Doubly reinforced beam
Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. This type of beam will be found necessary when due to head room consideration or architectural consideration the depth of the beam is restricted.
The beam with its limited depth, if reinforced on the tension side only, may not have enough moment of resistance, to resist the bending moment.
By increasing the quantity of steel in the tension zone, the moment of resistance cannot be increased indefinitely. Usually, the moment of resistance can be increased by not more than 25% over the balanced moment of resistance, by making the beam over-reinforced on the tension side.
Hence, inorder to further increase the moment of resistance of a beam section of unlimited dimensions, a doubly reinforced beam is provided.
Besides, this doubly reinforced beam is also used in the following circumstances:
The external live loads may alternate i.e. may occur on either face of the member.
A pile may be lifted in such a manner that the tension and compression zones may alternate.
The loading may be eccentric and the eccentricity of the load may change from one side of the axis to another side.
The member may be subjected to a shock or impact or accidental lateral thrust.