The Response of Framed Steel Structures to Fire

Abstract

The predication of nonlinear response of steel buildings under elevated temperatures from fire loading can be achieved by sequentially using heat-transfer analysis and stress analysis. A combined computational fluid dynamics (CFD) and structural stress investigation was conducted to study the 3D response of a two-story framed steel building. After an extensive literature review on the CFD modeling used in the heat transfer analysis, it is concluded that for practical considerations, the important factors such as thermal buoyancy, thermal radiation, and flow turbulence were considered while the effects of combustion and the associated chemical reactions were neglected. The fire was modeled as a high-temperature zone whose temperature changed with time according to a prescribed manner. Walls consisted of typical construction materials (gypsum, wood, and fiberglass) while the floor systems consisted of concrete slabs supported by steel beams. Free convection of air and thermal radiation were both included in the analysis. The governing differential equations of flow and energy were solved by FLUENT. The air turbulence was modeled by the standard k-e model and the radiation was simulated by the Discrete Transfer Radiation Model. The result of FLUENT analysis indicated that the air velocity was high in the middle of the room above the fire and adjacent to the ceiling. The temperatures of the beams and columns generally decreased away from the center, with a more rapid decrease toward the ends. It was also observed that radiation played an important role in fire modeling where 20–40% of the heat transfer took place by radiation. Subsequently, the temperature of the beams and columns obtained from the CFD analysis were used in the nonlinear stress analysis to obtain the complete structural response using ANSYS where the floor loads due to gravity were included. It was assumed that the stress analysis results had no bearing on the heat transfer analysis, which is a reasonable assumption for most conventional structures. The following research paper will provide a summary description of the structural model used in ANSYS, and the corresponding results obtained.