Two retrofit strategies, aiming at increasing the collapse resistance of simple connections by adding seat angles and steel plates with long-slotted holes, are proposed in order to address the vulnerability of steel gravity frames under column loss scenarios. A high-fidelity, detailed, finite element model for a planar composite frame is developed and calibrated against experimental data and is used to conduct numerical analysis to explore the effectiveness of the proposed retrofit strategies. The simulation results show that the planar composite frame with enhanced connections exhibits significantly higher collapse resistance and better ductility under column loss scenarios compared with the one with conventional connections. Meanwhile, it is also revealed that the proposed retrofit strategies have an insignificant impact on the behavior of the structural system under earthquakes. These two retrofit strategies are then implemented to retrofit the gravity system of a 10-story, seismically designed steel frame structure, which has been shown to be vulnerable to progressive collapse after an interior gravity column is forcibly removed or impacted by a heavy vehicle with high speed. Numerical simulations were performed using a 3-D micro-based model and the simulation results illustrate that progressive collapse of the structure with enhanced gravity systems is prevented under both scenarios. Therefore, the proposed retrofit strategies are effective in preventing the progressive collapse of existing steel structures employing simple connections.