Collapse of reinforced concrete structures under earthquakes is the main reason for life loss. Thus, avoiding structural collapse under strong earthquakes is the aim of seismic codes. The aim of the current study is to lead to an improved understanding of the seismic progressive collapse behaviour of reinforced concrete frame structures and to identify the most important parameters that should be considered in seismic progressive collapse analysis. The Applied Element Method, AEM, is an innovative method for direct progressive collapse simulation, in which strong geometric nonlinearity, element separation and collision can automatically be considered. Most previous studies focused on side-sway collapse modes only and indirectly checked for vertical collapse modes. A validation of the AEM for seismic progressive collapse simulation has been carried out. The AEM models of three different frame structures have been validated by comparing the analytical and experimental results. The results have indicated that the AEM can simulate the structure response from linear range up to collapse reasonably well. Sensitivity studies have been conducted to rank the material parameters most important to the collapse process in terms of the time at incipient collapse and to investigate their effects on the possible failure modes. The results show that the most important parameters are the parameters that can alter the failure mode. An investigation on the effect of inclusion of the vertical ground motions on the collapse capacity and the possible failure modes has been performed. Considering vertical ground motions in collapse assessment of irregular frame structures has led to a decrease in the collapse capacity and to modifications in the possible failure mechanisms resulting in vertical rather than side-sway collapse modes. A correlation study for investigation of the effect of using different intensity measures, fifteen spectrum and structure based intensity measures, for scaling far- and near-field ground motions for seismic assessment of mid-rise frame structures has been carried out. Employing intensity measures that account for the spectral shape has led to a considerably better correlation with the engineering demand parameters than utilizing intensity measures that are based on a single spectral value or a combination of two spectral values.