The doubly reinforced section is preferred in the following cases:
1) When the members are subjected to alternate external loads and the bending moment in the section reverse.
2) When the members are subjected to loading eccentric on either side of the axis.
3) When the members are subjected to shock impact or accidental lateral loads.
4) When overall size of beam section is limited.
5) When the beam section is continuous over several supports.
The theory of doubly reinforced section is based on the same assumptions as for singly reinforced section. In the steel beam theory of doubly reinforced concrete beams, it is assumed that
a) tension steel resists the tension and compression steel resists the compression,
b) stress in the compression steel is equal to to the stress in tension steel.
c) no stress is developed in tension and compression concrete.