Which of the following is NOT a common steel shape for structural members?

Square tube is not considered a common shape for structural members in the context of the options provided. In structural applications, steel shapes are typically defined by their cross-sectional profiles, which are designed to resist bending and shear forces effectively.

I beams, T bars, and channel sections are standard shapes used in construction due to their high strength-to-weight ratios and efficiency in carrying loads. I beams, for instance, are specifically designed for bending resistance and are widely used in bridges and buildings. T bars can provide both strength and versatility in certain configurations, while channel sections offer good resistance against bending forces in one plane.

Square tubes, while they can be used in some structural applications, are less common in traditional structural steel framing compared to the shapes mentioned above. They are typically found in applications that may require some aesthetic considerations or specific structural configurations, but they do not fall within the most frequently used shapes for primary structural members in construction. This distinction is what makes square tubes less appropriate as a common structural steel shape in comparison to the other options.