Describe what a 'T-beam' is.

A T-beam is defined by its cross-sectional shape, which resembles a "T". The design features a wide, flat top flange and a vertical web that extends downward, giving it both strength and stiffness. This shape is particularly advantageous for supporting loads while minimizing material use.

In construction and structural applications, T-beams are often used in reinforced concrete construction, where the top flange acts as the compression zone and the web resists shear forces. They can be utilized in various building scenarios, including floor systems and bridge constructions, where their unique shape provides effective load distribution.

The other choices do not accurately describe a T-beam: the description of an "I" shape refers to I-beams, and not all beams that resemble a "T" are restricted to steel structures or are necessarily hollow and lightweight. Thus, understanding that a T-beam is specifically characterized by its T-shaped cross-section is crucial for recognizing its applications in structural engineering.