______ is the primary material used for structural support in the construction of large buildings.

• A. Wood
• B. Cast Iron
• C. Steel
• D. Aluminum

Answer: (C)

Structural steel is used as the primary material for large buildings because of its high strength-to-weight ratio, ductility, and ease of fabrication into frames that can span wide distances. This combination lets engineers create tall, wide, and complex structures without excessive weight, while still resisting bending, shear, and dynamic forces from wind or earthquakes. The ductility of steel means it can deform and absorb energy before failure, providing warning and a safer failure mode under extreme loads.

Fabrication and erection of steel frames are highly efficient: beams and columns are manufactured to precise specifications and bolted or welded together on site, enabling rapid construction and consistent performance. Steel also offers predictable behavior, interchangeable components, and flexible design options that support long-span floors and open interior layouts.

In contrast, wood cannot reliably support the same heights and loads in large buildings. Cast iron is strong in compression but very brittle and lacks the ductility needed for dynamic loads, making it unsuitable for modern structural frames. Aluminum, while lightweight, generally cannot match the strength and stiffness required for large-scale structural systems at a reasonable cost, so it isn’t the default choice for major structural frames.