Structural Redundancy Analysis for Continuous Steel Two-Girder Bridges: Evaluation of Capacity and Failure Modes

This study presents structural redundancy evaluations of three two-girder steel bridge systems using finite element analysis to assess their capacity and behavior under fracture conditions. Following AASHTO System Redundant Members (SRM) Guide Specifications, detailed three-dimensional finite element analysis (FEA) models were developed in ABAQUS to simulate member fractures at critical locations and evaluate bridge responses under Redundancy I and II load combinations. The analysis includes two-girder bridges using geometries typical of real bridge applications. Multiple fracture scenarios across different spans were evaluated to identify controlling limit states and failure modes. Results demonstrate that two-girder systems may have redundancy challenges as a result of limited load sharing between intact and fractured girders. This limitation results from low torsional stiffness and relatively flexible lateral bracing and floor beams, which severely restrict load redistribution. When one girder fractures, the remaining intact girder carries substantially increased loads, causing the fractured girder to behave as a cantilever beam. The controlling failure mode was identified as buckling at section changes near piers on fractured girders, with high displacement differences between intact and fractured girders. These behaviors may reduce reserve capacity below Redundancy I and II thresholds. Analysis revealed that the evaluated two-girder bridges did not meet SRM qualification requirements because of inadequate load transfer mechanisms between the two main load-carrying members. This research provides bridge engineers with critical guidance on the need for careful evaluation when two-girder systems are evaluated for redundancy performance.