Evaluating the Redundancy of Steel Bridges: Effect of a Bridge Haunch on the Strength and Behavior of Shear Studs under Tensile Loading

AASHTO defines a fracture critical member (FCM) as a component in tension whose failure is expected to result in the collapse of the bridge. Bridges with FCMs must be inspected more frequently for this reason, which can lead to greater cost during the life of the bridge and a general reluctance to design new bridges with FCMs. However, evidence has shown that certain bridges with FCMs have redundant load paths and can withstand a fracture to an FCM.

There are many twin steel box girder bridges across the state of Texas, all of which are considered to be fracture critical because it is assumed that a fracture in one girder will initiate a total bridge collapse. In order to prevent collapse after the fracture of one box girder, the load that had been resisted by that girder must be transferred to the intact girder. The fractured girder will deflect so that the shear studs are loaded in tension and the deck slab is bending in double curvature. The shear studs and deck slab must both have the capacity to transfer the force over to the other girder.

The governing failure mode for the studs loaded in tension is a concrete breakout failure. This is a brittle failure in which the studs pull out with a large prism of concrete. When making these calculations, it was discovered that the bridge haunch may greatly reduce the concrete breakout strength of a single row of studs because it creates an edge effect. In order to determine the exact effect that the bridge haunch has on the tensile capacity of the shear studs, a series of laboratory tests were performed on bridge deck sections with and without a haunch.

The results of the laboratory tests showed that the bridge haunch greatly reduces the capacity of a row of studs grouped transversely across the top flange. More importantly the specimens with a haunch exhibited no ductility at failure, which may prevent redistribution of load during a fracture event. The specimens without a haunch did not suffer a reduction in strength when multiple studs were grouped across the flange width because there was no edge effect. In addition these specimens exhibited some ductility at failure because the studs extended into the bottom reinforcement mat, which forced the reinforcement bars to intersect the breakout failure plane.

The haunch is a necessary part of bridge construction, and despite the negative effects it has on the tensile behavior of the studs, it cannot be eliminated. With this in mind a series of techniques to improve the tensile behavior of the studs are recommended. These techniques include using haunch reinforcement bars, spacing studs longitudinally rather than grouping the studs transversely across the flange width, using longer studs, and developing a reduced diameter shear stud that will make yielding of the studs the governing failure mode. Yielding of the studs is the ideal failure mode because it would allow for the most redistribution of load during a fracture event.