Fatigue Performance and Shear Demand Distributions of Clustered Shear Connectors in Composite Bridge Girders

The current American Association of State Highway and Transportation Officials (AASHTO) Bridge Specifications assumes uniform shear flow demands at the steel-concrete interface of composite bridge girders. As stud pitch increases to beyond 24 in or as studs become clustered to account for pre-cast concrete decks, this assumed shear demand distribution may be unrepresentative. Understanding shear transfer and resulting demands on headed studs in composite beams are important for ensuring adequate composite design. This study investigates stud demands in composite bridge girders using large-scale fatigue testing and direct pressure measurements for stud force calculations. In this study, two large-scale composite beam specimens were fatigue tested to determine the effects of stud clustering on stud shear demands and fatigue life. One additional non-composite beam specimen was also fatigue tested to determine potential composite action performance and degradation following fatigue loading. All composite specimens were designed based on the stud strength limit state resulting in an expected finite fatigue life. Studs within the composite test specimens were instrumented with transverse pressure gauges capable of measuring concrete contact forces. Results from the two composite beam tests indicated that stud shear demands were lower than the AASHTO estimations (fatigue life exceeded code expectations by over 250%). Stud pressure measurements during fatigue testing indicated stud demands that were nearly 66% lower than those estimated by AASHTO. From the pressure measurements it was observed that the exterior rows of clustered shear studs felt a higher shear force than interior studs. Results from the non-composite specimen indicated composite behavior through alternative shear transfer mechanisms as a shift in the steel beam neutral axis toward the concrete slab was observed