How does structural steel behave under high heat without protection?

• A. It becomes stronger
• B. It resists heat completely
• C. It remains unchanged
• D. It loses strength and can buckle or fail

Answer: (D)

When structural steel is heated, it loses its strength and stiffness. As temperature rises, the steel’s yield and ultimate strengths drop and the material softens, so it carries less load. Under sustained load, heated steel can deform, buckle, or fail because its ability to resist bending and compression diminishes. Steel also expands with heat, which can introduce additional stresses and damage connections. Without protection, a fire can quickly raise temperatures to levels where the structure can no longer support its loads, risking collapse. Fireproofing or insulation slows heat transfer, helping maintain strength and keeping the structure safer for longer.