Bending behaviour of steel tubes has been examined in many research projects through the years, especially for empty ones. Also the infill and pressure effect in the tubes has been elaborately investigated but not much has been done on the effect that the sand infill has on the bending of the tubes. Combi-wall tubes are filled with sand during their installation on site not on purpose, since the sand enters through the lower open edge of the tube. The presence of the sand helps in achieving better post-yielding behaviour and local buckling capacity (failure at higher curvature), and also smaller ovalization of the steel tube cross-section. One of the advantages of the sand infill of combi-wall tubes is that it is free of costs and present at the application site. In order to have a better view of this favourable effect, the SBRCURnet organization performed an investigation programme, which included 4-point bending tests on empty and sand-filled tubes. In the current thesis, a numerical model of the experiments is designed which is verified and compared to the experimental test results and to analytical formulas. The numerical model is built with the ABAQUS software and consists of two approaches (models) on the issue. The first consists of a “slice” of the cross-section and it produces the moment-curvature and ovalization-curvature diagrams but it cannot predict the local buckling of the tube. The second is created in order to simulate experiment setting. It is a full scale model and it represents the mid-section of the set-up which has a constant bending moment. From this, the moment-curvature and the ovalization-curvature diagrams and the local buckling failure can be derived. The numerical curves and results are compared to curves derived from analytical formulas taken from literature research and to the results taken from the experimental tests. Additionally, the slice model is used to perform a parametrical study of the sand properties. By this, the influence of each sand property on the moment and ovalization can be examined. Generally, the final conclusion is that the numerical moment-curvature curves are a very good fit to the ones derived from the analytical formulas and they also have a good fit to the results of the experiment except for a very few of them. Furthermore, it can be clearly seen that the favourable effect of the sand is depicted in the numerical results as well as in the experimental ones. The sand infill contributes mainly to the post-yielding behaviour and the local buckling capacity since the buckle occurs in a later curvature value compared to the empty tubes. Also, the buckling shape of a sand-filled tube differs for the buckling shape of an empty tube due to the presence of the sand, which averts the inward local buckling and allows only an outward buckle to form.Additionally, by preventing the excessive ovalization of an empty steel tube, it can contribute to the bending moment capacity especially for the higher curvature values. In the curvature values close after the yielding of the tube, the contribution in the moment capacity is significantly smaller but nevertheless obvious. To sum up, filling the combi-wall steel tubes is an easy and very cheap manner in order to enhance their local buckling capacity.