The graduation thesis deals with a steel-concrete composite beam over two spans, being part of the floor structure of a buildnig. The following two scenarios examined at the construction stage during concreting: the beam is un- ropped, the beam is propped in the mid-span. The elastic and plastic analysis of the bending resistance of the composite beam, performed considering both construction stages. Internal forces have been calculated using the linear elastic global analysis. Furthermore, the thesis covers the presence of negative bending moments in the composite beam, which need to be paid particular attention in the design stage. The deflections were analysed in the serviceability limit state, taking into account both stages.