The R.C.C. beams in which the steel reinforcement is placed in the tension as well as compression zone are called as doubly reinforced beams. The moment of a balanced R.C.C. beam of dimension b x d is Rbd2. Sometimes due to head room constraints or architectural considerations the size of the beam is restricted and the same beam (b x d) is required to resist moment greater than Rbd2. There are only two ways in which it can be done.
- By using an over reinforced section.
- By using a doubly reinforced section
The option (i) is not a good choice because over reinforced sections are uneconomical and the failure of these beams is sudden without warning. Therefore, it is better to use doubly reinforced beam section in such circumstances. The extra steel provided in the tension and compression zone constitutes, the additional moment of resistance (greater than Rbd2) required.