Bending Moment in Beams

The correct answer is option 'D', which states that the couple moment produced in the beams is called the bending moment. The bending moment is a key concept in structural engineering, specifically in the analysis and design of beams.

Bending Moment Definition:
The bending moment at a particular section of a beam is defined as the algebraic sum of the moments about that section due to all the external forces acting on either side of the section. It represents the tendency of a beam to bend or deform under the action of external loads.

Explanation:
To understand why the correct answer is the bending moment, let's break down the concept further.

1. Beams and Load Distribution:
- Beams are structural members that are primarily designed to resist bending and carry loads.
- When a beam is subjected to external loads, such as point loads or uniformly distributed loads, it undergoes internal forces and moments.

2. Internal Forces and Moments:
- The internal forces in a beam include axial force (compression or tension) and shear force.
- The internal moments in a beam are called bending moments, which cause the beam to bend or deform.
- Bending moments are produced due to the couple of forces acting on either side of a section in the beam.

3. Bending Moment Diagram:
- The bending moment at any section of a beam can be determined by constructing a bending moment diagram.
- The diagram represents the variation of bending moment along the length of the beam.
- The bending moment diagram is essential for understanding the internal forces and moments in the beam and designing it accordingly.

4. Sign Convention:
- In structural engineering, a sign convention is followed for determining the positive or negative sign of bending moments.
- Conventionally, a positive bending moment is considered as sagging (concave up) and produces compressive stresses at the top and tensile stresses at the bottom of the beam.
- A negative bending moment is considered as hogging (concave down) and produces tensile stresses at the top and compressive stresses at the bottom of the beam.

Conclusion:
In summary, the couple moment produced in beams is called the bending moment. The bending moment represents the tendency of a beam to bend or deform under the action of external loads. Understanding the bending moment is crucial in the analysis and design of beams to ensure their structural integrity.