Structural Integrity of Composite Columns Subject to Fire

Abstract

This article investigates the thermal response of an axially restrained composite column, which is exposed to a heat flux due to fire. The heat damage, the charred layer formation and nonuniform transient temperature distribution in the column exposed to fire from one side are calculated by the thermal model developed by Gibson et al. [1]. For the thermal response analysis, the mechanical properties of the fire-damaged (charred) region are considered negligible, while the degradation of the elastic properties with temperature in the undamaged layer (especially near the glass transition temperature of the matrix) is accounted for using experimental data for the elastic moduli. Due to the nonuniform stiffness distribution through the thickness and the effect of the ensuing thermal moment, the structure behaves like an imperfect column, and responds by bending rather than buckling in the classical Euler (bifurcation) sense. Another important effect of the non-uniform temperature is that the neutral axis moves away from the centroid of the cross-section, resulting in an additional moment due to eccentric mechanical loading, which tends to bend the structure. The compressive behavior of a column subjected to simultaneous high intensity surface heating and axial compressive loading was investigated experimentally to verify the anticipated theoretical response. All specimens exhibited bending and subsequent catastrophic failure, even at compressive stresses well below these corresponding to the Euler load.