The present thesis deals with the structural instability of axially compressed thin-walled beams with open cross-section. Experimental and numerical (FEM) results will be shown for cruciform section beams with stiffeners placed in different positions. Such stiffeners are represented by longitudinal plates, which connect the flanges on a beam portion, thus providing intermediate warping restrictions. This work is structured into five chapters. Chapter 1 is an introductory chapter in which the problem is framed and early studies are mentioned. Chapter 2 gives some remarks on the instability of thin-walled beams subject to compressive axial load. Chapter 3 presents the experimental set-up and the instrumentation used to investigate buckling, as well as the programs used for data acquisition and processing.