Question

For a thin-walled C-channel, why does a load applied at the centroid produce twisting?

• A. Because the shear flow in the flanges is always zero
• B. Because the centroid is always the point where torsion is maximized
• C. Because C-channels lack a longitudinal axis of symmetry
• D. Because the centroid and the shear centre are at different locations in unsymmetrical sections

Hint:

For unsymmetrical thin-walled sections, \[ \boxed{ \text{Load through Shear Centre} \Longrightarrow \text{No Twisting} } \] whereas \[ \boxed{ \text{Load through Centroid} \Longrightarrow \text{Twisting}. } \]

Answer 1

The Correct Option is D

Step 1: Recall the concept of shear centre. The shear centre is the point through which the transverse load must act so that no twisting occurs.

Step 2: Explain the twisting in a C-channel. A thin-walled C-channel is an unsymmetrical section. For such sections, \[ \boxed{ \text{Centroid} \neq \text{Shear Centre}. } \] Therefore, when the load acts through the centroid instead of the shear centre, an additional twisting moment is produced. Hence, \[ \boxed{ \text{The centroid and the shear centre are at different locations.} } \] Thus, \[ \boxed{(D)} \] is the correct answer.