Stiffness reduction offers a practical means of considering the detrimental influence of geometrical imperfections, residual stresses and the spread of plasticity in the analysis and design of steel structures. In this paper, a stiffness reduction approach is presented, which utilises Linear Buckling Analysis (LBA) and Geometrically Nonlinear Analysis (GNA) in conjunction with developed stiffness reduction functions for the design of columns and beam-columns in steel frames. This approach eliminates the need for modelling geometrical imperfections and requires no member buckling checks. For columns, inelastic flexural buckling loads can be obtained using LBA with appropriate stiffness reduction, while GNA with stiffness reduction is required to determine an accurate prediction of beam-column failure. The accuracy and practicality of the proposed method is shown in several examples, including regular and irregular members. For the latter case in particular, it is found that the proposed approach provides more accurate capacity predictions than traditional design methods, when compared to results generated by means of nonlinear finite element modelling.