Learn : Necessity of Doubly reinforced beam, Doubly reinforced beam


Doubly reinforced beam

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 resistance of a balanced R.C.C. beam of dimension b×d is Rbd2 Sometimes due to head room constraints or architectural considerations the size of the beam is restricted and the same beam (b×d) is required to resist a moment greater than Rbd2 There are only two ways in which it can be done.

(i)        By using an over reinforced section.

(ii)       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 beam is sudden without warning. Therefore, it is better to use doubly reinforced beam section in such circumstances. The extra provided in the tension and compression zone constitute, the additional moment of resistance (greater than Rbd2) required.


Necessity of  Doubly reinforced sections due to the following conditions :

  1. When the dimensions (b×d) of the beam are restricted due to any constraints like availability of head room, architectural or space considerations and the moment of resistance of singly reinforced section is less than the external moment.
  2. When the external loads may occur on either face of the member i.e., the loads are alternating or reversing and may cause tension on both faces of the member.
  3. When the loads are eccentric.
  4. When the bam is subjected to accidental or sudden lateral loads.
  5. In the case of continuous beams or slab, the sections at supports are generally designed as doubly reinforced sections.