Up to now, most studies corresponding to building structures against progressive collapse are based on the column-removal scenario. However, other cases, i.e., falling-debris impact scenario, which may cause progressive collapse of building structures were limitedly studied. This paper presents numerical simulations and a simplified approach of steel-framed subassemblies with Reverse Channel connection with Extended End Plate (RC-EEP) under the falling-debris impact scenario. The numerical results demonstrate the ductility, load-carrying capacity, and energy-absorption capacity of the steel-framed subassemblies with different Span-to-Depth Ratio under various impacted locations. All specimens showed the same global deformation profile and same load-resistance mechanism under the impact load on the same locations. Under mid-span-impact scenario, axial elongation of connection primarily affected the ductility. Both catenary action and flexural action affected the load-carrying capacity. Failure displacement, maximum load-carrying capacity and energy-absorption capacity all decreased with the increase of the Span-to-Depth ratio. Under beam-end-impact scenario, both the ductility and the load-carrying capacity were primarily controlled by the shear deformation and shear strength of the connection close to the impacted location. The increase of the Span-to-Depth ratio had little effect on failure displacement while slightly reduced the maximum load-carrying capacity and energy absorption capacity. Smaller Span-to-Depth Ratio was recommended in the impact-resisting design for impact load on both impact locations. A simplified approach was also developed to predict the behaviour of the steel frame subassemblies up to total failure. The validation study indicated that the proposed models can represent the key responses of steel-framed subassemblies with different Span-to-Depth Ratio subjected to impact load on various locations, including the flexural action at pure bending stage, the development of flexural action and catenary action at the tension-bending stage, and the failure stage. The accuracy of the proposed approach was validated against the experimental tests and numerical simulations, in the aspects of internal force-displacement relationships, load-carrying capacity-displacement relationships, and energy absorption-displacement relationships. The proposed approach provided an accurate and efficient way to predict the impact resistance of specimens with various Span-to-Depth Ratios under both of the mid-span impact and the beam-end impact scenarios.