Behavior of Unrestrained and Restrained Bare Steel Columns Subjected to Localized Fire

Abstract

Real fires start from localized burning and will not develop to flashover in an open space or in large open-plan compartments. Temperatures of gas and exposed steel columns in localized fires are not uniform. Current structural fire design methods are based on uniform heating and, therefore, do not account for temperature gradients in real fires. This paper presents a numerical investigation of the behavior of bare steel columns subjected to a localized fire. The buckling behaviors of steel columns surrounded by and adjacent to a localized fire are investigated. Simple approaches are provided to calculate the temperatures of steel columns surrounded by and adjacent to a localized fire. Sequentially coupled thermal-mechanical simulations were conducted. Unrestrained, axially restrained, and rotationally restrained steel columns of various load ratios and dimensions subjected to different heating conditions were considered. The study found that the behavior of steel columns subjected to a localized fire may be completely different from that of columns subjected to a standard International Organization for Standardization (ISO) 834 fire. The steel columns surrounded by a localized fire fail by local buckling, while the same columns subjected to a standard ISO 834 fire fail by global buckling. The temperatures at which steel columns buckled (buckling temperature) when surrounded by a localized fire are higher than those of columns subjected to both a standard ISO 834 fire and an assumed uniform steel temperature condition. However, the buckling temperature of steel columns adjacent to a localized fire may be much lower than that of the columns subjected to both a standard ISO 834 fire and an assumed uniform steel temperature condition. The location of the localized fire relative to the column has a significant effect on the temperature and buckling behavior of steel columns. Structural fire design for steel columns based on a standard ISO 834 fire or a uniform steel temperature may be conservative if the potential real fires are localized fires surrounding the columns and may not be conservative if the potential real fires are localized fires adjacent to the columns.