Stress-resultant plasticity for frame structures

Two versions of a bounding surface plasticity model implemented in stress-resultant space and applicable to the analysis of steel, reinforced concrete, or composite beam-columns are discussed. One is a two-surface model appropriate for steel members with a finite elastic region. The second, which is developed for reinforced concrete and composite steel-concrete members, employs a single outer bounding surface with an infinitely small loading surface that is degenerated to a point. Plasticity-based assumptions employed in the formulation of these models are reviewed and predicted plastic flow directions are evaluated against data from more fundamental fiber-type analyses of the beam-column cross sections. Results of these comparisons support the use of Mroz's kinematic rule in stress-resultant space and lead to recommended improvements in the bounding surface formulation.