ABSTRACT
This paper aims at examining interaction phenomena between axial and lateral forces arising in steel columns in the plastic range of behaviour under multi-directional earthquake motion. Analyses have been carried out by means of two different models: a fiber one and a mathematical one. It is believed that the fiber model is more useful for a detailed response description while the mathematical model is more useful to draw more general conclusions, being its response dependent on a few number of parameters. Both models have clearly shown how axial and horizontal force interact in the plastic field, with a consequent development of important vertical displacements, even when axial force is due to gravity loads only. When considering the vertical earthquake component in addition to the horizontal ones, system response does not vary significantly in the inelastic range of behaviour.
