What is the typical structural failure mechanism for unprotected steel columns when exposed to a prolonged, high-temperature fire?

When unprotected steel columns are exposed to a prolonged, high-temperature fire, their typical structural failure mechanism is through buckling due to the severe degradation of the steel's mechanical properties. Initially, at ambient temperatures, steel possesses high yield strength and a high elastic modulus. Yield strength refers to the maximum stress a material can withstand before undergoing permanent deformation, while the elastic modulus, also known as Young's modulus, is a measure of a material's stiffness or resistance to elastic deformation. As the temperature of the steel column increases during a fire, both of these critical properties dramatically decrease. Above approximately 300°C, the steel begins to lose a significant portion of its yield strength and elastic modulus, with the degradation accelerating rapidly between 400°C and 600°C. For instance, at around 550°C, steel can retain only about 40-50% of its ambient temperature yield strength and even less of its elastic modulus. This reduction in stiffness makes the column far more flexible, and the reduction in strength means it can sustain much less axial compressive load. A column, by definition, is a structural element primarily subjected to compressive forces along its longitudinal axis. Under such compression, a slender column can fail not by direct crushing of the material, but by buckling, which is a sudden, large, lateral deflection or instability perpendicular to the applied load. This occurs when the compressive load reaches a critical value where the column can no longer maintain its straight form and instead bends or bows significantly, leading to a catastrophic loss of load-bearing capacity. In a fire, as the steel column heats up and its material properties degrade, its resistance to buckling significantly diminishes. The column's slenderness, which is a ratio comparing its effective length to its cross-sectional stiffness (radius of gyration), plays a crucial role; more slender columns are inherently more prone to buckling at lower loads and, consequently, at lower temperatures. The column reaches a critical temperature where its reduced strength and stiffness, combined with the applied service load, become insufficient to resist buckling. Once this critical temperature is reached, even if the applied load has not increased, the column's ability to support that load is compromised, and it undergoes rapid, unstable buckling deformation, leading to structural collapse.