Evaluating the Redundancy of Steel Bridges: Improving the Strength and Behavior of Shear Stud Connections under Tensile Loading

A fracture critical member in a bridge is a structural element whose failure is expected to result in collapse of the bridge. Bridges which contain fracture critical elements are required to be inspected frequently and in detail, at great expense to the owner. Evidence suggests however, that many bridges lassified as ‘fracture critical’ have substantial redundancy to overcome the loss of a fracture critical member.

Twin box girders are a type of fractural critical bridge frequently used in Texas, and for this research the Texas Department of Transportation sponsored a full-scale test of a twin box girder bridge to evaluate its redundancy. As part of the analysis to evaluate the capacity of the bridge’s redundant load paths, it was determined that the shear studs connecting the fractured girder to the bridge deck play a crucial role in the performance of the fractured bridge system. Once damaged, the fractured girder loses its stiffness and hangs from the bridge deck, loading the studs in tension. Due to the presence of the haunch over the bridge top flange, no reinforcement runs through the breakout cones of the shear studs, meaning no ductility can be mobilized to distribute the girder’s weight among the studs. Also, the haunch has the potential to greatly reduce stud breakout strength because of an edge effect between the studs and the haunch. Studs spaced closely to the edge of a haunch, as in current standard stud details, create stress concentrations in the concrete leading to premature cracking and very low tensile strengths.

A series of tests were performed on full-scale sections of shear stud details similar to those in the bridge deck, with and without a haunch. Loaded in tension, the tests confirmed that the haunch significantly reduced strength and had very limited ductility. Based on these results, it was clear that a stronger, more ductile detail was needed to improve connection behavior.

This research tests stud configurations spaced longitudinally down the bridge web, taller studs in a haunch, and dynamic loading of stud specimens to evaluate their strength and ductility. It is found that longitudinal spacing of studs greatly increases their strength over the traditional transverse spacing. Furthermore, when the studs are tall enough to engage reinforcing steel, significant ductility is achieved, even when a haunch is present. Dynamically loaded studs have a higher strength and slightly lower ductility compared to similar studs loaded statically. With slight modifications developed from this research, the existing ACI code equations are used to predict the strength of these alternate shear stud configurations. Implementation of ongitudinally spaced studs tall enough to engage reinforcement in twin box girder bridges will help ensure sufficient redundancy to reduce the need for the fracture critical designation.