Abstract: Progressive collapse refers to the disproportionate or complete collapse of a structure triggered by initial local damage. To study the progressive collapse resistance of reinforced concrete (RC) frame structures, a 9-story regular RC frame was modeled using fiber elements in OpenSEES. Static pushdown analysis and nonlinear dynamic alternate load path analysis were used to evaluate collapse-resisting mechanisms and structural resistance for various column removal scenarios. Results show that collapse resistance strongly depends on boundary constraints of the alternate load path: sufficient horizontal restraints enable the transition from flexural action to catenary action, thereby enhancing structural ductility and resistance; conversely, insufficient restraint limits collapse resistance to beam-end flexural moments. Dynamic analyses further reveals that top column failures pose higher collapse risks due to fewer redundant paths and limited force redistribution capacity, thereby requiring higher resistance reserves. Based on these findings, this study evaluated the capacity reserves stipulated in current design codes.