Blast-induced progressive collapse of steel moment-resisting frames: Numerical studies and a framework for updating the alternate load path method

The Alternative Load Path (ALP) method is widely used to assess progressive collapse resistance of steel framed structures. Code-based ALP is threat-independent methodology, implicitly focuses on a very special triggering event, i.e., a small near-field blast, that can lead to complete and sudden column loss. However, in a real blast-induced progressive collapse scenario, characteristics of the triggering event and subsequent initial damage control the structural response. To study these effects, a wide numerical investigation is carried out. First, the code-based ALP method is applied to assess the threat-independent dynamic column removal responses. The results emphasize the importance of initial damage location and building’s size. Then, the model structures were analyzed in different blast scenarios. A meaningful difference in the obtained results compared with code-based ALP is observed in both quantity and quality. Finally, a novel methodology (modified ALP) is suggested to update the code-based ALP method to capture the threat-dependent parameters, i.e., column removal time (CRT) and damage level. To serve this purpose, a substructure techniques, i.e., equivalent column model, is developed and validated. The results of three methods (threat-independent code-based ALP, threat-dependent blast analysis (BA) and the proposed modified ALP) are compared, and it is observed that the modified ALP method can effectively adjust the dynamic column removal response to reflect the blast effects.