Difference Between Singly Reinforced and Doubly Reinforced Beam

1. For a Beam, It is necessary to provide Reinforcement(Steel Bars) in Compression and Tension zone.
2. In a beam, If the reinforcement is only in tension zone then it is called Singly Reinforcement Beam and if the reinforcement is both at Tension and Compression zone then it is called Doubly reinforced Beam. 
3. In both case, there will be rod in tension and compression zone. This is because, it is not possible to form a Beam structure without stirrups.
4. To hold the stirrups in standing position, it is necessary to place two reinforcement in compression zone of singly reinforced Beam.
5. However, those two will never carry or pass loads in its body and it is just dummy. 
6. In a Beam, the top section is called Compression zone and the bottom section is called Tension zone. 

1. Singly reinforced section; 
   Steel at bottom of section to take TENSION force.
   
   
   Doubly reinforced section;
   Steel is provided at top & bottom zone i.e (Tension & 
   Compression zone resp.)
   
   
   
   Reason:
   
   Steel provided in top zone reduces cross sectional 
   dimension, section is safe against REVERSAL moment.