A conceptual note on the definition of initial failure in progressive collapse scenarios

Progressive collapse can be defined as a cascading phenomenon in which an initial failure is followed by the collapse of adjoining members which, in turn, is followed by further collapse that is disproportionate to the initiating failure. While extensive experimental and numerical studies have focused on the topic, little effort has been put forward in defining and redefining the underlying theory and philosophy. These theories and philosophies are of primary importance since they can shape the entire research methodology. The current definitions and approaches have been developed based on frame structures within a threat-independent methodology, although this aspect is not explicitly emphasized. This study tries to challenge this idea. It is shown that the initial failure (i) is not necessarily a local damage, (ii) is not necessarily a member loss, and (iii) cannot always be defined as a threat-independent damage scenario. The consequences of this insight are deeply discussed regarding the structural type and acting threat. In particular, it is shown that the current code-based approaches do not always lead to the most critical scenario. Finally, a rational framework for the definition of the initial failure is provided.