Mechanics-based collapse identification index and collapse story localization method for RC moment frames under seismic action

Current seismic design codes use drift ratios to define collapse-limit states in global structural response, making it difficult to accurately identify the initial collapse story and trace progressive collapse path. To address the above issues, this study proposes a mechanical index based on the effective secant stiffness and effective tangent stiffness (i.e., for evaluation of the static equilibrium state and dynamic stability of structures simultaneously and for coupling mechanism of strength degradation and instability). Meanwhile, two types of collapse assessment indices were introduced: (1) a displacement index based on the story drift ratio (with a clear threshold but insufficient precision and scientific rigor); (2) an energy stability function based on the energy balance relationship (i.e., verification for collapse assessment from the perspective of global energy input and dissipation). A tri-linear hysteretic backbone curve model including strength and stiffness degradation was adopted, which was also combined with the I-K hysteretic rules to describe the full-range mechanical behavior of structures. In comparison between the shaking table results of 1:2 scaled RC frame and predictions of ETABS refined model, the collapse times predicted by the proposed mechanical index and energy index (18.12 sec and 18.57 sec, respectively) were in good agreement with the actual collapse time of the test (18.367 sec), while the FEMA 350-based displacement index (17.48 sec) exhibited relatively poor precision. For simple evaluation, collapse assessment addressing the P -Δ effect and mechanical performance degradation was applied to single-degree-of-freedom and multi-degree-of-freedom systems. SDOF system revealed engineering applicability of the proposed mechanical and energy indices, and 2-DOF and 3-DOF systems could accurately identify the initial collapse story in vertically regular and irregular structures. This study establishes a set of accurate and practical seismic collapse identification and localization methods, providing a reliable theoretical basis and engineering reference for structural seismic safety assessment.