Thin-walled structures are interesting for applications in which lightweight structures with high stiffness and high resistance are especially advantageous or even required. The mechanical behaviour of thin-walled structures is, therefore, of high importance. Thin-walled structures which are internally stiffened are of particular importance, as internal reinforcements can be designed to maximize the inertia moment, leading to higher stiffness and strength. The present study investigates the mechanical behavior of sandwich beams using the Finite Element Method (FEM) software ANSYS Mechanical APDL. This study is in the field of solid mechanics, with a specific focus on analyzing structures' non-linear mechanical response. Structural analysis was conducted on internally reinforced hollow-box beams, and a non-linear static analysis was conducted under bending loads. The material considered was structural steel. The models were solved using the iterative Newton–Raphson method. A material curve, obtained from real tensile tests, was input into ANSYS Mechanical APDL software for the FEM simulations considering the plasticity of the material. A load–displacement curve was generated to characterize the non-linear behavior of the models. To ensure high precision in the results, a mesh convergence analysis was carried out. Additionally, a comparison was made between the stiffness characteristics of the different beams and those of conventional hollow-box beams.