A slender section buckles locally:
Try it first: pick an option, then read the explanation.
Correct answer: A. before reaching the yield moment
Asked in SSC JE 2012
Detailed Explanation
IS 800 classifies steel sections by the width-to-thickness ratio of their plate elements: plastic, compact, semi-compact and slender.
Why “before reaching the yield moment” Is Correct
In a slender (class 4) section the flange or web plates are so thin that they buckle locally before the yield moment is reached, so the section cannot even develop first yield.
| IS 800 class | Capacity reached before local buckling |
|---|---|
| Plastic (1) | Plastic moment Mp with large rotation (hinge forms) |
| Compact (2) | Mp, but little rotation capacity |
| Semi-compact (3) | Yield moment My only |
| Slender (4) | Buckles locally before My |
Why the Other Options Are Wrong
- B. as soon as it reaches the yield moment: That describes a semi-compact section (reaches My, then buckles).
- C. after reaching the yield moment: Compact and plastic sections go beyond My.
- D. as soon as it reaches the ultimate moment: Only plastic and compact sections reach the plastic (ultimate) moment.
Key Point
Slender section = class 4: local buckling before the yield moment.
Key Concepts for Students
- Classification depends on b/t and d/t ratios times ε = √(250/fy).
- Slender sections use an effective (reduced) section.
- Plastic sections are needed for plastic analysis.
Practice more: this is question 31 in Columns and Struts MCQs – Part 3. Try the whole set and check every answer instantly.