PSI - Issue 44

Ubaldo Saracco et al. / Procedia Structural Integrity 44 (2023) 721–728 Ubaldo Saracco et al./ Structural Integrity Procedia 00 (2022) 000–000

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Based on the above formulae, the most conservative d(t) value achieved from eq. (1) of the ISO 9224 standard, rounded to 1.30 mm, was used to reduce the thickness steel sections due to corrosion. These new structural sections were then imported in the FaTa-Next software and assigned to the different bridge elements. 3.4. Definition and placement of loads The structure, having no more dynamic vehicular traffic, is therefore solicited by a compact crowd only. The heaviest load condition is certainly that of the fully loaded deck. The Section 5.1.3.3 “Variable traffic actions - vertical loads Q1” of the Italian code NTC18 (2018) shows the different conditions of structures loaded by vehicular and/or pedestrian traffic. In the case of the structure in question purely used for pedestrian purpose, load diagram n°5 is identified with the value of 5kN/m 2 for “Compact Crowd”. 4. Push-down analysis 4.1. Framework The first type of analysis carried out was a static non-linear push-down one. This analysis aims to determine the multiplier of the collapse loads λ according to the following loading combination: S d = γ G G K + λ( γ Q Q 1K ) = R d (f kγ /FC ) (4) The verification is satisfied if λ max ≥ 1 , where the unit represents the totality of the load from compact crowds. To carry out the analysis for assessing λ max in cases of corroded structure, five different scenarios were assumed as explained below. 4.2. Scenario 0: Non-corroded structure The push- down analysis led to λ max greater than unity before the first structural element reaches the collapse. The failure occurs in the buckled lower chord tensioned in the plastic field. The total displacement of the structure was 443 mm. Analysis results of this scenario are shown in Figure 4.

b

a

c

λ max = 1.77 > 1

Fig. 4. (a) Push-down curve; (b) elasto-plastic deformation state of the bridge; (c) stress state in bottom chord.

4.3. Scenario 1: Corroded deck model In this scenario, as precautionary condition, the entire deck was hypothetically affected by corrosion. The structure exhibits a λ max greater than unity before the critical state of the structure is reached. The total displacement of the structure is 438 mm. The elasto-plastic state of the structure shows how the failure occurs with a different process compared to scenario 0, in which only the lower chord was the cause of the first collapse. This time the failure occurs because of both compression failure of the deck beams - actually damaged in this corrosion scenario - and tensile failure with subsequent instability of the lower chord beams. Analysis results of this scenario are shown in Figure 5.