For several years stainless steels have been specified in applications where the primary requirement is for corrosion resistance. However, nowadays stainless steels have also been recognised for other attributes such as durability, versatility, sustainability, hygiene and aesthetic appeal. For all of this, the use of stainless steel structural elements in construction has been increasingly spread in the last years. Despite the increase of the use of stainless steel for structural elements in construction, the behaviour of this material for structural purpose is not defined as accurately as for the carbon steel. The most importance difference between these types of steel is the shape of the stress- strain curve because stainless steel has a non-linear behaviour of stress and strain. Different material models were studied and some conclusions about their suitability reached. The primary aim of this thesis was to study the reliability of the different current approaches and if needed, to propose modifications for beam-column elements under combined load. A FE model was created and validated according existing experimental programs. From this model, a parametric study was done in order to generate a wide range of analysis for beam- column to have enough data to study and reach some conclusions about these studied approaches. Finally, it was studied the general method. The method that is very effective in carbon steel, but the behaviour for stainless steel is uncertain. For the development of this chapter the numerical results achieved in the previous chapters were used. Finally, some conclusions were made in order to summarize the work realized during the thesis and give a general view about the behaviour of stainless steel for beam-column elements under combined load with the main focus on Class 3 and Class 4 square hollow sections.