Doubly Reinforced Beam

A doubly reinforced beam carries steel in both the tension and the compression zones. It is adopted when the factored moment exceeds the limiting moment of resistance Mu,lim of a singly reinforced section of the available size — i.e. the section is depth-restricted (architectural or headroom constraints) and cannot simply be made deeper.

Design per IS 456 Annex G splits the moment: the balanced part is resisted as a singly reinforced section (Mu,lim), and the excess (Mu − Mu,lim) is resisted by an internal couple of additional tension steel and compression steel Asc placed near the top, the latter checked for the stress fsc at the strain corresponding to its depth d′. Compression steel also markedly improves long-term deflection, ductility and the moment-redistribution capacity, and is mandatory at supports of continuous beams and in ductile detailing (IS 13920).