The local buckling phenomenon presents one of the main premature failures often prohibiting the steel hollow-section columns to attend the ultimate strength capacity. A strengthening method is then required to extend the service life of the member. This paper presents the optimal retrofitting design of standard steel hollow-section columns using external steel plates, such that the ultimate strength of the post-retrofitted column sufficiently resists the design load imposed by an industrial crane. The optimal design adopts a so-called bi-directional evolutionary structural optimization (BESO) algorithm that determines the cost-effective distribution of steel plate topology welded to the column. The proposed method realistically considers the influences of inelastic material properties and geometric nonlinearity, simultaneously. The BESO algorithm is encoded within the MATLAB modeling framework providing a direct application interface to ANSYS (a commercial-purpose finite element analysis software), which models the retrofitting joint between steel column and corbel using the comprehensive 3D finite elements. The robustness of the proposed scheme is illustrated through standard steel warehouse applications. The accuracy and integrity of the resulting design are validated by the full elastoplastic responses of the post-retrofitted column under applied forces.