## 1 Introduction  
Most of the Italian public building heritage was built without seismic assessments in areas affected by a significant seismic hazard. This creates issues today and is particularly important where the building stock is antiquated. Consequently, the seismic assessment of existing buildings represents a topic of paramount importance, especially in the case of strategic structures [1]. In recent years, many research works were aimed at defining the fragility curves of existing school buildings in order to set an order of priority for structural retrofitting [2, 3, 4, 5]. Most of these buildings were built from reinforced concrete (RC) and designed in the 1950-70s without seismic considerations. The most common structural technique adopted for Italian RC buildings is a spatial frame with masonry-infilled walls along the external perimeter. The internal beams usually run in only one direction. Since such constructions were designed following non-seismic provisions, they are often vulnerable to horizontal loading [6]. In the last two decades, specific seismic rules have been introduced for the design of buildings following seismic codes: OPCM 3274 [7], NTC2008 [8] with its application commentary [9], NTC2018 [10] with its application commentary [11] and EN1998-3 [12]. In particular, structures are currently designed to achieve two main goals. The first requires that structures remain in the elastic range under seismic actions, with a return period comparable with the life cycle of the structure. The second is based on the consideration that to resist destructive earthquakes without totally or partially collapsing, structures must possess high deformation resources over their elastic limit [6]. To that end, fundamental principles of capacity design (CD) [13, 14, 15, 16] are applied. In particular, thedissipative parts of a building are designed according to the first principle of CD, i.e., as determined by structural analysis, while the non-dissipative zones are designed according to the second principle of CD, i.e., considering the movements of the dissipative parts in their ultimate conditions. It is clear to see that the previous considerations can easily be applied to the design of new buildings. Conversely, the retrofit of existing structures is more complicated than the design of new buildings. First, it is necessary to define the vulnerability level, to identify the structural criticalities. Subsequently, retrofitting interventions should be selected to mitigate the seismic risk and achieve a good compromise in terms of structural safety, cost and ease of execution. The logical process can be organized into four steps:  
* Step 1: Collection of historical information and geometrical and structural characterization of the building under investigation. * Step 2: Definition of mechanical properties characterizing the existing materials using destructive and non-destructive on-site tests. * Step 3: Evaluation of safety level according to different limit states using linear and/or non-linear analyses. * Step 4: Definition of the structural interventions needed to produce the most advantageous economic and structural solution. Particular attention should be focused on public and strategic buildings whose functionality is fundamental in a seismic emergency. After the occurrence of a school collapse in central Italy in 2003, an Ordinance of the Presidency of the Ministers' Council [7] gave five years maximum to evaluate the seismic vulnerability of the main public buildings (hospitals, schools, bridges, police stations, etc.). However, that timescale was extended several times and the seismic vulnerability of many strategic structures has not yet been evaluated. Among all the strategic buildings, special attention should be paid to hospitals, whose functionality in post-earthquake management is of primary importance. For such buildings, structural retrofitting interventions should be designed to ensure the continuous operation of medical activities.