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The working hypothesis, for the collapse of the 47-story WTC 7, if it holds up upon further analysis,
would suggest that it was a classic progressive collapse that included:
• An initial local failure due to fire and/or debris induced structural damage of a critical column, which supported a large span floor area of about 2,000 ft2, at the lower floors (below Floor 14) of the building,
• Vertical progression of the initial local failure up to the east penthouse bringing down the interior structure under the east penthouse, and
• Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7 that were much thicker and more heavily reinforced than the rest of the floors), triggered by damage due to the vertical failure, resulting in disproportionate collapse of the entire structure.
A vertical collapse appears to have occurred after interior columns 79, 80, and/or 81 failed.
The debris of WTC 7 was mostly contained within the original footprint of the building. From aerial photos, the debris visible on top of the pile is mostly façade structure. This failure sequence suggests that the interior of the building collapsed before the exterior.