portions of cracked or deteriorated masonry were consolidated with the traditional indenting procedure, many masonry panels were individually strengthened through the jacketing technique, by employing diffuse shotcrete and light steel net reinforcements, while several pairs of weakly-connected orthogonal walls were pinned together with diagonal steel fastening bars. The intervention was drafted according to the recommendations of the Italian national codes in force at the design time [33
]. Owing to the historical and architectural importance of the building, the intervention design had to be submitted for preliminary approval to the competent authorities of the Regional board of the Ministry of Cultural Heritage and Environmental Conservation.
In the principal body (Figure 1
), the intervention included local demolitions of the masonry walls for the realization of a modern lift shaft (Figure 2
a). In particular, a couple of load-bearing walls, adjacent to the ancient staircase, was partially hollowed to host a reinforced concrete box structure. To recover the structural redundancy and simultaneously improve the concrete-masonry cooperation, the new and old structural elements were anchored to each other by means of several stainless steel pins, inserted into holes drilled through the masonry and finally injected with cement grout. To comply with the modified requirements of the new functional distribution, a second staircase was built, with direct access from the courtyard. The reinforced concrete structure sustaining the stairs was built in adjacency to a pair of existing masonry walls (Figure 2
a), and properly connected to them through steel fastening pins. Together with these structural modifications, the remaining intervention were driven by the leading idea of improving, as much as possible, the global behaviour of the entire building and all its box-parts. To this purpose, a number of critical issues had to be solved, the most important being a marked loss of verticality of the principal façade, easily recognizable as an out-of-plane deformation of the masonry wall, characterized by an outward arch-shape between the ground and the roof. From a structural viewpoint, this crucial problem was certainly caused by a questionable, non documented, past intervention, during which the original second-level vaulted floor was completely removed to double the internal height of two noble halls at the first level. The consequent absence of any horizontal element able to offer a suitable out-of-plane stiffness was exposing the masonry wall to serious risks of overturning. The pre-existence of the problem is also testified by the consequent adoption of different palliative countermeasures over the years, including the addition of abutments and the thickening of the wall base. To mitigate the problem without heavy functional interferences, a light planar frame was realized in adjacency, and then strongly connected to the external face of the masonry wall (Figure 2
b). Thin steel profiles were employed to hide the entire frame in the thickness of the plaster exterior finish. The wall-frame system was finally anchored to the central spine wall of the building by several steel rods (Figure 2
c). The poor quality of the rubble masonry, made of irregular calcareous stones with weak adhesive bond, has been extensively treated using non-shrink cement grout injections at low pressure. Although recognized as highly vulnerable, the vaults covering the two noble halls were not object of intervention, due to limited financial resources. A complete overhaul of the roof structures suggested to maintain the existing structural scheme, made of traditional wooden trusses. Several steel ties were added to collaborate with the tensioned rods. A light reinforced concrete ring-beam was realized at the top of the masonry wall to the purpose of anchoring the steel ties (Figure 2
d,e). After careful revision, the large majority of the timbers was maintained in service, with only minor substitutions. The wooden slab sustaining the gable roof was instead entirely replaced, due to its advanced state of deterioration. During the works, an undocumented underground passageway was discovered, running from the building basement toward the Church.
In the secondary body, the structural intervention was generally heavier and affected all the floor levels. In particular, it included the addiction of a new underground level, which required the ground excavation and was founded on a new thick grid of reinforced concrete foundation beams. Reinforced concrete retaining walls were also designed to support the lateral pressure of soil. These retaining walls were orthogonally stiffened by shear walls, and a concrete slab was realized to ensure the structural redundancy. This stiff structural scheme served also as foundation of a self-sustaining three-dimensional reinforced concrete frame, raising up to the roof level, realized in strict adherence to the existing load-bearing masonry walls, and rigidly anchored to them through a system of fastening steel pins at each level. All the columns were abutted to the internal face of the existing masonry walls, and then connected to each other by flat beams at each level, running horizontally at the same height of the keystones of the vaulted floors. At the first level, the barrel vault was consolidated through a typical saddling intervention, consisting in, first, removing of the heavy but incoherent material constituting the original backfill, second, casting a thin curved concrete slab (saddle slab), reinforced by a steel mesh pinned to the vault extrados, third, replacement of the original backfill with light expanded clay aggregate and, finally, covering with a horizontal reinforced concrete slab at the floor level (Figure 2
f). At higher levels, some arched and vaulted structures, without significant architectural value, were evaluated as too seismically vulnerable and demolished. The excessive vulnerability was motivated by the evident vertical misalignment of the load-bearing masonry walls, with some arch abutments erected over the vaults of the first floors. Moreover, at the second and third levels, the horizontal thrust forces at the arch and vault springs were poorly confined. Therefore, the vaults were completely substituted with concrete slabs, while the original vaulted shape was formally reconstructed by a plasterboard false ceiling. A ring beam was added at the top of the masonry walls, with the twofold aim of ensuring the structural box behaviour and supporting the new reinforced concrete roof structure. The building has a mean height of about 14 m and is characterized by an almost rectangular plan, with side lengths of about 36 m and 28 m, respectively. The courtyard, which is also approximately rectangular, has side lengths of about 7.5 m and 10 m. A detailed geometric survey is reported in [18
During the inspections in the aftermath of the seismic main shock, a direct geometric relief of the building has been done, thus allowing the graphic restitutions of plans, prospects and sections, as well as the crack pattern for each prospect, and the most damaged room [18
Detailed graphical reconstructions of the northern and western prospects after the main shock are reported in [18
], with the corresponding crack patterns; in Figure 1
the plan of the first level and of the roof are reported. Since it was not possible to get access to all the private flats, in case of lack of data, the plans have been reported based on the acquired documents. In Figure 3
, the transversal section A-A is reported, and the vaults and structure typologies of the roof are highlighted.
With reference to the large hall at the second level (here called hall B), a high concentration of damage has been observed, both in the perimeter walls, and above all in the surmounting vault, which has partially collapsed (Figure 4
). The collapse of the vault represents the main damage of the building, above all for the artistic quality of its frescoes. The wall on S. Michele Street shows severe damage and partial collapse of the masonry medium as well as the loss of the perfect verticality close to the passage of the flue. This hall has been object of specific numerical simulations, the results of which will be described in the following paragraphs.
Concerning the masonry media, a visual investigation has lead to the observation of the following characteristics:
plastered masonry on the northern and western prospects, and on Via dei Francesi, made of calcareous ashlars with some bricks; on the southern front two different masonry typologies with visible leaf can be observed: the first is partially plastered and made of irregular calcareous ashlars, while the second has also some levelings with bricks (Figure 5
corner with ashlars (visible and not perfectly smoothed), with lime mortar (Figure 5
widest doors at the first level are surmounted by monolithic lintels, the smallest ones and the windows of the upper levels are surmounted by lintels made of several ashlars and brick arches (Figure 5
vaults of the two halls A and B are made of bricks ordered according to a knife arrangement, with some wooden ribs.