Mixed-Mode Fatigue and Fracture Assessment of a Steel Twin Box-Girder Bridge

Steel twin box-girder bridges are considered an attractive option in most of the world because of their basic design, simple form, and ease of construction. Despite their advantages, they are considered fracture critical, and as such, there is an additional mandate for the in-depth inspection of these bridges. This causes their inspection cost to be approximately two to five times greater than that of bridges with non-fracture-critical members. The required additional inspection in the United States is mainly driven by rare historical events of bridge collapse. In these events, the bridges were, however, not steel twin box-girder bridges. In addition, the mandated additional inspection does not reflect the inherent level of redundancy in most bridges. Therefore, it is important to quantify the potential for fracture and the level of redundancy in steel two-girder bridges to minimize their inspection cost. Recognizing the inherently large scatter in fatigue performance, the evaluation of crack propagation and the potential for fracture should be performed in a probabilistic manner using detailed models that represent accurate behavior of the bridge. In this study, a detailed numerical finite-element model of a steel twin tub-girder bridge was developed, and crack-growth analysis, potential for fracture of its main tubs, and its overall redundancy were evaluated. The crack-growth analysis was performed using multimode elastic fracture mechanics while accounting for uncertainties in the random variables associated with crack propagation and fracture. The results of the crack-growth analysis were utilized to develop fragility functions that specify inspection intervals versus probability of failure, where failure is characterized by dynamic crack growth. The analysis conducted to quantify the potential for fracture shows distinct possible failure modes that vary from brittle fracture to ductile fracture. The extreme loading case shows that the bridge overall is not at risk of collapse. It is important to note that this conclusion cannot be generalized for all tub-girder bridges because the level of redundancy is expected to vary between bridges depending on many factors, such as girder geometries, plate thickness, and fabrication, among others. The presented approach and the results provide a systematic method for evaluating fracture-critical bridges.