PSI - Issue 64

Donatella de Silva et al. / Procedia Structural Integrity 64 (2024) 1806–1814 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction In recent years, the risk of fire in transportation infrastructure has significant increased, mainly due to urbanization and the increased transportation of fuels and chemicals by Garlock et al. (2012). These fires can cause significant losses both economically, due to maintenance/reconstruction costs of the structure and traffic diversion to alternative routes, and in terms of human life by Naser and Kodur (2015). However, in bridges design, hazards such as earthquakes, wind and snow are considered, but the fire is not taken into current code and regulations, and building codes are not directly applicable due to significant differences in the fire conditions. Although, literature studies have shown that the likelihood of a fire occurring on a bridge is not as low as often believed compared to other risks. In particular. Lee et al. (2013) has shown that that the number of bridges damaged by fires is greater than the number of bridges damages by earthquake. A similar survey was conducted by the New York Department of Transportation (2008). At present, there are no specific fire design criteria for bridges in codes, and buildings codes are not directly applicable due to significant differences in fire conditions. For example, in the past, Miano et al. (2020) presented a probabilistic seismic and fire assessment of an existing reinforced concrete building and retrofit design. However, fires involving transportation infrastructure are often highly intense and explosive. Therefore, in the event of a fire, these structures could be particularly vulnerable to fire-induced damage. Given the increasing importance of this issue, this study proposes a methodology for evaluating the structural response of bridges under fire conditions, determining the fragility curves based on specific performance levels proposed by de Silva et al. (2023) in function to the vertical displacement of the midspan of the bridge deck. The bridges analyzed in this study are two, the first one is representative of prestressed reinforced concrete bridge while the second one is representative of composite bridge. These bridges were studied using a performance-based approach, for which a literature review was conducted on heat release rate (HRR) curves related to the most likely fire scenarios to which the bridges could be subjected. Ather obtaining the heat release rate curves, thermo-mechanical analyses of the structures were conducted by the SAFIR software developed by Franssen et al. (2017). Based on the results of these analyses, fragility curves were determined, considering two measures of fire intensity: the fire load and the peak of the heat release rate curves. These fragility curves obtained for an infrastructure based on a specific performance level can be a valuable tool for engineers, to be used both in the design phase of new infrastructure and in the evaluating phase of existing ones. 2. Methodology The starting point for developing the fragility curves of a structure is to identify the performance levels to be used ad reference for these curves. In the context to this study, these performance levels were established following the guidelines provided by de Silva et al. (2023), which include not only the maximum vertical displacement of the bridge deck’s midspan but also its residual vertical displacement, as reported in Table 1. Table 1. Performance levels for bridge

PL Description I

The bridge must hold for the time required for evacuation The bridge must withstand the duration of the fire

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III Max displacement should be limited to L/100 for the duration of the fire and residual displacement should be limited to 50% Δ in IV Max displacement should be limited to L/250 for the duration of the fire and residual displacement should be limited to 20% Δ in

The fragility curves represent the probability that a structure will experience damage beyond a certain level in response to a specific intensity measure. To assess the fragility of a structure, it is crucial to establish the relationship between the intensity measure of the action to which the structure is exposed and the probability exceeding a predefined limit state or performance level. Consequently, the involved process is divided into several consecutive phases. Initially, the geometric and mechanical parameters of the structure for which fragility curves need to be determined are defined. Subsequently, potential fire scenarios to which the structure may be subjected are identified, and these scenarios are