A closed-form analysis of perimeter member behavior in a steel building frame subject to fire

This paper outlines a closed-form methodology that can be used to predict the increase in demand experienced by the perimeter columns that are part of a fire-exposed steel building frame. The two-dimensional elevation-view subassembly considered for this study includes a two-storey length of the perimeter column and the floor beam framing into the column in the direction perpendicular to the building’s exterior. When heated, this beam will expand and induce bending moment and lateral deflection in the column as well as increased axial force in the beam itself. Our proposed approach has two primary components: (1) a material model that approximates nonlinearity and considers temperature effects, and (2) a mechanical model that represents the perimeter column and beam interaction. These models are used to develop a simplified closed-form solution for beam axial force and perimeter column bending moment that may be used as part of a performance-based design for fire exposure. The simplified model solutions are compared to the results of a more complex and detailed multi-story finite element analysis model. A comparison of these results shows that the simplified model results give good estimations of structural behavior.