Which of the given options represents a doubly reinforced beam?
- ASingle layer of reinforcement in only compression face
- BAt least single layer of reinforcement in both tension and compression faceCorrect
- CTwo layers of reinforcement in only tension face
- DDouble layer of reinforcement in only compression face
Solution
Concept
A doubly reinforced beam has steel in both the tension zone and the compression zone.
Step-by-step solution
1. A singly reinforced beam has steel only in the tension zone. 2. A doubly reinforced beam has steel in both the tension and compression zones. 3. The number of layers does not matter. What matters is steel on both faces. 4. So the answer is option B.
Explanation
A beam is called doubly reinforced when it has main steel on both the tension face and the compression face. Compression steel is added when the section depth is limited and the moment is more than Mu,lim. Many layers on one face only still make a singly reinforced beam.
Why the other options are wrong
A: Steel only in the compression face does not resist tension, so it is not a proper RCC beam. C: Two layers only in tension is still a singly reinforced beam. D: Steel only in the compression face is not doubly reinforced.
Shortcut
Doubly = steel on both faces (tension + compression), not two layers.
Common mistake
Thinking 'double' means two layers of bars on one face.
Correct answer: B) At least single layer of reinforcement in both tension and compression face