This paper deals with the stability evaluation of thin-walled steel tubular columns subjected to cyclic bidirectional loading. In comparison with the behavior under the conventional cyclic unidirectional loads, the deterioration in strength and ductility of thin-walled circular steel columns caused by the severe cyclic bidirectional loads is examined based on nonlinear finite element analysis and test results. The pseudodynamic bidirectional tests, available in the literature, are used to substantiate the accuracy of the finite element analysis. The results confirm the importance of considering the behavior of steel columns under bidirectional loading. The bidirectional tests and finite element analysis results showed that the behavior of a tubular column under bidirectional loading becomes complex and exhibits a circular trajectory once local buckling occurs. The local buckling bulge in the bidirectional loading case tends to develop monotonically due to the circular trajectory. As a result, the residual deformation becomes larger. The unidirectional loading test and analysis are likely to underestimate the damage and the residual displacements caused by an earthquake. It is concluded that the effects of bidirectional loading should be considered in ductility evaluation and seismic resistance design of steel structures.
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