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Doubly reinforced beam is defined as the beam in which the reinforcement is provided by the steel in both tension and compression zone of the beam. If it is known that the depth of the beam is fixed, then the best option is to provide the doubly reinforced beam for resisting the particular moment.
If the depth of the beam is restricted and it requires to increase the moment of resistance given by the bending member, there are mainly two options as below.
• The moment of resistance can be increased by designing the beam as over reinforced. This can be made only up to an extent of 25% on the strained side. This is done because with the increase in the amount of steel in the tension zone, the moment of resistance will not increase.
• The moment of resistance can be increased by providing the reinforcement in both tension and compression zone which is known as doubly reinforced beam.
The over reinforced beam is uneconomical, as it undergoes the brittle failure or sudden failure. Hence, doubly reinforced beam is preferred over the over reinforced beam.
The stress-strain variation for the doubly reinforced beam occurs.
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