In high-rise buildings utilizing reinforced concrete structures, the load-bearing capacity and energy absorption capability of the structure play a crucial role in ensuring the safety, stability, and durability of the building. Operational processes may cause local damages, altering internal forces and triggering progressive collapse mechanisms, which can lead to the risk of total structural failure. Assessing the probability of progressive collapse is an important tool for risk mitigation, design optimization, and cost reduction in construction. Probabilistic analysis methods based on reliability models, such as the First-Order Reliability Method (FORM), have been widely applied but face limitations in handling nonlinear functions and non-standard probability distributions. Recently, the Advanced First-Order Reliability Method (AFORM) has been proposed to overcome these limitations, enabling more accurate estimation of collapse probability in complex problems. This study focuses on developing appropriate limit state functions to simulate the progressive collapse process of reinforced concrete buildings, laying the groundwork for applying the AFORM in subsequent analyses. Additionally, a Robustness Index (RI) is proposed to measure the structure’s resistance to progressive collapse, based on the tolerance of local damages and the risk of overall failure. The research results provide both theoretical and practical tools to assist engineers in enhancing design efficiency, risk management, and safety assurance for buildings and constructions.