Proportionality of interventions to restore structural safety of existing bridges

The proportionality of interventions on existing structures comprises a comparison between effort (cost) and benefit of interventions with the objective of an efficient use of means. This contribution discusses the evaluation of interventions to restore sufficient structural safety of existing bridges as encountered in real case applications. The considered hazard scenarios include accidental actions and one extreme live load event. For all hazard scenarios, structural safety check could not be fulfilled. Consequently interventions to restore structural safety were developed and their efficiency was analysed by a comparison of risk reduction with respect to safety costs. In addition, safety requirements, operational availability of the structure, magnitude of damage as well as the preservation of material and cultural values were considered. In three of the four cases the safety interventions turned out to be disproportionate. This paper presents a review of the chosen approach and assumed values and numbers used to estimate risk reduction and corresponding safety costs. Finally, issues are raised regarding the decision to implement or not the intervention to restore structural safety.