How does composite action in bridges affect coefficient of thermal expansion and movement?

• A. Composite action helps synchronize deformations between steel and concrete, reducing differential movement and improving overall thermal response, provided expansion provisions are maintained.
• B. Composite action increases differential movement due to material mismatch.
• C. There is no effect of composite action on thermal movement.
• D. Composite action eliminates thermal movement completely.

Answer: (A)

Composite action makes the steel and concrete behave as a single unit under temperature changes. When the temperature rises or falls, both materials want to expand or contract, but their natural rates differ. Bonding them together with shear connectors forces the steel and concrete to share the same strain, so the differential movement between the two is minimized. This shared deformation changes the stiffness distribution of the section, helping the bridge respond more uniformly to thermal effects rather than one material pulling or pushing on the other. Expansion provisions, such as joints and bearings, are still needed to accommodate overall movement of the structure, but the presence of composite action reduces the relative movement between steel and concrete and improves the thermal response. Without proper expansion provisions, restrained movement could still generate higher stresses, even in a composite section.