PSI - Issue 78

Salvatore Mottola et al. / Procedia Structural Integrity 78 (2026) 623–630

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1. Introduction Across Europe, a significant portion of the social housing stock predates the 1980s and was constructed without compliance with modern seismic design codes. These buildings are often functionally obsolete, energy-inefficient, and structurally deficient. Endoskeleton and exoskeleton systems have emerged as viable strategies for comprehensive seismic retrofitting. Exoskeletons, in particular, offer an innovative approach to improving the seismic performance of older buildings through self-supporting external steel frameworks mechanically connected to the main structure. These frameworks often form a two- or three-dimensional lattice surrounding the building envelope. Such systems not only enhance seismic resilience by increasing lateral stiffness and redistributing seismic loads, but also present opportunities for energy retrofitting and architectural revitalization, thus contributing to sustainable urban regeneration. These retrofitting systems can be configured in either parallel or orthogonal layouts and may incorporate energy-dissipating devices – such as steel hysteretic dampers or shape memory alloys (SMAs) – to absorb seismic energy and limit structural damage (Ferraioli and Lavino, 2018; Ferraioli et al. , 2019, 2020, 2021, 2022, 2023a-b). Their design flexibility enables exterior enhancements without major interior alterations, allowing retrofitting to be carried out with minimal disruption to occupants. Beyond improving seismic performance, exoskeletons serve multiple roles. By integrating energy-efficient technologies into the retrofit design, exoskeletons simultaneously address structural and environmental challenges, aligning with broader goals of sustainability and the circular economy. Structurally, they enhance performance under seismic loading. However, despite their many benefits, the widespread adoption of exoskeleton-based retrofitting is still limited by the lack of streamlined, performance-based design methodologies. Simplified tools that assist engineers in scaling interventions to meet specific performance objectives are needed to support large-scale implementation. 2. Retrofit Design Procedure 2.1. Analytical model for combined system (RC structure-Exoskeleton) After adding the exoskeleton, the building behaves as a dual structural system, with both the original structure and the exoskeleton contributing to the overall seismic response. This combined system can be conceptually separated into two subsystems: the main structural framework ( S ) and the exoskeleton ( E ), as shown in Fig. 1. Gravity loads are primarily supported by the main structure, whereas seismic forces are resisted by both components. In many design approaches, exoskeletons are intended to remain within the elastic range during earthquakes. However, this often results in an increased overall stiffness, which can lead to higher seismic demands on the building. To counteract this, exoskeletons can be equipped with energy-dissipating elements such as braces, panels, or dampers – hysteretic, viscous, or based on shape memory alloys (SMAs) – which provide passive seismic energy dissipation. While some design methods assume the existing structure behaves elastically, many reinforced concrete buildings exhibit cracking and plastic deformations even under low levels of lateral displacement. The connection between the exoskeleton and the primary structure can be either rigid or incorporate linear elastic or nonlinear behavior, influencing the seismic response of the integrated system.

Retrofitted System (SE)

Main Structure (S)

Exoskeleton (E)

=

+

(SE)

(S)

(E)

V x

V x

V x

(SE)

(S)

(E)

V y

V y

V y

Fig. 1. Decomposition of the total structural system (SE) into two subsystems.