What is singly and Doubly Reinforced beam?

A Singly reinforced beam as the name suggests is a beam that can reinforce longitudinally only in the stress zone. Reinforcement carries the bending moment and the stress caused due to bending moment whereas concrete carries the compression. It is necessary to link the stirrups and because of this, it is not possible to provide the reinforcement only in the tension zone. Therefore, two rebars are being used to tie the stirrups in a compression zone

In addition to the tension reinforcement, the doubly reinforced beam also possesses compression reinforcement (on both sides of the beam). So, a beam is considered a doubly reinforced beam if it has both tension in the steel and the compression zone. The doubly reinforced beam is given when the depth/dimensions are limited as reinforcement only on the tension side is not able to endure the bending moment. Thus, a doubly reinforced beam is used in these situations to increase the strength of a beam.