This thesis compares manual 2D calculation and 3D calculation with SCIA Engineer 18 computer program. A multi-storey commercial composite structure was calculated. Composite columns, composite decks and composite structures in general are theoretically elaborated in the first part of the thesis, while the second part of the thesis is based on 2D manual calculation and comparison with 3D calculation with SCIA Engineer 18 computer program. Since composite structure calculation in computer programs is still in development, SCIA Engineer 18 has offered an adequate model for comparison with manual structure calculation. The object is positioned in Osijek and wind and snow loads were calculated in accordance with HRN EN 1990 norm. Dimensions of the structure plan are 27,0 𝑚×20,0 𝑚, with the total height of the structure ℎ=18 𝑚. The composite structure was calculated as a not supported structure and the calculation was carried out in two stages. Profile HEA 300 was used for the main beams, and for the columns profiles HEB 240 were used, which are concrete encased columns in composite performance. Composite decks were used in „Hoesch Additiv Decke“ system and decks with a mass approximate to „Hoesch Additiv Decke“ system mass were modelled in SCIA Engineer 18 computer program. Finally, the analysis and comparison of the results was done, depending on the method of calculation. The results of the comparison are relevant, that is, correspondence between 2D and 3D results is acceptable.