Miluccio, Giacomo (2023) Structural behaviour and fragility evaluation of existing prestressed concrete bridge decks under traffic loads. [Tesi di dottorato]

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Abstract

Despite prior research on the seismic fragility of existing bridges, recent collapses highlight the need to evaluate their vulnerability under gravity loads. Following the collapse of the Morandi highway bridge in Genova in 2018, the Italian government announced new guidelines (GL) for prioritising safety checks and retrofit actions on existing bridges. Those guidelines provided new traffic load models (TLMs) that may be used for existing bridges in the event of non-compliant safety checks according to the Italian building code (NTC). The initial purpose of this thesis is to examine a class of simply supported, beam-type prestressed concrete (PC) bridge decks built in Italy between 1970 and 1980. Based on data available in the literature and those collected on real bridges by the authors, a subset of random variables (RVs) was modelled through probability distributions, whereas other RVs were assumed to be statistically dependent upon the former RVs according to regression models. A sensitivity analysis was done in the first stage to identify the RVsthat most impact the structural response. Those variables were then probabilistically modelled and propagated through fragility analysis to assess the conditional failure probability of the selected bridge decks given a traffic load intensity. The Monte Carlo sampling approach was then implemented in MATLAB, assuming geometric, material, and load properties as RVs as well as capacity model uncertainty, to randomly produce deck models and evaluate their traffic-load fragility. A fragility study was performed to examine the levels of vulnerability of existing Italian bridges under different load patterns according to GL and NTC provisions. In addition to a defined TLM, the sensitivity of fragility to bridge usage restrictions, such as reduced distance of external load lane from kerb or reduced number of lanes, was assessed to support decision-making by road management companies. Furthermore, using a European weigh-in-motion database and the convolution of fragility and hazard, the yearly failure probability of the selected bridges was computed. Analysis results demonstrate that structural fragility is greatly affected by the load pattern, indicating that more realistic vehicle models should be designed to attain the goal safety level required by current construction regulations. The complete process is incorporated in a software to simplify application and determine fragility for a single bridge or a portfolio of bridges. In addition, a safety cross section module is used to assess structural safety of individual case-study decks. The thesis ends with a progressive collapse study of a 1:5 simply supported post tensioned PC bridge deck consisting of four beams, four cross girders, and a continuous RC slab. The analyses were carried out with the Extreme Loading for Structures software. The structural behaviour of a PC bridge deck is heavily impacted by the type of cross girder used. As a result, three types of cross girders are considered: (i) normal cross girders separated from the RC slab; (ii) prestressed cross girders separated from the RC slab; and (iii) normal cross girders connected to the RC slab. The results of the study reveal that cross girders can be employed as segmentation fuses in the bridge deck in some instances to bring the entire deck to a specific type of collapse. These analyses may also be used to create an experimental test on a PC bridge deck and validate the results in order to inform the design of bridge decks and cross girders.

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