The point of contraflexure is the point where:
Try it first: pick an option, then read the explanation.
Correct answer: A. bending moment changes sign
Asked in UKPSC AE (Paper I) 2007 / SSC JE 2012
Detailed Explanation
Along a beam, the bending moment may be sagging in one region and hogging in another. Between them it must pass through zero. The point where it changes sign is the point of contraflexure (or inflection), where the beam's curvature reverses.
Why “bending moment changes sign” Is Correct
By definition it is where the bending moment changes sign — option (a). It is common in overhanging, fixed and continuous beams.
Why the Other Options Are Wrong
- B. bending moment is maximum: The BM is maximum where the shear force is zero or changes sign.
- C. bending moment is minimum: A minimum BM is not the definition — the BM is zero and changes sign there.
- D. shear force is zero: Zero shear marks maximum or minimum BM, not contraflexure.
Key Point
Contraflexure = BM changes sign (BM = 0, curvature reverses).
Key Concepts for Students
- Bars in RCC beams are often curtailed or lapped near points of contraflexure.
- Simply supported beams under downward loads have no contraflexure.
- A point of inflection on the elastic curve coincides with the point of contraflexure.
Practice more: this is question 40 in Bending Moment and Shear Force MCQs – Part 3. Try the whole set and check every answer instantly.