Using reinforcement such as steel in concrete helps concrete resist tensile loads and prevent concrete from a sudden fracture. Reinforcement of concrete has its own characteristic graph which shows the proportion of the amount of steel which can be used to get a certain moment and axial load bearing from concrete. 

Using reinforcement such as steel in concrete helps concrete resist tensile loads and prevent concrete from a sudden fracture. Reinforcement of concrete has its own characteristic graph which shows the proportion of the amount of steel which can be used to get a certain moment and axial load bearing from concrete. 

simply, you can get the stiffness of the composite structure mainly from concrete section and ductility form steel section. 

Hi,

Concrete is good in compression and steel is good in tension. By joining the two materials together structurally these strengths can be exploited to result in a highly efficient and lightweight design. The reduced self weight of composite elements has a knock-on effect by reducing the forces in those elements supporting them, including the foundations.

In the case of columns, external concrete adds stability and fire resistance.