Pick the correct statement from the following:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 25/01/2018 (Evening)
Detailed Explanation
The shear centre (centre of twist) is the point in a cross-section through which a transverse load must pass if the beam is to bend without twisting. The line joining the shear centres along the beam is the bending axis.
Note: the commission cancelled this question in its revised answer key. On the merits, all three statements are standard properties of the shear centre, so we give (d).
Why “All options are correct” Is Correct
(a) is the definition: the bending axis cuts each cross-section at its shear centre. (b) In a doubly symmetric section such as an I-section the shear centre lies on both axes of symmetry, so it coincides with the centroid. (c) A channel is symmetric about one axis only; its shear centre lies on that axis but outside the web, away from the centroid. So all options are correct — option (d).
Why the Other Options Are Wrong
- A. The point of intersection of the bending axis with the cross-section of the beam is called the shear centre: True, but (b) and (c) are also true.
- B. For I-sections, the shear centre coincides with the centroid of the cross-section: True, but (a) and (c) are also true.
- C. For channels, the shear centre does not coincide with the centroid: True, but (a) and (b) are also true.
Key Point
Shear centre: load through it ⇒ bending without twist. I-section: S = G. Channel: S outside the web.
Key Concepts for Students
- For an angle section, the shear centre is at the junction of the legs.
- For a T-section, it is at the junction of flange and web.
- Channels used as purlins tend to twist because the load does not pass through S.
Practice more: this is question 29 in Bending Moment and Shear Force MCQs – Part 2. Try the whole set and check every answer instantly.