Revisiting the alternate load path method for impact-induced progressive collapse in steel moment-resisting frames

Ground-story columns are the most critical and exposed structural members in a frame system. Damage to these members under impact scenarios poses a significant risk to the structure, potentially leading to progressive collapse and severe socio-economic consequences. The predominant approach for assessing structural susceptibility to progressive collapse, both in research and practice, relies on the alternate load path method, which, in its original code-based methodology, follows a threat-independent framework. However, physical collapses are inherently threat-dependent. This study examines the parameters influencing the dynamic response and load-transferring mechanisms of impact-loaded steel moment-resisting frames. Various factors, including the mass and velocity of the impactor, impact height, and impact area, are analyzed and discussed. The findings highlight the differences between dynamic column removal and impact analyses, revealing that, in critical cases, the progressive collapse response can be up to seven times greater than the prediction based on dynamic column removal. This underscores the limitations of the code-based method in high-intensity impact scenarios. Moreover, the overall structural behavior differs, as the effects of initial failure location and energy dissipation patterns vary significantly between the two methodologies. In impact scenarios, at lower intensities, the response is governed by member-level mechanisms, particularly the local response of the column. As intensity increases, system-level performance becomes the dominant factor, reflecting the availability of alternate load paths.