The most efficient and economical section used as a beam is:
Try it first: pick an option, then read the explanation.
Correct answer: A. I-section
Asked in SSC JE 02/03/2017 (Morning)
Detailed Explanation
Bending stress is largest at the extreme fibres and zero at the neutral axis. The most economical beam puts its material where it is most stressed.
Why “I-section” Is Correct
The I-section concentrates material in the flanges at the top and bottom, with a thin web to hold them apart and carry shear. It gives the largest section modulus for a given weight about its major axis.
Why the Other Options Are Wrong
- B. circular section: A circle has most of its area near the N.A., where bending stress is small.
- C. angles: Angles are unsymmetric and weak in bending; they twist unless loaded through the shear centre.
- D. H-section: An H-section has wide flanges suited to columns (balanced Ix and Iy), not maximum Z per weight about one axis.
Key Point
I-section: maximum Z per unit weight — the standard beam section.
Key Concepts for Students
- ISMB = Indian Standard Medium-weight Beam.
- Shape factor of an I-section ≈ 1.12–1.14.
- Deep, thin webs need stiffeners against buckling.
Practice more: this is question 47 in Bending and Shear Stress MCQs – Part 4. Try the whole set and check every answer instantly.