NIST's Explanation of WTC 7's Collapse
In April of 2005, The National Institute for Standards and Technology (NIST) began releasing preliminary reports on WTC 7's collapse. It revealed its theory of the collapse in the form of a slide presentation named: Project 6: WTC 7 Structural Fire Response and Collapse Analysis. The slides excerpted here are from a revised version of that presentation released in June of 2005.
The earlier official report by FEMA, the World Trade Center Building Performance Study had blamed the collapse entirely on fires, raising the obvious question of how fires could have induced the total collapse of this steel-framed building when fires have never caused the total collapse of a steel-framed building. NIST's WTC 7 inquiry appears to have been motivated by the need to supply additional causative factors to make the collapse of this building without controlled demolition seem more plausible. Hence, it created a scenario of severe structural damage that cannot be easily confirmed or refuted.
NIST's theory of the collapse of Building 7 is summarized by the following five slides from its presentation.
NIST would have us believe that the failure of a single column near ground level led, first to a vertical progression of failures, causing the collapse of the East Penthouse, followed by a horizontal progression of failures leading the collapse of all of the building's 27 core columns, precipitating a total collapse.
In other words, NIST asserts that WTC 7 collapsed like a house of cards.