Channels get twisted symmetrically with regard to their axis ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 04/03/2017 (Evening)
Detailed Explanation
A channel has only one axis of symmetry — the one perpendicular to the web (parallel to the flanges). Its shear centre lies on that axis but outside the web, on the side away from the flanges. When the load acts in the plane of the web (parallel to the web) without passing through the shear centre, the flange shear forces form a couple and the channel twists as well as bends.
Why “parallel to web” Is Correct
The book's key is parallel to the web: loads applied parallel to the web (the usual purlin and lintel case) twist the channel, the twist being symmetric about the centre line of the section. Note: this question is ambiguous — the channel's axis of symmetry is the one parallel to the flanges, and different answer keys give different options. We keep the book's answer (b).
| Channel loaded… | Through shear centre? | Result |
|---|---|---|
| Parallel to the web, through the web centre line | No | Bends and twists |
| Parallel to the web, through the shear centre | Yes | Bends without twist |
| Parallel to the flanges (along the axis of symmetry) | Yes (lies on the symmetry axis) | Bends without twist |
Why the Other Options Are Wrong
- A. parallel to flanges: The axis parallel to the flanges is the axis of symmetry; loads along it pass through the shear centre and cause no twist.
- C. perpendicular to flanges: "Perpendicular to flanges" describes the same direction as parallel to the web; the book chose the wording in (b).
- D. perpendicular to web: "Perpendicular to web" is the symmetry axis again — no twist for loads along it.
Key Point
A channel loaded parallel to its web twists unless the load passes through the shear centre, which lies outside the web.
Key Concepts for Students
- Shear centre of a channel: e = 3b²/(6b + h) from the web centre line (thin, equal flanges).
- Doubly symmetric sections (I, box) have the shear centre at the centroid.
- Answer keys for this question differ.
Practice more: this is question 9 in Torsion MCQs – Part 1. Try the whole set and check every answer instantly.