**Doubly Reinforced Concrete Beam **

Analysis of Doubly reinforced concrete beam

Although concrete has high compressive strength, there might be a need to provide steel in the compression zone also in some circumstances e.g. reducing the size of cross-section, deflection control, difficulty in placing all the tension rebar within the width of the singly reinforced beam even by using two or more layers. A beam which has steel rebar in tension as well as compression zone is known as doubly reinforced concrete section. The compression reinforcement will do the following;

(i) increase in the the ductility of the beam .

(ii) raise the neutral axis and the depth of compression stress block

(iii) increase in the strain of tension reinforcement.

The nominal moment strength of a doubly reinforced concrete beam can be found in the manner given below;

With reference to the equivalent stress diagram as per Whitney,

**Force of tension due to tension rebar**;

T = (area tension rebar) * (stress of steel)

**T = ****A**_{s} *
f*

It is required that tension steel has yielded when failure takes place. This makes sure that the section is not controlled by concrete.

**Force of compression due to concrete**;

**C**_{C}** = 0.85***f _{C}^{'
}b a
acting at a/2 from the top edge *

* ***Force of compression due to compression
rebar**;

**C**_{S}** = A**_{S}^{'}** (****f**_{s}^{'
}**- 0.85f**_{c}^{'
}) acting at distance "d^{'} " form the
top edge.

* *for equilibrium of the section;
Compression = Tension.

**T
= C**_{C}**
+ ****C**_{S}

The above equation will give the value
of "**a**", and hence the nominal moment strength is
calculated by taking the moment of both the compressive forces about
the centroid of tension rebar area.

**M**_{n}** = C**_{c}** (d - a/2)
+ C _{S}
(d - d^{'})**

To learn more about calculating the strength of reinforced concrete beam please
visit solved
**
Example 9-1 ,
Example 9-2,
Example 9-3**

You can also use our free **online calculator for strength of Reinforced Concrete beam**.

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