Abstract: The existing analytical models of reinforced concrete (RC) frames against progressive collapse are only available to structures with equal spans. Therefore, a new modified tri-linear model is proposed for RC frames with unequal spans, which can consider the flexural action, compressive arch action, catenary action and tensile membrane action of the structure under different single column loss. A robustness ranking index is also established based on the proposed model to quickly identify the dangerous column loss scenarios. The results show that the proposed model is accurate and can provide a reference for the design of RC frames with unequal spans against progressive collapse. And the robustness ranking index can be used to quickly identify the worst column loss scenarios of RC frames.