The factor of safety of steel is less than concrete because the percentage air void in the steel is very less in comparison of concrete, and the main cause of development of crack are voids because the surface area of the cross section get reduces due to the presence of void (Stress = force/area) and stress in the material get increase because area get reduced and the material tends to failure. On the basis of this concept the factor of safety (FOS) of steel is less because of very less percentage air void in comparison of concrete .