This paper proposes a novel multiaxial plasticity model for 3-dimensional nonlinear static analysis of steel frame buildings with fiber-based beam-column elements. The proposed constitutive formulation is expressed within the framework of rate-independent metal plasticity and captures both the pre- and postpeak response of typical structural steel elements due to yielding and inelastic local buckling under monotonic loading. An initial yield criterion is selected along with newly developed evolution laws. The material response follows J2 plasticity under a tensile stress state. Under compressive loading, the developed constitutive relation incorporates softening to simulate the postpeak response of a member due to inelastic local buckling. The model relies on appropriate yield line mechanisms inferred from buckling analyses of steel plates with characteristic boundary conditions. The proposed constitutive formulation, which is implemented in an open-source frame analysis finite element program, is general and can be used to represent a wide range of softening phenomena. To tackle mesh dependency in the presence of a softening material response, a regularization procedure is developed for 3-dimensional fiber-based elements. Direct comparisons between the predicted and measured nonlinear monotonic responses of physically tested steel beam-columns suggest that the proposed formulation predicts accurately their deduced moment-rotation and the axial shortening-rotation relations. Moreover, the stress distributions across typical cross sections in the postpeak loading regime depict the importance of axial-shear-flexure interaction within a steel beam-column.