According to the FEMA report, fire within the buildings caused thermal expansion of the floors in a horizontal and outwards direction, pushing against the rigid steel columns — which deflected to an extent but resisted further movement. With the columns resisting movement, there was nowhere else for the concrete floors to expand to.
This led to an increased buildup of stress in the sagging floors, until the floor framing and connections gave in. The collapsing floors then fell on more floors below, leading to a progressive collapse. It is understood the South Tower collapsed sooner because it suffered more damage from the initial aircraft impact, which also dislodged more fireproofing material. While there are different theories regarding how the progressive collapse of Building 7 was initiated, there is consensus among investigators fire was the primary cause of failure.
Both official reports made a range of fire safety recommendations for other high-rise buildings, including to improve evacuation and emergency response. In , the National Institute of Standards and Technology also published a best practice guide recommending solutions to reduce risk of progressive collapse. There have since been changes to building codes and standards on improving the structural performance of buildings on fire, as well as opportunities to escape such as added stairwell requirements.
At the same time, the collapse of the twin towers demonstrated the very real dangers of fire in high-rise buildings. In the decades since the World Trade Center was designed, buildings have become taller and more complex, as societies demand sustainable and cost-effective housing in large cities. This can be partly attributed to the wide use of flammable cladding. Nonetheless, conspiracy theorists seized on a handful of "facts" to argue a very different scenario: The jets that struck New York and Washington, D.
And the lack of military intervention? Theorists claim it proves the U. They speculate that this "military pod" is a missile, a bomb, or a piece of equipment on an air-refueling tanker.
Greeley is an expert at analyzing images to determine the shape and features of geological formations based on shadow and light effects. After studying the high-resolution image and comparing it to photos of a Boeing ER's undercarriage, Greeley dismissed the notion that the Howard photo reveals a "pod.
In fact, the photo reveals only the Boeing's right fairing, a pronounced bulge that contains the landing gear. He concludes that sunlight glinting off the fairing gave it an exaggerated look. Culick, professor of aeronautics at the California Institute of Technology, gave a blunter response: "That's bull.
They're really stretching. Elsis of StandDown. None of the fighters got anywhere near the pirated planes. Why couldn't ATC find the hijacked flights? Similar theories surround other states, including Iraq and Iran, but no evidence of their direct involvement has ever been found. Image source, Getty Images. Online conspiracy-theory platforms continue to push claims of US involvement in the attacks. Uncontrolled fires. Footage of 7 World Trade Center vertically collapsing on itself is cited as evidence of a controlled demolition.
Go viral. Business jet. Theories suggest a missile, not a Boeing , actually hit the Pentagon. Image source, Reuters. A theory claims wreckage of United Airlines Flight 93 proves it was shot down. Related Topics. Published 29 August
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