Doubly Reinforced Beam

A doubly reinforced beam is a reinforced concrete beam that has reinforcement in both the compression and tension zones. The reinforcement in the tension zone is called the main reinforcement and the reinforcement in the compression zone is called the compression reinforcement.

Economy of Doubly Reinforced Beam

The economy of a doubly reinforced beam is determined by comparing it with a singly reinforced beam, where reinforcement is provided only in the tension zone. The economy of a doubly reinforced beam is considered less than that of a singly reinforced beam because of the following reasons:

Compressive Steel Under-Stressed

The compressive steel in a doubly reinforced beam is under-stressed because the depth of the compression zone is less than the depth of the tension zone. As a result, the compression steel is not able to develop its full strength, which leads to wastage of steel and increases the cost of construction. This is the main reason why a doubly reinforced beam is considered less economical than a singly reinforced beam.

Tensile Steel Requirement

The amount of tensile steel required in a doubly reinforced beam is more than that required in a singly reinforced beam. This is because the depth of the tension zone in a doubly reinforced beam is more than that in a singly reinforced beam. As a result, more tensile steel is required to resist the bending moment.

Shear Reinforcement

The amount of shear reinforcement required in a doubly reinforced beam is also more than that required in a singly reinforced beam. This is because the depth of the compression zone in a doubly reinforced beam is less than that in a singly reinforced beam, which reduces the shear capacity of the beam.

Concrete Not Stressed to Its Full Value

The concrete in a doubly reinforced beam is not stressed to its full value because the amount of compression steel is more than that required for a balanced section. This results in an increase in the cost of construction, as more concrete is required to cover the additional compressive steel.

Conclusion

In conclusion, a doubly reinforced beam is less economical than a singly reinforced beam because of the under-stressed compressive steel, the increased requirement of tensile steel and shear reinforcement, and the concrete not being stressed to its full value. Therefore, the design of a doubly reinforced beam should be avoided unless it is absolutely necessary.