GENERALIZED YIELD SURFACE REPRESENTATIONS IN THE ELASTOPLASTIC 3-DIMENSIONAL ANALYSIS OF FRAMES

The yield surface equations for three-dimensional frames subjected to combined actions of an axial force, shear forces, bending moments, warping moment, and a torque are derived based on two different approaches: (i) continuum-based description in terms of stress components and (ii) stress resultants. For the former approach, the well-known von Mises model for the stress components of any material point on the cross-section is utilized. Based on the latter approach, two approximated forms for yield surface equations, semi-quadratic and linear, are suggested as reasonable (upper and lower) bounds for two representative cases; i.e., rectangular and wide flange sections. The one-step, fully implicit method of backward Euler is adopted to facilitate the implementation algorithms for the consistent material tangent stiffness and stress updating. The validity of the proposed equations has been clearly demonstrated by a number of numerical simulations for planar and spatial structures.