Analitycal fire fragility assessment for bridges considering fire scenarios variability

The Italian infrastructure network of roads and bridges is one of the most complex in the world, due to the orography of the territory. Italy is strongly interested by seismic and hydrogeological hazards and, in addition, degradation and obsolescence phenomena are common on infrastructures approaching the end of their nominal life. Furthermore, during their service life these infrastructures can be subjected also to extreme actions such as the fire. In particular, the past examples of damages on infrastructures as consequence of fire event and the high dependency on uncertain factors of fire have motivated the study for developing models to estimate the possible consequences that this phenomenon can cause on infrastructures. In particular, the paper sets a methodology to assess the structural fire fragility of infrastructures, based on fire hazard scenarios, developed specifically for bridges. Then, two existing pilot case studies (a reinforced concrete bridge and a composite steel-concrete one) are modelled, by deriving the structural response under different natural fire scenarios. By means of Cloud Analysis, a linear regression about the key parameters in the fire action definition and the structural response is assessed, by considering different performance levels, specifically proposed for bridges. Finally, the fragility curves are obtained, measuring the capacity of the case study bridges under the selected fire scenarios. The study also shows which of the fire scenarios parameters can be better correlated to the structural response of the bridge, that is useful for a proper fire resistance assessment and design.