In which situation is a doubly reinforced beam preferred over a singly reinforced beam?

Question

Question

This question was previously asked in

RRB JE CBT 2 (Civil Engineering) ReExam Official Paper (Held On: 04 Jun, 2025)

When the beam is short and deep
When the beam is subjected only to compressive forces
When moments are expected to reverse, such as in multi-storeyed frames under lateral loads
When minimum reinforcement is required

Answer 1

Explanation:

Doubly reinforced beam

A doubly reinforced beam has steel reinforcement in both tension and compression zones, typically at the bottom and top respectively.
It is adopted when the beam section is restricted in size and cannot accommodate all the tension with steel at the bottom alone.
Also used when moment reversal is expected (like in earthquake-prone zones or multi-storey frames) where both faces of the beam undergo tension alternatively.
Compression reinforcement enhances ductility, resists reversal of stresses, and controls long-term deflection and cracking.

Additional InformationSingly Reinforced Beam

A singly reinforced beam has tension reinforcement only, placed in the bottom zone (in simply supported beams), where tensile stresses develop.
These beams are used where bending moment causes tension in only one zone, typically in cases of simply supported or cantilever beams.
Concrete resists compression, and steel resists tension; hence only the tension side is reinforced in this design.
It is economical and sufficient when the depth of the beam is adequate to resist the moment using only tension steel.