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, war ping moment, and a torque are derived based on two different approache s: (i) continuum-based description in terms of stress components and ( ii) stress resultants. For the former approach, the well-known von Mis es model for the stress components of any material point on the cross- section is utilized. Based on the latter approach, two approximated fo rms for yield surface equations, semi-quadratic and linear, are sugges ted as reasonable (upper and lower) bounds for two representative case s; i.e., rectangular and wide flange sections. The one-step, fully imp licit method of backward Euler is adopted to facilitate the implementa tion algorithms for the consistent material tangent stiffness and stre ss updating. The validity of the proposed equations has been clearly d emonstrated by a number of numerical simulations for planar and spatia l structures.