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A cantilever beam with an unsymmetric cross-section is subjected to a transverse shear force (P) at its free end. P acts at the shear center of the beam cross-section.

Which one of the following statements is TRUE about the deformation of this beam?

  • A.The beam undergoes only torsion
  • B.The beam undergoes only bending
  • C.The beam undergoes both torsion and bending
  • D.The beam undergoes neither torsion nor bending

Answer: B

The beam undergoes only bending.

The shear centre is, by definition, the point in the cross-section through which a transverse load must act for the section to bend without twisting. For an unsymmetric section it generally does not coincide with the centroid, which is why a load through the centroid of a channel, for example, makes it twist. Here P is applied at the shear centre, so no torque about the shear centre arises.

The beam still bends because P is a transverse force on a cantilever, so 'neither torsion nor bending' is wrong. Torsion alone, or torsion with bending, would require the load to be offset from the shear centre.

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