In a doubly reinforced beam, reinforcement is provided in the tension zone and compression zone. (as shown in the above figure).
The moment of resistance can not be increased by increasing the amount of steel in the tension zone. It can be increased by making the beam over-reinforced but not more than 25 percent on the strained side. Hence, a doubly reinforced beam is provided to enhance the moment of resistance of a beam having restricted dimensions.
A doubly reinforced beam increases the moment of resistance by providing steel in both tension and compression zone in the beam. The depth of this type of beam is restricted to fulfill architectural requirements and building serviceability.
Purpose to Provide Doubly Reinforced Beam
When the cross-section of the beam can not increase due to architectural considerations then we use a doubly reinforced concrete beam. In this type of beam, steel bars are provided in the compression zone to improve their strength.
Doubly reinforced beam provided a high moment of resistance due to steel in compression and tension zone. Singly reinforced concrete beams have less moment of resistance for the same cross-section, steel grade, and concrete.
Another purpose to provide a doubly reinforced beam is that it prevents long-term deflection of the concrete beam.
When Doubly Reinforced Beam is Provided?
It is suitable for the following conditions.
- If the size of the beam is confined.
- If the section of the beam is subjected to inversion stress.
- If the beam is nonstop more than a few backings.