Thermal buckling of a fire-damaged composite column exposed to heat flux

This paper deals with the thermal buckling of a fire-damaged composite column, which is exposed to heat flux from one side. The column is composed of an undamaged layer and a fire-damaged layer (char layer), which is due to the resin material decomposition during fire. Two end fixity cases are considered: an axially restrained column (constrained, immovable ends case) and a column free to move axially (unconstrained case). The column is exposed to heat flux from the fire-damaged layer side, and this results in a nonuniform transient temperature distribution. To simplify the thermal conduction problem, we treat it as one-dimensional in the direction of thickness. For the thermal buckling analysis, the mechanical properties of the fire-damaged region (char) are considered negligible; the degradation of the elastic properties with temperature in the undamaged layer (especially near the glass transition temperature of the matrix) is accounted for, by using experimental data for the elastic modulus of the glass/vinylester material as a function of temperature. Furthermore, the formulation includes transverse shear. Because of the nonuniform stiffness, the effect of the ensuing thermal strains and the resulting eccentric loading, the structure behaves like an imperfect column and responds by bending rather than buckling in the classical bifurcation (Euler) sense. Simple equations for the response of the column are derived and numerical results are presented for the deflection with the variation of time. Two possible transverse deformation modes are identified, one is the column bending toward the heat source, and the other is the column bending away from the heat source. Which direction the column bends is determined by the competition between the thermal moment and the eccentricity moment. Finally, the effect of the fire-damaged (char) layer is assessed by comparison with the original (without tire-damage) column.