Bridge Failure during the October 2018 Flooding in Iran

The significant rainfall in October 2018 in northern Iran caused devastating flooding that caused considerable economic losses and claimed at least nine lives. Following the flooding, several bridges have suffered different levels of damage. Immediately after the event, the authors conducted a field investigation to study the performance of RC bridges during the flood. This paper presents the failure modes of flood-affected bridges and provides reasons for every failure case. The results revealed that scour around foundations, pier tilting, flexural failure of piers, and superstructure unseating were the most significant failure modes observed. Pile-supported bridges performed notably better than those with shallow foundations. In addition, observations have revealed that structural integrity plays a crucial role in the general stability of bridges during flooding. Pier ductility is another significant parameter that affects the performance of bridges during flood events. Finally, fragility curves were developed based on the empirical data set provided from the post-flood status of the bridges in the affected region. The river flow velocity ($V_s$), maximum flow discharge ($Q_p$), and $Q_p$ normalized to the 10-year average river discharge ($Q_m$) were considered intensity measures of the fragility analysis. The correlation analyses revealed that despite $V_s$ and $Q_p/Q_m$, the flow discharge was not an appropriate intensity measure for generating fragility curves.