Direct Redundancy Evaluation of Bridges Designated as Fracture-Critical

This paper describes the procedure recommended in two specific standards to define and quantify structural redundancy using a direct analysis of bridges. The procedure is illustrated using two typical bridge configurations. The first example is a simply-supported truss bridge superstructure. The second example is a continuous three-span two-girder steel box bridge. These examples are selected because these types of structures are generally considered to be fracture-critical nonredundant bridges. The object of analysis is to investigate the reserve strength redundancy of the structures, defined as their ability to continue to carry loads after the limiting strength of one member is reached. The analysis also investigates the loads that the structures could still carry after brittle damage to one of their members. The results of the analysis show that both structure types can provide adequate levels of redundancy for overloading, assuming that the bridge members have been designed to satisfy the applicable specifications. Also, the analysis demonstrates that neither bridge is necessarily fracture-critical in the traditional sense because the failure in any of their steel tension members is not expected to result in a partial or full collapse of the bridge. However, the truss bridge may be labeled as “damage-critical” because the failure of a compression chord would significantly reduce the bridge’s ability to carry vehicular traffic in a damaged state.