Question

With reference to shear force and bending moment diagrams, along the length of a beam subjected to loads, at the point of contraflexure, bending moment is:

• A. Zero
• B. Constant
• C. Minimum
• D. Maximum

Hint:

Think of "Contraflexure" as "Counter-Flexing." It is the spot where the beam stops curving one way and starts curving the other. To make that switch, the internal bending stress must momentarily hit zero.

Answer 1

The Correct Option is A

1. Definition and Significance: A point of contraflexure occurs at a section along the beam where the nature of the bending moment changes. This means the bending moment transitions from sagging (positive) to hogging (negative), or vice-versa.

2. Mathematical Condition: Because the bending moment is changing its mathematical sign (crossing the zero line on a graph), it must pass through a value of

zero at that exact point. $$\text{At point of contraflexure, } M = 0$$

3. Structural Behavior: At this location, the curvature of the beam's elastic curve changes its direction. For example, the top fibers of the beam transition from being in compression to being in tension. In reinforced concrete design, this point is critical for determining where to terminate or bend reinforcement bars. Note: While the bending moment is zero at the point of contraflexure, the shear force at this same location is generally not zero.