Fire vulnerability assessment of bridges: a performance-based approach procedure with unconventional fire scenarios

This paper proposes a methodological evolution in the field of performance-based approach for the assessment of infrastructure fire vulnerability such as bridges. Criteria for assessing vulnerability to thermal and mechanical actions induced by fire events are standardized only in the case of buildings with specific intended uses. In addition, prescriptions are often lacking in performance-based approach, preferring verifications in prescriptive-based one, which is easier to apply but provides, in many cases, conservative results. In the case of infrastructures, on the other hand, such criteria are not explicitly defined by the relevant technical standards although experience have shown how the fire action can have impactful consequences on the functionality of the infrastructure. This work extends the definition of the criteria defined for structures by proposing four performance levels that the bridge must satisfy according to its level of importance. The proposed approach aims at the utilization of Fire Safety Engineering (FSE) principles by highlighting how this approach, although a higher computational burden, provides non-negligible performance differences, allowing for an advanced modelling of both the fire (natural fire scenarios) and the structure (advanced thermo-mechanical analyses). Finally, fire fragility curves are provided, using the Cloud linear regression method, based on different fire scenarios whose temperature curves were evaluated through a CFD analysis, for two levels of performance varying the initial utilization factor. The novelty of this work is to propose a general methodology that can be applied by varying the fire scenarios. Indeed, also unconventional fire scenarios were considered, since, if viaducts pass through densely populated urban areas, they can be susceptible to fires triggered by factors beyond just vehicles.