Lateral-torsional buckling of steel beam-columns under fire exposure

Steel beam-columns under fire may fail by either complete plasticizing of the section or in a premature stability failure. The type of stability failure in beam-columns under fire ranges from local buckling of section (plates) to global buckling of the overall member. Lateral-torsional buckling of beam-columns is influenced by many factors including unbraced length, load level, initial imperfections, degree of restraint, high-temperature properties of steel, and fire scenario. A set of numerical studies, using finite element computer program ANSYS, was carried out to study the global buckling response of steel beam-columns under realistic fire, load and restraint scenarios. Results from the parametric studies indicate that unbraced length, load level, degree of end-restraint, and fire scenario have significant influence on the stability of beam-columns under fire conditions. Stability of beam-columns is severely affected with increasing unbraced length. The type of fire scenario plays a critical role in the development of fire induced forces and thus affects the response of the laterally-unbraced steel beam-column. Increased load level leads to premature stability failure resulting in lower fire resistance of beam-columns. Rotational restraint enhances the fire resistance of laterally-unbraced steel beam-columns, while the axial restraint has detrimental effect on fire resistance. Overall, the effect of fire induced restraint forces on the stability of beam-columns under fire must be taken into consideration for better evaluation of fire resistance.