An index-based methodology for multi-hazard risk assessment of bridge portfolios

In recent years, several catastrophic collapses of existing bridges have highlighted the need for rapid risk analysis methods aimed at supporting infrastructure managers in the prioritization of detailed assessments, and if any, risk mitigation actions. A large percentage of existing road bridges were built between the 1960s and 1980s, and so many of them have already reached or even exceeded their design lifetime. Several studies have also statistically shown that bridges often suffer collapse due to either natural or human- related events such as floods, collisions, and overloading that, in addition to earthquakes, should be taken in due consideration in risk assessment. This calls for multi-hazard assessment methodologies, which provide a comprehensive view on the risk of bridge portfolios, allowing the identification of critical structures and effectively supporting decision-makers. This study proposes an index-based methodology for quantitative risk assessment of existing bridge portfolios at regional scale, considering multiple types of natural and anthropic hazards, as well as the potential impact of their out-of-service/collapse on the road network. The risk-based approach proposed herein allows incorporating the condition for which structural vulnerability and consequences of failure are significantly influenced by each hazard under consideration. Single-hazard risk indicators associated with traffic loading, seismic, fire, and impact hazards, while also accounting for the structural condition of each bridge, are combined into a multi-hazard risk indicator according to different approaches and compared to each other. The methodology is applied to a case-study portfolio of road bridges located in northern Italy, producing a risk-based ranking that is critically discussed. Analysis results are then compared to those obtained according to methods available in the literature and current guidelines for existing bridges.