The thesis deals with seismic analysis of the 22-storey reinforced concrete building in Ljubljana. The impact of modelling uncertainties, such as the impact of secondary seismic members, impact of effective span of slabs and the impact of taking into account cracked reinforced concrete elements is examined. Five models of the building were developed and analyzed. The first model, with reinforced concrete cores, columns and slabs, is most realistic. In the second mode, the slabs were replaced with beams to illustrate the impact of the effective span of slab. Third model includes core and columns without beams. We anticipate that the impact of the columns is small and they can be considered as a secondary seismic members. That was checked by a fourth model. However, it is convenient to take them into account in the seismic model, which simplifies to obtain deformations in the columns which are needed for dimensioning of these elements, while their impact on the primary seismic members is insignificant. By the fifth model, we examine the impact of cracked reinforced concrete elements. The results of the seismic analysis were evaluated with global seismic requirements, such as period of vibration and forms of oscillation, story displacements, story shears and with loads. We came to the conclusion that the choice of structural model of a building with strong concrete core and plates on the columns is important. Each has advantages and disadvantages. Models with consideration of secondary seismic elements make it possible to better capture the behaviour of the structure and simplify the dimensioning of secondary elements. Based on the results of the study it was concluded that it is most appropriate to make a model with cores, columns and beams with consideration of the effective span of slab. This gives a sufficiently accurate results, but modelling and design remain simple. It is important to be aware that this affects the course of further design.