Question

Point of contra-flexure is a

• A. Point where Shear force is maximum
• B. Point where Bending moment is maximum
• C. Point where Bending moment is zero
• D. Point where Bending moment is zero but also changes sign from positive to negative

Hint:

Do not confuse “zero bending moment” with “contra-flexure”. Contra-flexure occurs only when bending moment becomes zero {and} changes sign across that point.

Answer 1

The Correct Option is D

Step 1: Define the point of contra-flexure.
The point of contra-flexure is the point on a beam where the bending moment changes its sign.
This means the curvature of the beam changes from sagging to hogging (or vice-versa).
Step 2: Connect sign change with bending moment value.
For bending moment to change sign, it must pass through zero at some location.
So, at the point of contra-flexure:
- Bending moment is zero, and
- It changes sign across that point (positive to negative or negative to positive).
Step 3: Analyze the options.
(A) Point where Shear force is maximum: Incorrect, because contra-flexure is related to bending moment sign change, not maximum shear force.
(B) Point where Bending moment is maximum: Incorrect, because contra-flexure occurs at zero bending moment, not maximum.
(C) Point where Bending moment is zero: Incomplete, because bending moment can be zero at supports in simply supported beams, but that does not necessarily indicate a sign change.
(D) Point where Bending moment is zero but also changes sign from positive to negative: Correct, this captures both necessary conditions for contra-flexure.
Step 4: Conclusion.
Hence, the point of contra-flexure is correctly described by option (D).