PSI - Issue 44

Alessandra Gubana et al. / Procedia Structural Integrity 44 (2023) 512–519

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A. Gubana and A. Mazelli / Structural Integrity Procedia 00 (2022) 000–000

From this point of view, the fragility curves represent a powerful tool in the vulnerability assessment procedures of existing building stocks, and the consequent mitigation of seismic risk. They estimate the probability of exceeding a damage threshold (or a limit state), as the probability of the failure intensity to be equal to or lower than the possible seismic intensity at the site (Kiremidian et al. 1985), but they can also provide information about the operational seismic action, as the greatest Peak Ground Acceleration that would allow a hospital building to maintain its function during and immediately after earthquakes. For healthcare facilities the problem is complex, as they are characterized by several safety issues, such as operation of internal medical and electrical equipment, including medical gas plants, and fire safety. This last threshold has recently been under investigation and reference data are needed. Several studies and evaluations have been conducted to derive fragility curves, based on either empirical methods or analytical–mechanical methods. Empirical methods use information from post-earthquake surveys (Dolce et al. 2003; Rossetto and Elnashai 2005; Del Gaudio et al. 2020; da Porto et al. 2021), while analytical–mechanical methods evaluate the relationships among structural responses, seismic intensity, and the expected damage by means of Finite Element, Equivalent Frame, Discrete Element models (Vamvatsikos and Cornell 2002; Lagomarsino and Cattari 2015). These analyses can be performed in static or dynamic fields (Masi et al. 2021). Due to the necessity of investigating large numbers of facilities, simplified methods have been adopted, especially for masonry buildings (Angioilli et al. 2021), while for reinforced concrete structures simplified typical frames have been initially investigated, often redesigned on the basis of the codes in force in the past (Masi 2003; Masi et al. 2015). Attention was then devoted to residential buildings (in Italy Borzi et al. 2021), schools (Masi et al. 2021), and churches (Leite et al. 2013). More recent studies consider degradation phenomena, such as the deterioration of material properties, damage from previous earthquakes (Réveillere et al. 2012; Pitilakis et al. 2014) or model uncertainties (D’Ayala and Meslem 2013). In Karapetrou et al. (2016) the FEM model based on the available construction plans was updated by means of modal identification results with ambient noise field measurements. In the present work a set of Fragility Curves (FCs) for an actual reinforced concrete hospital building is presented. To date, many examples in the literature refer to two-dimensional structures or simplified 3D frames. The fragility curves are derived by means of Incremental Dynamic Analysis (IDA) technique, following the indications in CNR-DT 212/2013 (2013). Nonlinear dynamic analyses under increasing levels of several seismic excitations are performed. A three-dimensional nonlinear model is built, based on design documentation, as-built detailed plans, and concrete and steel bar test results (Gubana et al. 2019). First, Modal Response Spectrum analyses and Pushover analyses are performed to obtain a preliminary comprehension of the structural behaviour. Subsequently, two nonlinear dynamic analyses are run to validate the model under the Friuli ’76 earthquake, which struck the structure with no damage. Then, Incremental Dynamic Analysis (IDA) is performed to evaluate FCs, choosing the Interstorey Drift Ratio (IDR) as the Damage Measure (DM). The second goal of the study is a comparison between two different choices of Intensity Measures (IMs), Peak Ground Acceleration and Spectral Pseudo Acceleration (5% damping) in correspondence of the fundamental period of the structure. The choice of the best IM for IDA analyses and for the evaluation of FCs is still debated (Kostinakis et al. 2015). 2. Case study The case study is a hospital facility located in Friuli Venezia Giulia (Italy). It consists of two units, which are connected by a structural joint. The first unit was designed in 1967, the structure was completed in 1971, and it suffered no damage from the seismic sequence that hit the area in 1976. The second unit was designed in 1975 and completed some years later. The structure has a 112 m × 35 m rectangular plan section, while the basement is 10 m longer. It is a seven-storey r.c. structure with interstorey heights of 3.20 m, except for the basement storey, which is 5.13 m height. The structure is organized with four principal longitudinal frames, two interrupted frames in the middle of the building and six lift and stairs cores, labelled A to H (Fig. 1). As it was not conceived as a seismic building, no transversal frames are present, and all the structural elements were designed only for gravity and wind loads.