PSI - Issue 78

Gregory Santilli Di Luia et al. / Procedia Structural Integrity 78 (2026) 1513–1520

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3. Modelling and standard safety assessment The structural assessment of the bridge was conducted in Midas Civil software (Figure 2a). Here, beam elements were used to model the piers, crossbeams, and girders, whereas plate elements were employed to model the deck slab. Elastic links were adopted to model the bearing supports. To adequately capture the structural response under severe loading conditions, material nonlinearity was incorporated through a fibre-based modelling approach (Figure 2b), in which the cross-section of each beam element was discretised into distinct fibres for concrete and steel, both assumed to undergo axial deformations only. According to this approach, the section force – deformation relationship was derived by integrating the stress – strain response of each individual fibre. The nonlinear behaviour of the elements thus resulted from the nonlinear stress – strain relationships of both the concrete and steel fibres. The formulation of the fibre element follows the flexibility-based approach described by Spacone et al. (1996). The models used for the materials and their values are shown in Table 1.

(a)

(b)

Fig.2. FEM model: (a) axonometric view; (b) detail of the subdivision in fibres of a section.

Table 1: Mechanical parameters for confined and unconfined concrete and steel.

Park et al. model (1982)

Menegotto and Pinto model (1973)

Parameter

Unit

Unconfined

Confined

Parameter

Unit

K fc 0 1

[-]

1.00 7.70

1.089 7.70 0.0025 0.0060 0.0070

fy

[MPa] [MPa]

170

[MPa]

E b

200000 0.0052

[%] [%] [%]

0.002 0.0034 0.0035

[-]

The bridge was analysed by considering both permanent loads (self-weight of the structures, superstructures, road pavement, etc.) and accidental loads, including vehicular traffic, wind actions, thermal variations, and seismic actions. The definition of the load combinations was carried out in accordance with the current Italian regulations (NTC 2018). First, a preliminary modal analysis was performed, followed by the application of the response spectrum method, taking into account modal amplification and the distribution of participating masses. Subsequently, a more detailed analysis was carried out through direct time-domain integration, using nine artificial accelerograms compatible with the design spectrum, which means three sets of recording ( x , y and z directions), considering the maximum response. The assessment was conducted in accordance with standard procedures (MIT, 2020), revealing that the bridge is inadequate to support static loads, primarily due to deficiencies in the superstructure. Moreover, it fails to meet seismic performance requirements, with pier failure governed by flexural-compression mechanisms. In summary, for the relevant load combinations, the structure exhibited a static safety factor of 0.4 and a seismic safety factor of 0.37. Additionally, on-site inspections identified a widespread degradation of the bridge, that is not accounted for in standard assessment procedures. This is expected to further reduce the current safety of the structure and highlighted