PSI - Issue 78

Lorenzo Audisio et al. / Procedia Structural Integrity 78 (2026) 1277–1284

1278

occupant safety, post-earthquake functionality, and potential economic losses (Braga et al., 2011; Filiatrault et al., 2001; O’Reilly & Calvi, 2021; Perrone et al., 2019) . Recent earthquakes have highlighted how damage to NSEs can critically impair the usability of buildings, even in the absence of significant structural damage. Such damage may obstruct evacuation routes, compromise the operation of critical equipment, and lead to prolonged downtime particularly in hospitals, schools, and public buildings. The consequences associated with NSEs damage include risks to occupant safety (Life Safety), direct economic losses (Property Loss), and disruptions to building functionality (Functional Loss). The economic losses attributed to NSEs, relative to the total building cost, can reach up to 92% for hospitals, 87% for hotels, and 82% for office buildings (Taghavi & Miranda, 2003), while exceeding 60% in school buildings (O’Reilly et al., 2018) . NSEs can be broadly classified into three main categories: architectural components, mechanical/electrical systems, and building contents. Current design codes (MIT - Italian Building Standard 2018, MIT - Commentary of Italian Building Standard 2019, CEN 2004) primarily focus on the performance of the structural system, while checks for NSEs are generally limited to inter-story drifts and floor accelerations. However, such criteria are often insufficient to ensure the functional integrity of these elements during seismic events (Filiatrault & Sullivan, 2014). The seismic response of NSEs is strongly influenced by the dynamic properties of the structure, as well as by the component’s location, boundary conditions, and damping ratio. Furthermore, in the presence of significant dynamic interaction, the coupling between the structure and the component can substantially alter the overall system response. Lastly, nonlinear behavior either of the component or the supporting structure introduces additional complexity, significantly modifying the system’s dynamic performance. In the literature, two main analytical approaches are commonly used to evaluate the seismic response of NSEs (Figure 1): the coupled approach, modelling the structure and the component as a single integrated system and, by contrast, the decoupled approach using the floor motion of the primary structure as input for the NSE. The latter is the most widely adopted due to its simplicity and its validity in cases where the NSE mass is negligible compared to that of the supporting structure.

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b

Fig. 1. (a) Coupled approach; (b) Uncoupled approach.

This work provides the background for the formulation provided by the Instructions for the application of the Italian Standards NTC 2018 (MIT - Italian Building Standard 2018) adopted for determining the Floor Response Spectra (FRS). To this scope, firstly the theoretical basis is explained. Afterwards, the numerical formulation proposed is applied to a case study and compared with other numerical formulations adopted in some international design codes (e.g., Eurocode 8) as well as with relevant contributions from the scientific literature. 2. Floor Response Spectrum (FRS) provided by the Italian Standards NTC 2018 The Italian Building Code (NTC 2018) classifies NSEs into two main categories, that are: • NSEs with stiffness, strength, and mass sufficient to significantly influence the dynamic response of the main structural system; • NSEs that affect the structure primarily through their mass but are relevant in terms of occupant safety.