The elastic-plastic behavior of concrete filled steel columns subjected to concentrical or eccentrical compression is analyzed under high temperature. The new analytical model and design formulas of columns in the fire are proposed. In the new model, the strength of steel tube is negelected, because the unprotected outer steel tube loses the strength and the stiffness gradually in the fire. The concrete is divided . into two parts in the model, one is an un-effective portion near the edge of the column-section. In this portion, the strength of concrete is neglected. The other is an effective inner portion. It is assumed that the filled concrete maintains the strength under room temperature in the effective portion. On the basis of this simple model, a simple but rational design formula of columns is proposed.
