To design a structural member made up of reinforced concrete it is very important to study the concept of Structural Analysis which describe how the structure will behave under different types of loads. The theory of reinforced concrete depends upon the bond between steel and concrete. Therefore the steel re-bar used are deformed or ribbed to make a strong bond and avoid chances of slip.
If we consider a simply supported beam subjected to downward forces, the beam will bend in such a way that the bottom fibres will have tensile force and the top fibre compressive. Therefore it is required to provide steel in the bottom fibres of such beams or slabs.