Plastic bifurcation analysis of a two-layer shear-deformable beam-column with partial interaction

This paper deals with the plastic buckling behavior of two-layer shear-deformable beam-columns with partial interaction. The Timoshenko kinematic hypotheses are considered for both layers and the shear connection (no uplift is permitted) is represented by a continuous relationship between the interface shear flow and the corresponding slip. A set of differential equations is obtained from a general 3D plastic bifurcation analysis, using the above assumptions. Original closed-form expressions of the buckling loads and the corresponding modes of the composite beams under axial compression are derived for various boundary conditions, considering that both layers (or possibly only one) behave plastically at the critical point. The particular case of Euler-Bernoulli beams can be deduced from these general expressions by neglecting the influence of shear deformability. The proposed analytical solutions are favorably compared against the predictions of a FE model based on a co-rotational two-layer beam formulation which accounts for interlayer slip and inelasticity. Parametric analyses are performed and the effect of the elastoplastic moduli is particularly emphasized.