Risk-consistent calibration of load factors for the design of reinforced concrete bridges under the combined effects of earthquake and scour hazards

• A multi-hazard bridge design framework is proposed.
• A new multi-hazard probabilistic seismic demand model is developed.
• The joint failure probability is obtained by the multi-hazard convolution method.
• Load combination factors are determined based on the annual failure probability.
• The derived scour load factor is consistent with the practical LRFD methodology.

Current bridge design specifications deal with various extreme hazards independently, which may lead to less economic design and construction practices, and may also underestimate failure probabilities. Therefore, a multi-hazard bridge design framework is required to guide the future design of new bridges or the retrofit of existing ones. This paper lays the foundation toward a risk-based design approach to combine earthquake and scour hazards. First, the development of a new multi-hazard probabilistic seismic demand model is proposed, which is the basis for calculating a combined fragility surface as a function of earthquake and scour hazards. Then, the joint failure probability of the bridge can be obtained by convolving the combined bridge fragility surface with the earthquake and scour hazard curves at a given site. Load combination factors for design are then determined by comparing the joint failure probability and the failure probability of the bridge for a certain scour depth. Results for the case studies considered suggest the use of a scour load factor equal to 0.59 to combine with the earthquake hazard. This risk-consistent multi-hazard bridge design framework provides a basis for exploring combinations of earthquake and scour loads for additional bridge types and geometries, while being consistent with the practical load and resistance factor design (LRFD) methodology.