PSI - Issue 62

Paolo Borlenghi et al. / Procedia Structural Integrity 62 (2024) 468–475 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Risk-based classification with the Lombardia Regional Guidelines The assessment of the structural and hydraulic risks of the Candia bridge was firstly performed by using the RL Guidelines (Arena et al. 2022, Biondini et al. 2022). The first step of the RL Guidelines regards the collection of available information from documents and past inspections. The local administration officers – with the support of Politecnico di Milano’s working groups – were in charge of completing a bridge form, including data on localization, census, main structural/hydraulic characteristics, construction period, traffic related parameters (e.g. average daily traffic, and percentage of heavy vehicles), and results of periodic inspections. The bridge form – that consists of a spreadsheet – is then uploaded to StradeNet system, the regional bridge inventory. Subsequently, the TPI is defined from the combination of the attention classes k s (structural) and k i (hydraulic), and the inter-class subdivisions α k . The attention classes, as well as the inter-class subdivisions, are divided into 4 levels of intensity: the highest level corresponds to 1 and the lowest level corresponds to 4. The structural attention class k s is defined by the structural damage class k d (representing the damage level) and the class k t related to historical information (i.e. the time passed from the last inspection and the year of construction). In case of Candia bridge, k d is equal to 2 due to the presence of cracks on the arches – identified in past inspections – and k t is equal to 2 due to the construction period prior to 1962 and the last inspection performed in the last 5 years. Therefore, the structural attention class is assigned according to the aggregation matrix of Fig. 2 and it is assumed as k s = 2. The hydraulic class k i considers the effects of three processes: (1) hydrodynamic loads on the structure; (2) riverbed erosion; (3) river banks erosion. The parameter k i is equal to the higher value obtained for the three phenomena. In case of Candia bridge, the presence of scour at piers – known from past inspections – directly determines the higher attention class of k i = 1. The overall structural-hydraulic attention class is defined as k = min( k s , k i ) = 1. Consequently, despite the results of the inter-class level α k and the FPI (that are not herein commented), the high attention class is assigned to the Candia bridge, requiring an immediate action. Firstly, on-site visual inspections and documentary research were performed. Subsequently, according to owner’s policies and results from onsite inspections, some possible actions are: (a) monitoring, (b) structural safety evaluation, (c) repair, or (d) traffic restrictions.

Fig. 2. Aggregation matrix of the structural attention class k s determined by the damage class k d and the class k t related to historical data (Biondini et al. 2022).

3. Preliminary analysis 3.1. Hydraulic analysis

The Candia bridge is located 5 km upstream of the confluence between Sesia River and Po River. The Sesia River has a meandering channel pattern, which appears planimetrically stable from aerial images observation since 1954. In addition, the flow regime of the Sesia River is highly variable, with flow discharges ranging between 70 m 3 /s and 5000 m 3 /s. In case of a 200-year flood event, water can reach an elevation of about 105 m above sea level (asl) and 1 m above the skewback of the bridge arches.