Due to the significant advantages of composite moment frames with Concrete-Filled Tube columns (CFT) and steel beams for being used as a seismic load-resistant system in buildings, increasing in number of terrorist attacks and the important role of infill panels in the resistance of the structures against loads, in this research, the effect of concrete sandwich infill panels on the progressive collapse of the steel structures with CFT columns has been investigated. For this purpose, first the models are analysed and designed by ETABS software; next, the numerical models are analysed by alternate load path method under nonlinear dynamic analysis by SAP2000 software. The parameters of column removal location, the ratio of span length to floor height, the number of stories, the percentage of opening and the presence or absence of gap between the frame and the infill panel are considered as the main parameters of this research. The effect of these parameters on the response of the structure has been investigated by evaluating the change in the vertical displacement of the structure as well as the distribution of stress in the infill panels.
The results of this research show that by modelling the concrete sandwich infill panels, the vertical displacement of the 3-story and 6-story structures has been reduced at least 93.2% and 82.3%, respectively; and the vertical vibration of the structures with infill panels, after removing the column and the infill panels connected to it, has been reached to its equilibrium sooner than bare frame. However, by increasing the percentage of opening of infill panels from 30% to 50% the vertical displacement of the 3-story and 6-story structures in the location of the removed column has been increased up to 7.8 times and 2.4 times, respectively. Also, by increasing the ratio of span length to floor height from 1 to 2, the resistance of the structures has been decreased and the vertical displacement of the 3-story and 6-story structures in