Question

In a loaded beam, the point of contra flexure occurs at a section where

• A. bending moment is minimum
• B. bending moment is zero or changes sign
• C. bending moment is maximum
• D. shearing force is maximum

Hint:

"Contra" means opposite and "flexure" means bending.
Therefore, contraflexure literally translates to "opposite bending", representing the location where the bending switches direction, which mathematically means the bending moment must pass through zero.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the physical definition and structural condition associated with the "point of contraflexure" in a loaded structural beam.

Step 2: Detailed Explanation:

• Bending Moment Behavior in Beams:
Under different transverse loads, beams experience bending.
The curvature of the beam can bend downwards (sagging, positive bending moment) or upwards (hogging, negative bending moment).

• Point of Contraflexure:
In continuous or overhanging beams, there are regions where the nature of the bending moment transitions from sagging to hogging, or vice versa.
The point where the bending moment diagram crosses the zero axis—meaning the bending moment is exactly zero and changes its mathematical sign—is defined as the point of contraflexure (or point of inflection).

• Structural Significance:
At this location, the curvature of the elastic curve of the beam changes its direction of concavity.
This point is highly critical in concrete structural design because the placement of tensile reinforcement (rebar) must switch from the bottom of the beam (for sagging) to the top of the beam (for hogging) around this region.

Step 3: Final Answer:
The point of contraflexure occurs at a section where the bending moment is zero or changes sign.