TUTORIALS

Process for Design of Double RC (Reinforced Concrete) Beam

The neutral axis divides the beam section into 2 zones, which are compression zone and tension zone. Therefore, the steel utilized in tension zone is called tensile steel, while steel utilized in compression zone is called compressive steel.

Therefore, at the time steel is utilized in only in tension zone, the beam is called single RC beam. At the time steel is used in both zones, the beam is called doubly RC beam. At the time a beam cross section is limited because of architectural or other type of consideration, it could occur that concrete can’t develop the compression force needed to oppose the given bending moment.

In such case, reinforcing steel bars are added in compression zone, which is known as doubly reinforced beam having compression and tensile reinforcement. One more reason for placing bars in compression zone is that at the time the beam is continuous beam, both negative and positive bending moment are going to exist.

Positive moment exists at all the points and as a result, the main tension reinforcement is going to be placed at the bottom of the beam. Negative moment exist at the bottom of the beam. The tensile reinforcement is going to be placed close to the top of the beam.

Usefulness of compression bars:
a. Such bars act as stirrup support beams and support shear reinforcement.
b. Such bars can put minimum limit on bending moment.
c. Such bars decrease long-term member deflection.

Parts of double RC beam:
Neutral axis: Denoted by ‘NA’, it is an axis in the beam cross section along which bending stresses are zero.

b is the breadth of the beam,
As is steel area in tension zone,
A’s is steel area in compression zone,
d is effective depth of tension steels,
d’ is depth of compression steel with respect to compression fibre,
h is total height of the beam.

Let’s check out the procedure:

A) Suppose d = h – 3 inches
B) And compute total design moment = Mu
C) We are going to verify if doubly RC beam is needed or not.
D) Maximum moment for single RC beam = Maximum ϕ MN = ϕ bd2k (k is design coefficient)
E) In case ϕ MN ≥ Mu the beam will be designed as single RC beam. In case ϕ MN < Mu the beam will be designed as double RC beam.
F) Offer a concrete steel couple with steel ratio, ρ = 0.9, ρMAX = 0.9(0.75ρb).
By making use of the steel ratio ‘ρ’, we compute the value of k
G) We are going to find out the moment capacity of concrete steel couple.

H) We are going to find the remaining moment that should be opposed by steel – steel couple
I) Find the needed compressive force in the steel
J) In case εs’ ≥ εy , the compressive steel has relinquished at the ultimate moment and stress at compression is equal to yield stress. Find out stress using Hook’s law.
K) It will assist up us to find out area of steel in compression zone. After that, we will find out the number of bars.
L) Then, we find out the total tension steel area.
M) Then, choose number of bars for compressive steel and bar number for tension steel also.
N) Lastly, make sure the section is all right or not. Also, sketch the design.

To learn details process, go through the following construction video tutorial.

Lecturer: SL Khan