PSI - Issue 78

Alessandro Fulco et al. / Procedia Structural Integrity 78 (2026) 2046–2053

2047

1. Introduction Existing r/c and masonry buildings, characterized by high seismic vulnerability, constitute a significant portion of the Italian building stock. This condition is linked to multiple factors, including durability of materials, deficiencies resulting from inadequate criteria in seismic design, and lack of knowledge of the actual seismic hazard of the territory in terms of expected seismic intensity. In recent years, multiple methodologies have been developed to mitigate the seismic risk of existing buildings, Mezzi et al. (2008, 2013). The main strengthening techniques can be summarized as local approaches, global approaches, or a combination of both. The first, in the case of framed r/c structures, consists of the local consolidation of beams, columns, nodes, while for masonry structures, it involves the insertion of elements against first- and second-mode mechanisms. In the second (global) approach, the structure is strengthened through the insertion of new earthquake-resistant elements. The choice of intervention strategy is narrowed when certain architectural/functional conditions are introduced that prevent the interruption of activities during construction and/or prevent modifications to the original structure. These conditions are generally required for strategic buildings, public utilities, or production facilities, and in all cases where the interruption of operations could result in significant costs and/or social consequences, Fulco et al. (2020). Similarly, constraints on the choice of intervention also arise in the context of the conservation of the asset, both from an artistic and administrative perspective. Comprehensive interventions that allow the structure to function include all interventions performed inside/outside the building through additive structures, appropriately connected to existing ones and equipped with independent foundations. These systems are called "exoskeletons" or "endoskeletons," or systems that, when installed, are capable of protecting the existing building by primarily increasing its capacity for lateral forces [Foraboschi et al. to (2017)]. Additional systems can be conceived with alternative solutions: the adoption of bracing integrated within the exoskeleton (dissipative wall or tower solution) as reported in Bladucci et. al. (2015) or, innovatively, through designing the new envelope as an earthquake-resistant box-like system (shell solution) or with bidirectional frames within the existing voided structure to form an endoskeleton. The choice of the structural solution depends on the building initial stiffness and can be conceived as over-resistant or dissipative. Wall solutions include, for example, the use of bracing walls with rigid or dissipative connections, dissipative bracing, base-hinged walls, rocking walls, or adaptive seismic walls, Dall'Asta et. al. (2008). One of the most delicate aspects of applying this technique concerns the method of coupling the additional systems to the existing structure. The type of coupling depends on the strategy adopted: exoskeletons or endoskeletons. In the former case, the coupling adopted must ensure the transfer of horizontal seismic forces from the existing structure to the exoskeleton without generating abnormal stresses that the original structure would not be able to withstand without strengthening interventions. This coupling may be of the punctual type at the frame nodes or of the diffuse type across the floor elements. In the latter case, the connection system must ensure decoupling from horizontal forces between the existing envelope and the new endoskeleton while also avoiding the transfer of any new vertical floor loads (and therefore masses) present on the endoskeleton to the existing alignments. However, in the case of an endoskeleton, the connection system must be an effective constraint with respect to the primary and secondary overturning mechanisms of the external walls. In the following of the paper the general design principles for the systems previously introduced are presented and some examples of their design and construction within the Umbria Region (Italy) are illustrated. 2. General design rules for additional earthquake-resistant systems The design principle is based on the transfer of seismic action, either partially or completely, to the additional structures. The transfer of seismic action or the direct transfer of masses is calibrated by the connection solution between the existing structure and the additional structure. Generally, exoskeleton systems are transferred a portion of the seismic action by the existing structure, while endoskeleton systems are transferred the entire seismic action. The latter, in fact, completely replace the existing structure, which therefore acts only as an envelope without seismic masses induced by the decks.