Principles and Practice of Engineering (PE) Civil: Structural Practice Exam

To understand why answer B is the right choice, it's essential to consider how each type of cross-section behaves under torsional loads. The ability to withstand torsion is largely determined by the moment of inertia, which indicates how resistant a shape is to twisting.

Starting with the solid circle, it has a high polar moment of inertia because it distributes material evenly around the center, which provides excellent resistance to torsion. This is why it ranks high in the ability to withstand torsional loads.

Next is the circular tube, which, while also effective in resisting torsion, is not as strong as the solid circle due to the hollow nature which reduces the material in the inner area. However, it still provides good torsional capacity due to its shape.

The solid square comes next. Its moment of inertia is decent because all sides are equal, but the arrangement of the material does not leverage the shape as effectively as the round shapes do when resisting torsion.

The rectangular tube typically has a lower resistance to torsion than solid shapes due to its geometry, especially if the dimensions are not favorable (i.e., a large width compared to height). Thus, it ranks lower in torsional resistance.

Finally, the open section has the least inherent resistance to twisting