Elasto-plastic finite element models for angle steel frames

The paper compares two elasto-plastic models for incorporating the effects of material nonlinearity into a finite element procedure in the large deflection analyses of angle beam columns and trusses. These are the fiber and the lumped plasticity models. In the fiber model, the element cross section is divided into a finite number of elementary areas, from which the sectional rigidities, resisting forces, and moments, allowing for material yielding and strain-unloading, are calculated. In the lumped plasticity model', the inelastic behavior is defined for the whole cross section. The stress resultants in the cross section interact to produce yielding for the section. Yielding in the element is treated in a lumped manner, in which the plasticity effects are assumed to occur only at generalized locations (plastic hinges), while the element between the hinges remains elastic. Numerical studies of the large deflection behavior of restrained imperfect angle columns, K-braced angle frames, and of single-angle web member trusses using the two elasto-plastic models are presented. Tests on two pairs of angle trusses with web members on either the same side or opposite sides of the chord are described.