In which type of collapse might the structure not show visible signs of distress before collapsing?

The correct answer is that a structure may experience a progressive collapse without displaying visible signs of distress prior to the event. A progressive collapse occurs when the failure of one structural element leads to the failure of other elements, resulting in a chain reaction. This means that initial failures can occur in a way that does not manifest noticeable distress observable by firefighters or observers from the outside.

Structures may appear stable even when critical components have lost integrity, making it difficult to assess the building's safety status effectively. This is particularly dangerous during fire situations, as the firefighters may not be aware of the compromised structural elements until it is too late.

In contrast, global collapse typically presents evidence of distress, such as significant deformation or structural failure that can be seen in various parts of the building. Similarly, localized collapse generally involves a specific area of the structure that shows signs of failure, making it more apparent. A silent collapse refers specifically to sudden events, often in specific types of structures or situations that can occur without prior warning, but still may involve some underlying issues becoming visible over time.