PSI - Issue 78

Gianluca Fagotti et al. / Procedia Structural Integrity 78 (2026) 2070–2077

2074

This solution avoids the use of traditional isolation systems applied to individual buildings, which, due to the significant horizontal displacements, would require a highly complex and expensive network of joints, given the architectural complexity of the historic centre characterized by very close building aggregates. The "artificial ground" solution by-passes the aforementioned problems because the base plate acts as a rigid connecting element between all the buildings, which, during an earthquake move all together as a single structural body. The isolation system allows for an increase in the horizontal period of oscillation of the whole body, reducing the horizontal accelerations to which the buildings are subjected. The seismic isolation system consists of a certain number of isolators distributed across the entire surface of the slab and positioned in such a way as to ensure adequate dynamic efficiency while simultaneously limiting the stresses on the slab generated by the static loads of the buildings. The use of curved-surface sliding isolators (friction pendulums) is envisaged. These are characterized by a stiffness that depends substantially on the load acting on them and therefore ensure, except for approximations related to the influence of the friction coefficient, a substantial coincidence of the centres of mass and stiffness and therefore a containment of the torsional contribution to the displacements of the isolated system. The stepped isolated slab has a constant thickness and is located at a distance from the foundation slab, allowing for inspection of the insulation system, as required. The isolated slab consists of a total of 10 steps, positioned at heights that restore the pre-existing profile of Castelluccio, not only in terms of the buildings but also of all other elements of the urban fabric (alleys, streets, and squares). Given that the elevations of the various aggregates are significantly different, to limit the number of steps on the slab, rigid reinforced concrete box-shaped bases are provided, allowing for the modulation of the elevation of the buildings relative to the insulated slab. The development of the bases is therefore linked to the planimetric and altimetric development of the buildings. At the perimeter road of Via degli Sciatori, a shaft will be created on the intrados of the slab, continuing the one already provided in the areas outside the slab. The main characteristics of the structural system are listed in the following, making reference to the preliminary studies since the actual final/executive design is currently under development and optimization, as it will be detailed in a successive paragraph. The number of isolators originally defined was 301 (during the executive design the number is being modified in 306). They are arranged on r/c pedestals having a section of 1.20 x 1.20 m set on a foundation consisting of a plate or a grid of beams 0.70 m high. The space between the lower edge of the isolated plate and the upper edge of the foundation was limited to a height of 0.80 m (in the executive design the value is being increased up to 1,20 m), allowing for inspection operations and devices' substitution. Devices are sliding isolators with curved surface having diameter D g =880 mm, height H=173 mm, radius of curvature R=3700 mm, friction coefficient 2,5% (at maximum load) and 4% under dynamic action, maximum vertical load N Ed =6000 kN, maximum displacement 400 mm, effective lateral stiffness for mean axial force K d = 591 kN/m. For the seismic action it has been considered a design value corresponding to a return period T R = 2475 years. The subsoil category was assumed as B. Further investigation for the executive design lead to define a category A, so, from this aspect, the initial evaluation can be considered prudential for the isolated structural system. The elastic acceleration spectrum for the collapse limit state (SLC) is characterized by bedrock acceleration a g = 0.441 g and horizontal ground acceleration PGA = 0.529 g. The total isolated seismic mass is approximately equal to M iso = 45000 t. The average load acting on the isolated plate is Q = 70 kN/m 2 . The maximum response parameters at SLC limit state are: displacement 335 mm, axial force 4201 kN (no tensile forces), horizontal force 360 kN. Some emerging situations concern the intervention area. One issue consists on the fact that among the buildings of the intervention area there are two buildings of worship, registered as Assets of Cultural Interest. One of them is the church of the Most Holy Sacrament Oratory that was completely destroyed and will be rebuilt ex-novo on the isolated plate. The other one is the church of Santa Maria Assunta, of which only portions of the perimeter walls and of the apse stand, for which a controlled disassembly is provided, with a successive reassembly at the site, above the isolated plate. Another issue consists of the provision to create a multi-purpose vaulted hypogeum rooms hosting museal and educational spaces, corresponding to a useful surface of more than 200 sqm that will be obtained in an underground portion under the stepped isolated platform thanks to a deepening of the foundation level. Due to the relevance of the work and to the non conventional seismic-resistant system, as well as taking into account the near-fault situation characterizing the site, attention must be paid to the vertical component of the seismic action to be considered in the design and to the related analysis methods. Indeed, this aspect was taken into