What is the definition of progressive collapse in structural engineering?

Progressive collapse in structural engineering refers to a scenario where the failure of a primary structural element leads to a successive and systemic failure of other elements, potentially resulting in substantial portions or even the entire structure collapsing. This concept highlights the chain reaction that can occur when a critical component fails, causing additional stresses on other parts of the structure that were not designed to sustain such loads, ultimately leading to a catastrophic failure.

The definition focuses on the failure mechanism being initiated by a single point of failure, which could be due to various reasons, such as design flaws, insufficient redundancy, or unforeseen load conditions. This characteristic sets progressive collapse apart from other forms of structural failures that may stem from entirely different causes, like design exceeding load capacity or environmental factors affecting stability.

In contrast, other choices refer to different failure types or stability concerns. For instance, becoming less stable under load pertains to a structural element's capacity rather than a failure leading to a cascading effect. Similarly, exceeding the design load is about the limit of a structure itself and does not imply subsequent failures. Lastly, exposure to environmental conditions may cause localized failures, but it doesn’t define the systemic failure mechanism that progressive collapse entails.