The fire resistance of concrete filled square steel tube columns (CFT columns) without fire protection under a constant axial load has been previously examined. The purpose of this present study is to investigate the fire resistance of CFT columns when they are subjected to combined loads. This paper presents a nonlinear thermal stress analysis method for predicting the mechanical behavior and fire resistance of CFT columns under eccentric axial load (axial load and flexure moment). This method is based on the stress-strain characteristics of materials at high temperatures and on the mechanics of column deflection curves. From the results of the developed computational technique, it was demonstrated that as the yield stress and rigidity of the steel tube rapidly decreased for approximately 30 minutes, the decrease of flexure moment capacity increased significantly more than that of the axial load capacity. In addition, as the load eccentricity ratio increased, the fire resistance time drastically decreased. However, the time of maximum expansion under eccentric axial load was independent of the load eccentricity ratio.

中文翻译：

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偏心轴向荷载作用下方钢管混凝土柱的耐火性

先前已经研究了在恒定轴向载荷下无防火保护的方钢管混凝土柱（CFT柱）的耐火性。本研究的目的是研究CFT柱承受组合载荷时的耐火性。本文提出了一种非线性热应力分析方法，用于预测CFT柱在偏心轴向载荷（轴向载荷和挠曲力矩）下的力学行为和耐火性。该方法基于材料在高温下的应力应变特性以及立柱挠曲曲线的力学原理。从开发的计算技术的结果可以证明，随着钢管的屈服应力和刚度在大约30分钟内迅速下降，弯矩承载力的下降幅度明显大于轴向承载能力。另外，随着负载偏心率的增加，耐火时间急剧减少。但是，在偏心轴向载荷下的最大膨胀时间与载荷偏心率无关。