What describes composite action between a concrete deck and a steel girder and what is required to achieve it?

• A. Composite action occurs when no interaction occurs between steel and concrete.
• B. Composite action requires chemical bonding between deck and girder; bolts alone are enough.
• C. Composite action occurs when deck and girder act as a single unit without any connectors.
• D. Composite action between a concrete deck and steel girder occurs when shear connectors transfer shear and ensure full composite behavior.

Answer: (D)

Composite action means the concrete deck and steel girder work together as a single bending unit, which requires transferring shear between the two so they don’t slip relative to each other. This transfer is achieved with shear connectors, such as headed studs welded to the steel and embedded in the concrete deck. When these connectors are properly spaced and bonded, the deck and girder share bending stresses, increasing overall stiffness and strength because the concrete handles compression while the steel carries tension. Without these connectors (or with only relying on chemical bonding or bolts without shear transfer), slip would occur and the two members would not act as a single unit. So the essential point is that shear connectors transfer shear and guarantee full composite behavior.