Design of Double Reinforced RCC Beams

R.C.C Beams

R.C.C beams are cast in cement concrete reinforced with steel bars. Beams take up compressive and add rigidity to the structure.

Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (i.e., loads due to an earthquake or wind). The loads carried by a beam are transferred to columns, walls, or girders, which then transfer the force to adjacent structural compression members. In Light frame construction the joists rest on the beam.

Doubly Reinforced Beam

In this article, we are going to discuss types of beam construction and RCC design of Doubly reinforced beam…

RCC beam construction is of two types:

• Singly reinforced beam
• Doubly reinforced beam

Singly reinforced beam

A singly reinforced beam is a beam provided with longitudinal reinforcement in the tension zone only.

Doubly reinforced beam

• Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. This type of beam will be found necessary when due to head room consideration or architectural consideration the depth of the beam is restricted.

• The beam with its limited depth, if reinforced on the tension side only, may not have enough moment of resistance, to resist the bending moment.

• By increasing the quantity of steel in the tension zone, the moment of resistance cannot be increased indefinitely. Usually, the moment of resistance can be increased by not more than 25% over the balanced moment of resistance, by making the beam over-reinforced on the tension side.

• Hence, inorder to further increase the moment of resistance of a beam section of unlimited dimensions, a doubly reinforced beam is provided.

Besides, this doubly reinforced beam is also used in the following circumstances:

• The external live loads may alternate i.e. may occur on either face of the member.

For example:

• A pile may be lifted in such a manner that the tension and compression zones may alternate.

• The loading may be eccentric and the eccentricity of the load may change from one side of the axis to another side.

• The member may be subjected to a shock or impact or accidental lateral thrust.

Design procedure for doubly reinforced beam

Step 1

Determine the limiting moment of resistance for the given c/s(Mulim) using the equation for singly reinforced beam

M_ulim = 0.87.f_y.A_st1.d [1 – 0.42X_umax]

Or

Balanced section

A_st1 = (0.36.f_ck.b.X_umax)/(0.87fy)

Step 2

If factored moment M_u > M_ulim, then doubly reinforced beam is required to be designed for additional moment.

M_u – M_ulim = f_sc.A_sc (d – d’)              [f_sc value from page no. 70]

Step 3

Additional area of tension steel Ast₂

Ast₂ =Asc.fsc/0.87fy

Step 4

Total tension steel Ast, Ast = Ast₁ + Ast₂