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Unstiffened elements affect markedly the resistance of commonly used open steel sections under fire conditions. The distinct non-linear material behavior of steel at elevated temperatures requires large strains to activate the increase of cross-sectional capacity due to plastification. The yield line theory in conjunction with temperature dependent second order elastic theory including equivalent geometric imperfections has been used to describe the load-carrying behavior of unstiffened elements in compression and bending at elevated temperatures in fire. Using both theories, the complete load carrying behavior in the pre- and post-buckling range can be analyzed. In addition, the results obtained from these analytical solutions were compared to numerical results using a geometrical and physical non-linear finite element approach.