# Numerical Assessment of the Seismic Vulnerability of Bridges within the Italian Road Network
## 1 Introduction  
The safety of existing bridges represents a serious problem since these structures are fundamental for the transportation system of a nation and, at the same time, they can be subject to significant deterioration phenomena linked to the fact that the construction period typically dates back to the 1960s. Considering that at the time durability was not one of the goals in projects and that the service life of the structures has been, in most cases, already reached or exceeded, proper maintenance and retrofit interventions are needed. In the last few decades, a significant increase in earthquake-related losses has been observed. The first studies on the effects of earthquakes on structures and the estimation of losses date back to the second half of the 1900s [1]. To effectively deal with a possible seismic event, it is important to identify the areas and structures that will most likely be affected by the earthquake. In this way, it is possible to determine the potential losses associated with the seismic event [2]. The definition of an earthquake loss model for a given region is also fundamental in view of the mitigation of seismic risk and, consequently, of losses. A loss model calculates the seismic hazard at sites of interest and predicts the damage distribution of the structures by combining the hazard with the vulnerability of the exposed structures. Damage ratios are then used to calculate the loss by relating repair and replacement costs to demolition and replacement costs. The seismic vulnerability, which is a fundamental component of an earthquake loss model, indicates the susceptibility todamage of a structure given a certain intensity of ground shaking [3]. Several methods to assess seismic vulnerability have been proposed in the literature over the years. The approaches can be divided into empirical or analytical. Hybrid approaches represent a combination of empirical and analytical methodologies. The work of [PERSON] et al. [3], in which this classification was proposed, describes the evolution of vulnerability assessment procedures over the years, highlighting the main developments for each methodology. The behavior of bridge structures is complex and requires attention to a number of important details. To gain a thorough understanding of these systems, researchers have explored various fields in bridge engineering. [PERSON] et al. [4] studied the bridge network of the Province of Quebec (Canada). Many bridges in the region were not designed for seismic loads and therefore may have a high seismic vulnerability. The authors developed fragility curves related to the bridges in the province to assess the seismic vulnerability of the structures. Considering the variability in terms of structural systems, it was found that concrete girder bridges are more vulnerable than steel girder bridges. The fragility curves developed in the study are useful to determine potential losses from seismic events and prioritize retrofit interventions but can be improved by collecting more information on the response of individual bridge components and integrating the definitions of the limit states with experimental tests. Fragility curves are common representations of the vulnerability of structures. Considering the extension of road networks, which count thousands of bridges, it is not possible to individually study every structure. Given that every bridge is different, a 2013 study [5] proposed a method to calculate fragility curves based on geometry, materials, and data recorded by a monitoring system. The proposed method allows changes in structural parameters to be tracked during the life of the structure, updating the fragility curves to obtain more accurate vulnerability assessments. It has to be pointed out that the structures involved in the research were not subject to a strong earthquake at the time of the study, so the updating of the fragility curves in post-earthquake condition had not been assessed in the study.