## 1 Introduction  
Recent earthquakes have highlighted the vulnerability of bridges and highway infrastructures in areas of medium to high seismicity [1; 2; 3; 4]. In the past, infrastructure networks and particularly bridges were constructed before the introduction of modern European and International seismic standards [5; 6; 7; 8; 9]. As such, existing bridges are characterized by an increased vulnerability to earthquake ground-shaking events [10; 11; 12; 13; 14; 15]. Furthermore, in Italy, the critical condition of ageing infrastructure has recently received international attention due to the recent collapse of the Morandi bridge [16; 17] and other infrastructures [18]. For the mitigation of further physical, human and economic loss, a proper vulnerability assessment coupled with the identification of adequate retrofitting strategies for the structural system should be carried out [19; 20; 21; 22; 23]. Recently, experimental campaigns highlighted the effectiveness of seismic isolation as a major retrofitting technique for damage prevention in reinforced concrete piers. This is due especially to limiting the maximum base shear at the base of the piers, decreasing the lateral drifts, and preventing the onset of inelastic deformation in the frame sections [24; 25]. Furthermore, significant residual drifts sustained following earthquake events render concrete bridge piers more vulnerable to strong ground motions, subsequently reducing the overall seismic resilience of the infrastructure network. A broad range of mitigation techniques is presented in the literature to reduce the vulnerability of bridge piers located in high seismic zones. After a numerical procedure, many retrofitting techniques can be designed and applied.  
Moreover, the overall seismic performance assessment of bridges is a critical research topic of paramount importance. Fragility functions are typically the end result of vulnerability assessment and are inherently governed by quantities subject to numerous sources of both aleatory and epistemic variables, such as modelling uncertainties and record-to-record variability, for example. Much literature highlighted advanced [26; 27] or simplified [28; 29; 30] methodologies for the aim of carrying out fragility assessment on infrastructure networks. Commonly, when actual bridge damage and ground motion data or the full geometrical properties of the stock are unavailable (or are partially so), analytical fragility curves are used to evaluate highway bridge performance. Furthermore, to improve the ability of numerical models to capture the static and dynamic behaviours of bridges, advanced finite element models were calibrated based on empirical data derived from tests and real-time monitoring techniques applied to existing bridges [31; 32; 33]. In many applications, analytical fragility functions also derived from simplified approaches [34; 35], and were compared to existing empirical fragility functions to highlight which direction of the bridge was more prominent or vulnerable to vertical and/or horizontal excitation effects [36; 37]. Additionally, nonlinear dynamic effects on bridges can affect the ultimate limit state behaviours of the bridge by amplifying the maximum stresses experienced by its different members. Furthermore, risk assessment plays an important role in quantifying earthquake loss and has been widely used in the field of bridge design and seismic engineering [38; 39; 40]. However, current studies of risk assessment rarely consider the different variable loads and seismic actions on piers. To this end, this study develops a multi-modal approach for the comparative assessment of variable loads and seismic actions applied to bridges. The multi-modal procedure is presented and applied to an existing 342 m long bridge in Italy, namely the Vizzana-Zampogna viaduct of the A15 Parma-La Spezia motorway (central part of Italy).