Question

1_List the circumstances in which a singly reinforced and doubly reinforced beam sections are used in a structure.

2_State Necessity of Reinforcement in R.C.C Structures.

Answer 1

DOUBLY REINFORCED BEAM:

The beams in which reinforcement is provided in both compression and tension zones are known as doubly reinforced beams. the compressive forces are handled by both concrete and reinforcement in the compression zone.

SINGLY REINFORCED BEAM:

A singly reinforced beam is the beam in which the longitudinal reinforcement is provided only in the tension zone. whereas Compressive forces are handled by the concrete in the compression zone of the beam.

Circumstances of the singly reinforced beam;

From the architectural point of view, if the specified width and depth are sufficient for withstanding the external bending moment by providing reinforcement only in the tension zone then the singly reinforced beam is provided.

Circumstances of the Doubly reinforced beam;

  1. When the size of the beam (both width and depth) of the beam is restricted. (singly reinforced beam is not enough in withstanding external loads by providing reinforcement only in tension zone)

2. When the moment of resistance required is too high that it results in very large beams(depth) if designed as a singly reinforced beam.

3.   When there is a reversal of load in beams(due to earthquake loads, impact loads, etc)

4 The deflection due to shrinkage effects can be reduced by designing doubly reinforced beams ( since singly reinforced beam results in differential shrinkage thereby causing larger deflections)

NECESSITY OF REINFORCEMENT IN RCC BEAMS;

RCC MAIN COMPONENTS :

1.CONCRETE: concrete is very good in resisting compressive forces but it has negligible tensile strength.

2 REINFORCEMENT BARS: it is good at taking tension as well as compression.

Reinforced concrete is designed for three types of stresses:

1.tensile,

2.compressive,

3.shear & torsion.

So the primary role of reinforcements is to provide what is lacking in concrete alone. Steel is a strong material when talking about tensile forces in which concrete lacks. Basically, reinforcements in RCC carry almost all the tensile forces that occur within a structure.

However, steel has high compressive strength as well. Compressive forces occurring within a structure is shared by concrete and steel reinforcements.

To summarize, reinforcements in RCC compliments the tensile/flexural capacity and supplements the compressive, shear and torsional capacity of a structure

When it comes to shear and torsional forces, transverse reinforcements (e.g. stirrups, ties, spirals) are provided and supplement the shear and torsional capacity of a structure.