<p>Reputable standards suggest that the Post-Elastic Stiffness Ratio (PESR) in the load-displacement curve for concrete members should range from zero to ten% of the elastic portion’s slope. Since the behavior of a concrete structure under progressive collapse depends on the ductility of its members, a change in their load-displacement curves can change the response of the structure to this phenomenon. Moreover, as progressive collapse is a dynamic phenomenon, a change in the column removal duration can be expected to affect structural behavior significantly. This study evaluates how the PESRs of materials and the column removal duration, as two independent variables, affect the Dynamic Increase Factor (DIF). The impact of these two parameters on the DIF has not been addressed so far. Three ten-story and two three-story reinforced concrete (RC) structures with the Moment-Resisting Frame (MRF) system at different strengths were designed. DIF variations were obtained as a set of points based on the normal rotation of the critical beam for PESRs of 1%, 3%, 6%, and 9% by removing a set of columns in different positions in structures using the alternate load path (ALP) method. Four equations were extracted for four PESRs through curve-fitting in SPSS. These equations were integrated into a new DIF formulation that relied on both critical beam ductility and the PESR. Furthermore, changing the removal duration of each column produced a set of points for DIF, the curve-fitting of which led to another empirical DIF equation. The results indicated that each of these variables would significantly affect DIF. Finally, these two equations were integrated into a formulation consisting of both variables to improve the accuracy of DIF estimation.</p>