When do we need doubly reinforced beams ?

As the name implies doubly reinforced beams means we are providing the reinforcement in both the zones. In the case of a beam the zone is basically compression and tension zone. At the analysis stage if the value of moment of resistance is less than the required bending moment and also if there is a restriction in breadth or depth of section because of architectural constraints. Then the way out for this problem is :

• We can think of a over reinforced section
• We can go for a doubly reinforced section

Over reinforced section

An over reinforced section is a type of section in which we use the concrete for its ultimate compressive strength strength. The yield strain of concrete is reached before the ultimate strength of steel. Over reinforced beam section undergoes a compressive failure. The percentage of tensile reinforcement is more than the amount of reinforcement provided for a balanced section. As in this type of section concrete fails first i.e it undergoes a brittle failure. The main disadvantage of this section is it undergoes a sudden failure without warning. We will not advice to select this type of section for the structure as it adds an extra cost, by increasing the percentage of reinforcement.

It is always advisable to go for a under reinforced beam section as it is safer when we compare all the three beam section. Because of its ductile failure it provides sufficient time before collapse and reduces the intensity of hazard.

Doubly reinforced section

When we provide reinforcement in compression and tension zone, such beams are called as doubly reinforced beam. The extra value of moment of resistance (M – Mu,lim) is taken care by this additional steel which we provide in the compression zone.

Different conditions where we go for doubly reinforced beam sections:

• The dimension of the beam is a constrain, it may be because of any reason (architectural or for head space).
• The loads are reversible in nature, and may cause tension in both the faces of beam.