The subject of progressive collapse assessment in RC structures has been concerned in various studies. In contrast, most of these studies have attended to a scenario in which an instantaneous removal of a column due to unexpected impact or explosion has occurred. The present study addresses progressive collapse in RC structures resulting from both instantaneous and gradual removal of columns. The scenario for a gradual removal is the result of slow decreasing strength due to fire propagation in a specific zone of structure which is partially fire-proved. A five-story building model was selected as a case study for which the nonlinear model is later developed. Vertical displacement in the upper node of the removed column, redistribution of forces after removing the column, plastic deformations in adjoining elements, and the stress imposed on the sections of the beams adjacent to the removed column in both instantaneous and gradual cases are studied.