PSI - Issue 78

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

2048

2.1. Exoskeleton-type solution The exoskeleton solution typically involves increasing seismic capacity by creating an external earthquke-resistant system of adequate stiffness, capable of withstanding most seismic action and ensuring that the forces acting on the existing structures remain below its capacity levels without strengthening. The overall objective is therefore to activate the collapse mechanisms of the existing structural elements at a seismic demand level greater than the current state. The exoskeleton system typically consists of braced steel frames, arranged externally to the building, close to the façades. The bracing systems are positioned planimetrically so as to also recenter the center of gravity of the stiffness with that of the masses, which typically presents significant eccentricities in reinforced concrete buildings. Therefore, the additional systems not only take the amount of seismic shear necessary to achieve the required performance levels, but also correct the dynamic response of the structure by mitigating eventual torsional effects. The external braced frames are made with large profiles to ensure the required stiffness and are founded on reinforced concrete slabs with deep foundations consisting of piles or micro-piles. The connection system is composed of two parts, one fixed to the exoskeleton and the other connected to the existing building. The two parts are interconnected by the shear-resistant connector profile. The connector is the element capable of transferring the seismic shear load from the existing building to the exoskeleton. However, it is essential to provide connection systems that allow for vertical sliding between the two structures to avoid the onset of compression or traction in the building columns affected by the connection system. The design principle of exoskeleton-type systems is based on an increase in the overall stiffness of the strengthened system ΔK , such as to achieve the spectral demand accelerations S a,d . Therefore, given that the capacity of the existing structure is known in terms of capacitive displacement S d , ext and spectral acceleration S a,ext , and that the seismic mass of the structure M s is known, it is possible to determine the stiffness to be conferred to the structure using the additional systems. The increase in stiffness translates into an increase in the natural oscillation period of the structure ΔT with respect to the original period T . This procedure represents a pre-design in the elastic field useful for the subsequent phases of project optimization. In fact, it is: ΔK = M s ∙ (S a,d – S a,ext ) / S d,ext Fig. 1a reports an ADSR diagram showing, by way of example, the capacity curves of the existing (red line) and reinforced (green line) building, and highlighting the quantities described above. In reality, during the design phase, it is possible to reduce the stresses acting on the exoskeletons and therefore the design spectral acceleration through the use of dissipative connectors. These allow the target displacement ( S d,ext ) to be achieved in the plastic range, reducing the stress on the additional structures and therefore on the foundations. This principle is illustrated in Fig. 1b by the orange line.

Fig. 1 General design rules using ADSR diagrams: a) response comparison between existing (red line) and strengthened building (green line); b) use of the elastic-plastic capacity of the additional system (orange line).