What is double R.C. beam?
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The beam, which is reinforced from both zones i.e., compression and tension, that beam is said as Doubly Reinforced beam. It is usually given when the depth of the beam is constricted. It is obviously not safe to provide a singly r/f beam with min depth resulting in failure due to insufficient resistance to oppose the bending moment.
Moment of resistance could not be increased by increasing the steel in the tension zone. Rather it can be increased by increasing the reinforcement, but it should not exceed 25% of the strained side. That’s why a doubly reinforced beam is provided to increase the moment of resistance of a beam having limited dimensions.
Double Reinforced beam or doubly reinforced beam is the beam having reinforcements both the bottom and top.
The singly reinforced beam has tension reinforcements placed only at the bottom of the beam. In a singly reinforced beam, tension is carried by steel, and compression is carried by the concrete.
In doubly reinforced beams, tension reinforcements are provided at the bottom and the compression reinforcements are provided at the top. This type of beam is mostly used when the depth of the beam is restricted.
Doubly Reinforced Beam:
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.
Doubly reinforced sections are used in the following conditions: 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.